In recent years, predation has been determined to be a seriously limiting factor in the reproduction of the endangered California least tern (Sterna antillarum browni) at many of its nesting colonies. Among them is a major colony at Camp Pendleton Marine Corps Base near Oceanside, CA. Early efforts to control predation were limited in effectiveness. In 1988, the U.S. Department of Agriculture, Animal Damage Control Program was contracted to provide control of mammalian and avian predators. The development of the successful strategy that has evolved over four years is discussed, with emphasis on the development and application of techniques, and the timing and areas of control

Butchko, Peter H.

Small, Maynard A.

English

DigitalCommons@University of Nebraska

University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Proceedings of the Fifteenth Vertebrate Pest Conference 1992 Vertebrate Pest Conference Proceedings collection March 1992 DEVELOPING A STRATEGY OF PREDATOR CONTROL FOR THE PROTECTION OF THE CALIFORNIA LEAST TERN: A CASE HISTORY Peter H. Butchko USDA-APHIS-ADC Maynard A. Small USDA-APHIS-ADC Follow this and additional works at: https://digitalcommons.unl.edu/vpc15  Part of the Environmental Health and Protection Commons Butchko, Peter H. and Small, Maynard A., "DEVELOPING A STRATEGY OF PREDATOR CONTROL FOR THE PROTECTION OF THE CALIFORNIA LEAST TERN: A CASE HISTORY" (1992). Proceedings of the Fifteenth Vertebrate Pest Conference 1992. 14. https://digitalcommons.unl.edu/vpc15/14 This Article is brought to you for free and open access by the Vertebrate Pest Conference Proceedings collection at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Proceedings of the Fifteenth Vertebrate Pest Conference 1992 by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. DEVELOPING A STRATEGY OF PREDATOR CONTROL FOR THE PROTECTION OF THE CALIFORNIA LEAST TERN: A CASE HISTORY PETER H. BUTCHKO, USDA-APHIS-ADC, 1942 South Court, Suite 2, Visalia, California 93277 MAYNARD A. SMALL, USDA-APHIS-ADC, Santa Maria, California 93454 ABSTRACT: In recent years, predation has been determined to be a seriously limiting factor in the reproduction of the endangered California least tern (Sterna antillarum browni) at many of its nesting colonies. Among them is a major colony at Camp Pendleton Marine Corps Base near Oceanside, CA. Early efforts to control predation were limited in effectiveness. In 1988, the U.S. Department of Agriculture, Animal Damage Control Program was contracted to provide control of mammalian and avian predators. The development of the successful strategy that has evolved over four years is discussed, with emphasis on the development and application of techniques, and the timing and areas of control. Proc. 15th Vertebrate Pest Conf. (J. E. Borrecco & R. E. Marsh, Editors) Published at University of Calif., Davis.  1992 INTRODUCTION Predation has been seen by some in recent years as a significant limiting factor in the recovery of endangered spe-cies and as a result predator control has been undertaken to benefit several endangered species (Butchko 1990). One of these is the California least tern which is a seasonal resident of California where it traditionally nests in colonies on the coastal estuaries and beaches. While much of its preferred nesting habitat has been lost to development, approximately 35 colonies remain, most of which are controlled by public agencies. One of the largest and most important is in northern San Diego County at Camp Pendleton Marine Corps Base. The Base Environmental and Natural Resources Manage-ment Office (ENRMO) has conducted a comprehensive least tern protection program including habitat restoration, habitat protection and monitoring, which revealed that both avian and mammalian predators frequently limited reproduction. Such was the case in 1987, when predators - primarily coy-otes - destroyed approximately 106 of 234 nests, resulting in the production of only 65 fledglings (Keane 1989). Early control efforts, including fencing and occasional shooting, were limited in effectiveness (C. Winchell, pers. comm.). Consequently, the U.S. Department of Agriculture, Animal Damage Control Program (ADC) was contracted by the U. S. Navy to provide predator control for the 1988 nesting season. The program was coordinated with the ENRMO and con-ducted with Section 7 approval by the U. S. Fish and Wildlife Service (Kaufman 1988). This paper documents the development and results of the predator control strategy in 1988 through 1991. It is offered as a case history which may be useful to resource managers as they plan a predator control program for a wildlife re-source of their concern. LOCATION There are actually two tern colonies on Camp Pendleton but for the purposes of this paper, discussion will focus on the colony within the Santa Margarita River estuary. The estuary is an area of approximately 400 acres consisting of sandy dunes, tidal flats and salt marsh habitat. The dominant vege-tation of the estuary is pickleweed (Salicornis virginica) while the beach was dominated by sea-rocket (Cakile maritima). The Santa Margarita colony consists of three sites: North Beach, Salt Flats and Salt Hats Island. The North Beach site is the largest—approximately 30 acres—with approximately 80% of the 200+ nests occurring there in recent years (D. Boyer, pers. comm.). It is bounded by an extensive, solar-powered electric fence. The Salt Flats and Salt Flats Island are unfenced and are used by approximately 20% of the terns. Areas adjacent to the estuary include a recreational beach, military facilities, agricultural fields, riparian vegetation, upland vegetation and a freeway/railroad corridor. The estuary itself is off limits to people but the surrounding areas have considerable military and civilian activity. The only people, other than ENRMO personnel, authorized to enter the colony itself were those individuals under contract to monitor the tern nesting. ADC investigated predation signs within the colony when notified by the contract monitors. 1988 PROGRAM Analysis An analysis of the site and the terns revealed the follow-ing factors as significant in developing a strategy: 1) Due to the endangered status of the terns, the ENRMO considered the protection of the terns from predators the highest priority. 2) Many potential predators were known to be in or near the estuary, including coyotes (Canis latrans), raccoon (Pro- cyon lotor), opossum (Didelphis virginianus), striped skunk (Mephitis mephitis), ground squirrel (Spermophilius bee- chyii), raven (Corvus corax), crow (Corvus brachyrhynchos), kestrel (Falco sparverius), and loggerhead shrike (Lanius ludovicianus). Based on previous observations of predation at the Santa Margarita colony, it was expected that the most serious threat to the terns were coyotes, kestrels, shrikes and ravens. 3) The electric fence around the North Beach site, while necessary, was not 100% reliable in excluding mammalian predators. 4) The regular presence of predators in or near the nest ing sites can deter site selection or result in nest abandonment through harassment. 5) Even one predatory episode can result in extensive losses. This was illustrated in 1987 when coyotes caused severe nest destruction, most of which occurred in three nights (C. Winchell, pers. comm.). 29 Goal As a result of this analysis, it was mutually agreed that the goal of the program was to make the colony free of all known and potential predators during the tern nesting season. Strategy The following elements were selected to effect the strat-egy necessary to accomplish the goal: 1) The timing of control efforts would be phased in to coincide with tern breeding stages. Because mammals can disrupt adult terns during site selection and nest construction, mammalian control began prior to the anticipated tern arrival. Because ravens can disrupt nesting activities, ravens were controlled when nest selection began. Because kestrels and shrikes are most threatening to tern chicks, control of these species was initiated just prior to tern hatching. 2) Control methods would include steel leghold traps, cage traps, shooting, gas cartridges (for ground squirrels), raptor trap (Channing and Bal-chatri) and DRC-1339. DRC- 1339 was used to control ravens and crows, under a state research authorization, by injecting 1 milliliter of a 10% solu- tion into hard-boiled chicken eggs. The baited eggs were delivered to ravens and crows on the ground near the estuary after acceptance of untreated eggs. A minor modification was made in the use of steel traps near the colony. Tension de- vices, routinely used elsewhere to eliminate smaller, non- target animals, were not used in order to catch all predators in close proximity to the nesting colony. Control efforts were conducted seven days a week. 3) Control efforts would be conducted not only in the colony but also in a buffer zone surrounding the colony. This was accomplished by beginning control efforts within 50 yards of the tern colony. As these control efforts were suc- cessful in removing predators, control efforts were expanded outward in an effort to prevent predators from entering the colony. In an effort to remove the coyotes, the most mobile and threatening mammalian predator, this perimeter was ex- tended to a distance of approximately 1 mile. There were many (at least seven) mated pairs of coyotes coming into the estuary from surrounding areas to forage in the estuary. To make avian control more selective, ADC extensively moni- tored their presence and movements in areas adjacent to the estuary. Once avian predators entered the estuary they were the object of control efforts. 4) A system of communication between tern monitors and ADC personnel was critical. Because only monitors rou- tinely entered the tern colony itself, they were most likely to observe or suspect predation which would be information vital to ADC personnel. However, since both monitors and ADC personnel had irregular schedules, face-to-face communication was very unpredictable. To facilitate this communication, a log book was kept at the estuary for both parties to record and receive pertinent information regarding predators or nesting activities. Results The 1988 tern nesting results were outstanding. An esti-mated 246 pairs fledged an estimated 365-409 birds. This was by far the most productive colony in the state in 1988 (Massey 1988) and the most productive the Santa Margarita colony had been in many years (Keane 1989). Virtually no nests or birds were lost to predators. The ENRMO attributes the successful season primarily to the effective predator con-trol program (C. Winchell, pers. comm.). 1989 PROGRAM Strategy In 1989, the predator control program for the protection of the least terns was resumed with the same elements as the 1988 program with the following exceptions: 1) DRC-1339 treated hard-boiled chicken eggs were again used to control corvids. However, instead of presenting the eggs on the ground, the eggs were placed on elevated platforms and secured to the platform with a wire. This was done to eliminate the caching of eggs by ravens, which was observed in 1988, and to reduce the hazard to non-target animals. 2) Raptor control efforts were expanded. Shooting, Channing traps and Bal-chatri traps were often effective but there were many cases, such as with owls, when they were neither effective and/or feasible. Therefore, an adaptation of a pole trap was developed and found to be quite effective. It involved affixing a small (1 inch) elevated perch to the pan of a small (#0 or 1/2) padded jaw steel trap. The trap was mounted on a post or sections of aluminum conduit with a wire for the trap to slide to the ground when it was sprung. The pole trap assembly was built so as to be easily portable and was used almost continually once terns began hatching. The threat of raptor depredation increased with the rec-ognition that Northern harriers (Circus cyaneus) were prey-ing on terns. A harrier, shot while inside the North Beach site, revealed upon examination that it had consumed five em-bryos - three tern and two snowy plover embryos - in ten minutes. 3) Control efforts, in one instance, were begun after pre- dation was occurring. As a result of a misunderstanding, con- trol of ground squirrels was delayed until significant egg predation was occurring at North Beach. Upon discovery of this, control of ground squirrels was attempted with shooting, gas cartridges, conibear traps and steel traps. It took approxi mately 14 days of control until egg losses were halted. By this time approximately 50 nests were destroyed. The ground squirrel predation, in combination with severe harrier preda- tion, seemed to reduce the vigor of the terns defensive mob- bing and perhaps increased nest abandonment. Results The 1989 tern nesting results were significantly reduced from 1988. An estimated 163 nests producing only 67 fledg-lings (Massey 1989). The primary reason for this was consid-ered to be that ground squirrel and harrier control was done reactively rather than proactively. 1990 AND 1991 PROGRAM Strategy The predator control program for the protection of the least terns was resumed with the elements as in previous years with the following modifications: 1) Reduction in pole trap use. In 1990, in an instance that was considered virtually impossible, a tern landed on and was caught in a pole trap. The incident was immediately reported to the Fish and Wildlife Service and the consensus was that the bird should be released on site. To further mini- 30 mize the unlikely event of this reoccurring, no pole traps are set when monitors enter the colony, which causes significant numbers of terns to become airborne. Gradually, pole traps were generally limited to nighttime use after owls were ob-served predating on terns or other shorebirds within the estu-ary. This was feasible only because ADC personnel and contract monitors were able to provide extensive surveillance of the estuary. 2) Increased use of nonlethal methods on harriers. Northern harriers, while common elsewhere in California, are uncommon breeders in coastal Southern California and are becoming a species of special concern. Efforts are now made to disrupt harrier nesting efforts, which seems to divert their attention from the tern colony, before lethal control of the adults is attempted. 3) Increased use of relocation of target animals. Because of the large size of Camp Pendleton, it affords unusual oppor- tunities to relocate animals within the property boundaries. At the request of ENRMO, this option was occasionally used, particularly for barn owls and bobcats. The fate of the relo- cated animals is unknown but their return to the tern colony has not been documented. 4) Reduce lethal control of ravens. The Navy has con- tracted with the Denver Wildlife Research Center to study raven behavior and taste aversives as it pertains to deterring predation upon terns. To accommodate the research, ravens were only controlled when they foraged in a colony. Results - Tern nesting was very successful. In 1990, 293 pairs fledged and an estimated 275 - 335 birds (Belluomini 1991a). In 1991, 328 pairs fledged an estimated 377 birds, which was the highest ever recorded at Camp Pendleton (Belluomini 1991b). While many factors contributed to this success, the reduction of predation is considered to be a major factor (D. Boyer pers. comm.). DISCUSSION California ADC personnel had not been involved in any operational program protecting endangered birds, such as the least tern, until 1988. After four years, we believe we have developed a strategy that is effective if thoroughly applied. Two principles for effective predator control can be summa-rized from this case history: 1) Effective predator control must be of sufficient scale and intensity. Buffer zones and the removal of all known and potential predators ought to be considered. 2) Effective predator control must be implemented prior to the onset of predation. This was clearly proven in the inadvertent delay in the control of ground squirrels in 1989. Obviously, there are other factors that pertain to the for- mation of a successful predator control program. However, wildlife managers unfamiliar with predator control would be inclined to violate these principles by underestimating preda-tor control requirements. Wildlife managers may tend to un-wisely restrict the scale and intensity of control or delay control efforts in order to minimize the number of predators removed. Such restrictions or delays reduce the margin of safety and increase the risk of predation to the resource, which is ill-advised, if the resource is truly deserving of protection. ACKNOWLEDGEMENTS We are deeply indebted to the U.S. Navy and the Camp Pendleton ENRMO who assisted and supported our efforts: T. Burr, S. Buck, C. Winchell, D. Boyer, and GySgt M. Murphy. We wish to recognize the following ADC personnel who have participated significantly and professionally: C. Adams, E. Knittle, S. Little, W. Millsap, R. Moschetti, S. Moyles.P. O'Neil, R. Scott, and J. Turman. Special thanks to G. Ferreira and V. Dice for preparing this manuscript. LITERATURE CITED BELLUOMINI, L. A. 1991a. The Status of the California Least Tern at Camp Pendleton, California, During the Breeding Season of 1990. Unpubl. report prepared for the Natural Resources Management Branch, Southwest Division of Naval Facilities Engineering Command, San Diego, California. 77 pp. BELLUOMINI, L. A. 1991b. Summary of the Status of the California Least Tern at Camp Pendleton During the 1991 Breeding Season. Agreement Number: N68711-91-LT-1009.6pp. BUTCHKO, P. H. 1990. Predator control for the protection of endangered species in California. Proc. Vertebr. Pest Conf. 14:237-240. Univ. of Calif., Davis. KAUFMAN, N. M. 1988. Field Supervisor, U. S. Fish and Wildlife Service, Laguna Field Office, Laguna Niguel, California 92656. Letter to Mr. Ron A. Thompson, State Director, USDA-APHIS-ADC, dated April 5,1988. KEANE, K. 1989. Nesting Efforts of the California Least Tem at Camp Pendleton California, 1987. Unpublished report prepared for Natural Resources Management Branch, Western Division Naval Facilities Engineering Command, San Bruno, CA. 66 pp. MASSEY, B. W. 1988. California Least Tern Field Study, 1988 Breeding Season. Final Report to CA Department ofFish&Game.22pp. MASSEY, B. W. 1989. California Least Tern Field Study, 1989 Breeding Season. Draft Report to CA Department of Fish & Game. 24 pp.  31 

DEVELOPING A STRATEGY OF PREDATOR CONTROL FOR THE PROTECTION OF THE CALIFORNIA LEAST TERN: A CASE HISTORY

eScholarship - University of California

UC Agriculture & Natural ResourcesProceedings of the Vertebrate Pest ConferenceTitleDeveloping a strategy of predator control for the protection of the California least tern: A case studyPermalinkhttps://escholarship.org/uc/item/69k0g18pJournalProceedings of the Vertebrate Pest Conference, 15(15)ISSN0507-6773AuthorsButchko, Peter H.Small, Maynard A.Publication Date1992eScholarship.org Powered by the California Digital LibraryUniversity of CaliforniaDEVELOPING A STRATEGY OF PREDATOR CONTROL FOR THE PROTECTION OF THE CALIFORNIA LEAST TERN: A CASE HISTORY PETER H. BUTCHKO, USDA-APHIS-ADC, 1942 South Court, Suite 2, Visalia, California 93277 MAYNARD A. SMALL, USDA-APHIS·ADC, Santa Maria, California 93454 ABSTRACT: . In ~t yearn, predation has been detennined to be a seriously limiting factor in the reproduction of the endangered CaliforlW! least tern (Sterna amillarU11f brown£) at many of its nesting colonies. Among them is a major colony at Camp Pendleton Marine Coips Base near Oceanside, CA. Early efforts to control predation were limited in effeetiveness. Jn 1988, ~ U.S. Department of Agriculture, Animal Damage Control Program was conttacted to provide control of mammalian and aVL\lll predators. The development of the successful strategy that has evolved over four years is di&:ussed, with emphasis on the development and application of techniques, and the timing and areas of control. INTRODUCTION Predation has been seen by some in recent years as a significant limiting factor in the reeovery of endangered spe-cies and as a result predator control has been undertaken to benefit several endangered species (Bulehko 1990). One of these is the California least tern which is a seasonal resident of California where it traditionally nests in colonies on the coastal estuaries and beaches. While much of its preferred nesting habitat has been lost to development, approximately 35 colonies remain, most of which are controlled by public agencies. One of the largest and most important is in nonhem San Diego County at Camp Pendleton Marine Crups Base. The Base Environmental and Natural Resources Manage-ment Office (ENRMO) bas conducted a comprehensive least tern protection program including habitat restoration, habitat protection and monitoring, which revealed that bolh avian and mammalian predators ftequently limited reproduction. Such was the case in 1987, when predators - primarily coy· ot.es • destroyed approximately I 06 of 234 nests, resulting in the production of only 65 fledglings (Keane 1989). Early control efforts, including fencing and occasional shooting, were limited in effectiveness (C. Winchell, pers. comm.). Consequently, the U.S. Depanment of Agriculture, Animal Damage Control Program (ADC) was contracted by the U.S. Navy to provide predator control for the 1988 nesting season. The program was coordinated with lite ENRMO and con· ducted wilh Section 7 approval by the U. S. Fish and Wildlife Service (Kaufman 1988). This paper documents the development and results of the predator control strategy in 1988 through 1991. It is offered as a case history which may be useful to resoun:e managers as they plan a predator control program for a wildlife re-soun:e of their concern. LOCATION There are actually two tern colonies on Camp Pendleton but for the pwposes of this paper, discussion will focus on the colony within the Santa Margarita River estuary. The estuary is an area of approx.imalely 400 acres consisting of sandy dunes, tidal flats and salt marsh habitat. The dominant vege-tation of the estuary is pickle weed (Salicornis virginica) while the beach was dominated by sea-rocket (Ca/die maritima). The Santa Margarita colony consists of three sites: North Beach, Salt Flats and Salt Flats Island. The North Beach site 29 Proo. 15th Vertebrate Pest Conf. (J.E. llorrecoo & R. E. Marsh, Editors) Published at University of Calif., Davis. 1992 is the largest-approximately 30 acres-with approximately 80% of the 200+ nests occurring there in recent yearn (D. Boyer, pers. comm.). It is bounded by an extensive, solar-powered electric fence. The Salt Flats and Salt Flats Island are unfenced and are used by approximately 20% of the terns. Areas adjacent to the estuary include a recreational beach, military facilities. agricultural fields, riparian vegeta-tion, upland vegetation and a freeway/railroad corridor. The estuary itself is off limits to people but the surrounding areas have considerable military and civilian activity. The only people, other than ENRMO personnel, authorized to enter the colony itself were those individuals under contract to monitor the tern nesting. ADC investigated predation signs within the colony when notified by the contract monitors. 1988 PROGRAM Analysis An analysis of the site and the terns revealed the follow-ing factors as significant in developing a strategy: l) Due to the endangered status of the terns, theENRMO considered the protection of the terns from predators the highest priority. 2) Many potential predators were known to be in or near the estuary, including coyotes (Canis /a1rans), raccoon (Pro-cyon /otor), opossum (Didelphis virginianus), striped skunk (Mephitis mephitis), ground squirrel (Spemwphilius bee· chyi1), raven (Corvus corax), crow (Corvus brachyrhynchos), kestrel (Falco sparverius). and loggerhead shrike (Lanius /udovici1JJ1us). Based on previous observations of predation at the Santa Margarita colony, it was expeeted that the most serious threat to the terns were coyotes, kestrels, shrikes and ravens. 3) The electric fence around the North Beach site, while necessary, was not l 00% reliable in excluding mammalian predators. 4) The regular presence of predators in or near the nest· ing sit.es can deter site selection or result in nest abandonment through harassment 5) Even one predatory episode can result in exiensive losses. This was illustrated in 1987 when coyot.es caused severe nest destruction, most of which occurred in three nighlS (C. Winchell, pers. comm.). Goal As a result of this analysis, it was mutually agreed that the goal of the program was to make the colony free of all known and potential predators during the tern nesting season. Strategy The following elements were selected to effect the strat-egy necessary to accomplish the goal: I) The timing of control efforts would be phased in to coincide with tern breeding stages. Because mammals can disrupt adult terns during site selection and nest construction, mammalian control began prior to the anticipated tern arrival. Because ravens can disrupt nesting activities, ravens were controlled when nest selection began. Because kestrels and shrikes are most threatening to tern chicks, control of these species was initiated just prior to tern hatching. 2) Control methods would include steel leghold traps, cage traps, shooting, gas cartridges (for ground squirrels), raplor trap (Channing and Bal-chatri) and DRC-1339. DRC-1339 was used to control ravens and crows, under a state research authorization, by injecting 1 milliliter of a 10% solu-tion into hard-boiled chicken eggs. The baited eggs were delivered to ravens and crows on the ground near the estuary after acceptance of untreated eggs. A minor modification was made in the use of steel traps near the colony. Tension de-vices, routinely used elsewhere to eliminate smaller, non-target animals, were not used in order to catch all predators in close proximity to the nesting colony. Control efforts were conducted seven days a week. 3) Control efforts would be conducted not only in the colony but also in a buffer zone surrounding the colony. This was accomplished by beginning control efforts within 50 yards of the tern colony. As these control efforts were suc-cess-fol in removing predators, control efforts were expanded outward in an effort to prevent predators from entering the colony. In an effort to remove the coyotes, the most mobile and threatening mammalian predator, this perimeter was ex-tended to a distance of approximately 1 mile. There were many (at least seven) mated pairs of coyotes coming into the estuary from surrounding areas to forage in the estuary. To make avian control more selective, ADC extensively moni-tored their presence and movements in areas adjacent to the estuary. Once avian predators entered the estuary they were the object of control efforts. 4) A system of communication between tern monitors and ADC personnel was critical. Because only monitors rou-tinely entered the tern colony itself, they were most likely to observe or suspect predation which would be information vital to ADC personnel. However, since both monitors and ADC personnel had irregular schedules, face-to-face communication was very unpredictable. To facilitate this communication, a log book was kept at the estuary for both parties to record and receive pertinent information regarding predaLOrs or nesting activities. Results The 1988 tern nesting results were outstanding. An esti-mated 246 pairs fledged an estimated 365-409 birds. This was by far the most productive colony in the state in 1988 (Massey 1988) and the most productive the Santa Margarita colony had been in many years (Keane 1989). Virtually no 30 nests or birds were lost to predators. The ENRMO attributes the successful season primarily to the effective predator con-trol program (C. Winchell, pers. comm.). 1989 PROGRAM Strategy In 1989, the predator control program for the protection of the least terns was resumed with the same elements as the 1988 program with the following exceptions: 1) DRC-1339 tteated hard-boiled chicken eggs were again used to control corvids. However, instead of presenting the eggs on the ground, the eggs were placed on elevated platfonns and secured to the plalf orm with a wire. This was done to eliminate the caching of eggs by ravens, which was observed in 1988, and to reduce the hazard to non-target animals. 2). Raptor control efforts were expanded. Shooting, Channmg traps and Bal-chatri traps were often effective but there were many cases, such as with owls, when they were neither effective and/or feasible. Therefore, an adaptation of a pole trap was developed and found to be quite effective. It involved affixing a small (1 inch) elevated perch to the pan of a small (#0 or 1/2) padded jaw steel trap. The trap was mounted on a post or sections of aluminum conduit with a wire for the trap to slide to the ground when it was sprung. The pole trap asrembly was built so as to be easily portable and was used almost continually once terns began hatching. The threat of raplOr depredation increased with the rec-ognition that Northern harriers (Circus cyaneus) were prey-ing on terns. A harrier, shot while inside the North Beach site, revealed upon examination that it had consumed five em-bryos - three tern and two snowy plover embryos - in ten minutes. 3) Control efforts, in one instance, were begun after pre-dation was occurring. As a result of a misunderstanding, con-trol of ground squirrels was delayed until significant egg predation was occurring at North Beach. Upon discovery of this, control of ground squirrels was attempted with shooting, gas cartridges, conibear traps and steel traps. It took approxi-mately 14 days of control until egg losses were halted. By this time approximately 50 nests were destroyed. The ground squirrel predation, in combination with severe harrier preda-tion, seemed to reduce the vigor of the terns defensive mob-bing and perhaps increased nest abandonment Results The 1989 tern nesting results were significantly reduced from 1988. An estimated 163 nests producing only 67 fledg~ lings (Massey 1989). The primary reason for this was consid-ered to be that ground squirrel and harrier control was done reactively rather than proactively. 1990 AND 1991 PROGRAM Strategy The predator control program for the protection of the least terns was resumed with the elements as in previous years with the following modifications: 1) Reduction in pole trap use. In 1990, in an instance that was considered virtually impossible, a tern landed on and was caught in a pole trap. The incident was immediately reported to the Fish and Wildlife Service and the consensus was that the bird should be released on site. To further mini-mize !he unlikely event of !his rooccuning, no pole traps are set when monitors enter !he colony, which causes significant numbers of terns to become aitbome. Gradually, pole traps were generally limited to nighttime use after owls were ob-served predating on terns or olher shorebirds wilhln the estu-ary. This was feasible only because ADC personnel and contract monitors were able to provide extensive surveillance oflhe estuary. 2) Increased use of nonlelhal methods on harriers. Norlhem harriers, while common elsewhere in California, are uncommon breeders in ooastal Southern California and are becoming a species of special concern. Efforts are now made to disrupt harrier nesting efforts, which seems to diven their attention from the tern colony, before lethal control of the adults is attempted. 3) Increased use of relocation of target animals. Because of the large size of Camp Pendleton, it affords unusual oppor-tunities to relocate animals within the property boundaries. At therequestofENRMO, thisopr.ion was occasionally used, particularly for barn owls and bobcats. The fate of the relo-cated animals is unknown but their return to the tern colony bas not been documented. 4) Reduce lethal control of ravens. The Navy has con-tracted willt the Denver Wildlife Research Center to study raven behavior and taste aversives as it pertains to deterring predar.ion upon terns. To accommodate the research, ravens were only controlled when they foraged in a colony. Results. Tern nesting was very successful. In 1990, 293 pairs fledged and an estimated 275 - 335 birds (Belluomini 199la). In 1991, 328 pairs fledged an estimated 377 birds, which was the highest ever recorded at Camp Pendleton (Belluomini 1991 b). While many factors contributed to !his success, !he reduction of predation is considered to be a major factor (D. Boyer pers. comm.). DISCUSSION California ADC personnel had not been involved in any operational program protecting endangered birds, such as the least tern, until 1988. After four years, we believe we have developed a strategy that is effective if thoroughly applied. Two principles for effective predator control can be summa· rized from !his case history: l) Effective predator control must be of sufficient scale and intensity. Buffer zones and the removal of all known and potential predators ought to be considered. 2) Effecr.ive predator control must be implemented prior to the onset of predation. This was clearly proven in the inadvertent delay in the control of ground squirrels in 1989. Obviously, there are ollter factOrs that pertain to the for-31 mation of a successful predator control program. ffilwever, wildlife managers unfamiliar willt predator control would be inclined to violate lhese principles by underestimar.ing preda-tor control requirements. Wildlife managers may tend to un-wisely restrict the scale and intensity of control or delay control efforts in order to minimize the number of predators removed. Such restricr.ions or delays reduce the margin of safety and increase theriskofpredar.ion to !he resource, which is ill-advised, if the resource is truly deserving of protecr.ion. ACKNOWLEDGEMENTS We are deeply indebted to the U.S. Navy and the Camp Pendleton ENRMO who assisted and supported our efforts: T. Burr, S. Buck, C. Winchell, D. Boyer, and GySgt M. Murphy. We wish to recognize the following ADC personnel who have participated significantly and professionally: C. Adams, E. Knittle, S. Little, W. Millsap, R. Moschetti, S. Moyles, P. O'Neil, R. Scott, and J. Turman. Special thanks to G. Feneira and V. Dice for preparing !his manuscript LITERATURE CITED BELLUOMINI, L. A. 199la. The Status of the California Least Tern at Camp Pendleton, California, During the Breeding Season of 1990. Unpubl. report prepared for the Natural Resources Management Branch, Southwest Division of Naval Facilities Engineering Command, San Diego, California. 77 pp. BELLUOMINI, L.A. 1991b. Summazy of the Status of the California Least Tern at Camp Pendleton During the 1991 Breeding Season. Agreement Number. N68711-91-LT-l009. 6pp. BUTCHKO, P. H. 1990. Predator control for the protection of endangered species in California. Proc. Vertebr. Pest Conf. 14:237-240. Univ. of Calif., Davis. KAUFMAN, N. M 1988. Field Supervisor, U. S. Fish and Wildlife Service, Laguna Field Office, Laguna Niguel, California 92656. Letter to Mr. Ron A. Thompson, State Director, USDA-APHIS-ADC, dated April 5, 1988. KEANE, K. 1989. Nesting Efforts of the California Least Tern at Camp Pendleton California, 1987. Unpublished report prepared for Natural Resources Management Branch, Western Division Naval Facilities Engineering Command, San Bruno, CA. 66 pp. MASSEY, B. W. 1988. California Least Tern Field Study, 1988 Breeding Season. Final Report to CA Department of Fish & Game. 22 pp. MASSEY, B. W. 1989. California Least Tern Field Study, 1989 Breeding Season. Draft Report to CA Department of Fish & Game. 24 pp. 

Developing a strategy of predator control for the protection of the California least tern: A case study

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Proceedings of the Fifteenth Vertebrate Pest 
Conference 1992 
Vertebrate Pest Conference Proceedings 
collection 
March 1992 
DEVELOPING A STRATEGY OF PREDATOR CONTROL FOR THE 
PROTECTION OF THE CALIFORNIA LEAST TERN: A CASE 
HISTORY 
Peter H. Butchko 
USDA-APHIS-ADC 
Maynard A. Small 
USDA-APHIS-ADC 
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Butchko, Peter H. and Small, Maynard A., "DEVELOPING A STRATEGY OF PREDATOR CONTROL FOR THE 
PROTECTION OF THE CALIFORNIA LEAST TERN: A CASE HISTORY" (1992). Proceedings of the Fifteenth 
Vertebrate Pest Conference 1992. 14. 
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Lincoln. 
DEVELOPING A STRATEGY OF PREDATOR CONTROL FOR THE 
PROTECTION OF THE CALIFORNIA LEAST TERN: A CASE HISTORY 
PETER H. BUTCHKO, USDA-APHIS-ADC, 1942 South Court, Suite 2, Visalia, California 93277 
MAYNARD A. SMALL, USDA-APHIS-ADC, Santa Maria, California 93454 
ABSTRACT: In recent years, predation has been determined to be a seriously limiting factor in the reproduction of the 
endangered California least tern (Sterna antillarum browni) at many of its nesting colonies. Among them is a major colony at 
Camp Pendleton Marine Corps Base near Oceanside, CA. Early efforts to control predation were limited in effectiveness. In 
1988, the U.S. Department of Agriculture, Animal Damage Control Program was contracted to provide control of mammalian 
and avian predators. The development of the successful strategy that has evolved over four years is discussed, with emphasis on 
the development and application of techniques, and the timing and areas of control. 
Proc. 15th Vertebrate Pest Conf. (J. E. Borrecco & R. E. Marsh, 
Editors) Published at University of Calif., Davis.  1992 
INTRODUCTION 
Predation has been seen by some in recent years as a 
significant limiting factor in the recovery of endangered spe-
cies and as a result predator control has been undertaken to 
benefit several endangered species (Butchko 1990). One of 
these is the California least tern which is a seasonal resident 
of California where it traditionally nests in colonies on the 
coastal estuaries and beaches. While much of its preferred 
nesting habitat has been lost to development, approximately 
35 colonies remain, most of which are controlled by public 
agencies. One of the largest and most important is in northern 
San Diego County at Camp Pendleton Marine Corps Base. 
The Base Environmental and Natural Resources Manage-
ment Office (ENRMO) has conducted a comprehensive least 
tern protection program including habitat restoration, habitat 
protection and monitoring, which revealed that both avian 
and mammalian predators frequently limited reproduction. 
Such was the case in 1987, when predators - primarily coy-
otes - destroyed approximately 106 of 234 nests, resulting in 
the production of only 65 fledglings (Keane 1989). Early 
control efforts, including fencing and occasional shooting, 
were limited in effectiveness (C. Winchell, pers. comm.). 
Consequently, the U.S. Department of Agriculture, Animal 
Damage Control Program (ADC) was contracted by the U. S. 
Navy to provide predator control for the 1988 nesting season. 
The program was coordinated with the ENRMO and con-
ducted with Section 7 approval by the U. S. Fish and Wildlife 
Service (Kaufman 1988). 
This paper documents the development and results of the 
predator control strategy in 1988 through 1991. It is offered 
as a case history which may be useful to resource managers 
as they plan a predator control program for a wildlife re-
source of their concern. 
LOCATION 
There are actually two tern colonies on Camp Pendleton 
but for the purposes of this paper, discussion will focus on the 
colony within the Santa Margarita River estuary. The estuary 
is an area of approximately 400 acres consisting of sandy 
dunes, tidal flats and salt marsh habitat. The dominant vege-
tation of the estuary is pickleweed (Salicornis virginica) while 
the beach was dominated by sea-rocket (Cakile maritima). 
The Santa Margarita colony consists of three sites: North 
Beach, Salt Flats and Salt Hats Island. The North Beach site 
is the largest—approximately 30 acres—with approximately 
80% of the 200+ nests occurring there in recent years (D. 
Boyer, pers. comm.). It is bounded by an extensive, solar-
powered electric fence. The Salt Flats and Salt Flats Island 
are unfenced and are used by approximately 20% of the terns. 
Areas adjacent to the estuary include a recreational beach, 
military facilities, agricultural fields, riparian vegetation, 
upland vegetation and a freeway/railroad corridor. The 
estuary itself is off limits to people but the surrounding areas 
have considerable military and civilian activity. The only 
people, other than ENRMO personnel, authorized to enter the 
colony itself were those individuals under contract to monitor 
the tern nesting. ADC investigated predation signs within the 
colony when notified by the contract monitors. 
1988 PROGRAM 
Analysis 
An analysis of the site and the terns revealed the follow-
ing factors as significant in developing a strategy: 
1) Due to the endangered status of the terns, the ENRMO 
considered the protection of the terns from predators the 
highest priority. 
2) Many potential predators were known to be in or near 
the estuary, including coyotes (Canis latrans), raccoon (Pro- 
cyon lotor), opossum (Didelphis virginianus), striped skunk 
(Mephitis mephitis), ground squirrel (Spermophilius bee- 
chyii), raven (Corvus corax), crow (Corvus brachyrhynchos), 
kestrel (Falco sparverius), and loggerhead shrike (Lanius 
ludovicianus). Based on previous observations of predation 
at the Santa Margarita colony, it was expected that the most 
serious threat to the terns were coyotes, kestrels, shrikes and 
ravens. 
3) The electric fence around the North Beach site, while 
necessary, was not 100% reliable in excluding mammalian 
predators. 
4) The regular presence of predators in or near the nest 
ing sites can deter site selection or result in nest abandonment 
through harassment. 
5) Even one predatory episode can result in extensive 
losses. This was illustrated in 1987 when coyotes caused 
severe nest destruction, most of which occurred in three nights 
(C. Winchell, pers. comm.). 
29 
Goal 
As a result of this analysis, it was mutually agreed that 
the goal of the program was to make the colony free of all 
known and potential predators during the tern nesting season. 
Strategy 
The following elements were selected to effect the strat-
egy necessary to accomplish the goal: 
1) The timing of control efforts would be phased in to 
coincide with tern breeding stages. Because mammals can 
disrupt adult terns during site selection and nest construction, 
mammalian control began prior to the anticipated tern arrival. 
Because ravens can disrupt nesting activities, ravens were 
controlled when nest selection began. Because kestrels and 
shrikes are most threatening to tern chicks, control of these 
species was initiated just prior to tern hatching. 
2) Control methods would include steel leghold traps, 
cage traps, shooting, gas cartridges (for ground squirrels), 
raptor trap (Channing and Bal-chatri) and DRC-1339. DRC- 
1339 was used to control ravens and crows, under a state 
research authorization, by injecting 1 milliliter of a 10% solu- 
tion into hard-boiled chicken eggs. The baited eggs were 
delivered to ravens and crows on the ground near the estuary 
after acceptance of untreated eggs. A minor modification was 
made in the use of steel traps near the colony. Tension de- 
vices, routinely used elsewhere to eliminate smaller, non- 
target animals, were not used in order to catch all predators in 
close proximity to the nesting colony. Control efforts were 
conducted seven days a week. 
3) Control efforts would be conducted not only in the 
colony but also in a buffer zone surrounding the colony. This 
was accomplished by beginning control efforts within 50 
yards of the tern colony. As these control efforts were suc- 
cessful in removing predators, control efforts were expanded 
outward in an effort to prevent predators from entering the 
colony. In an effort to remove the coyotes, the most mobile 
and threatening mammalian predator, this perimeter was ex- 
tended to a distance of approximately 1 mile. There were 
many (at least seven) mated pairs of coyotes coming into the 
estuary from surrounding areas to forage in the estuary. To 
make avian control more selective, ADC extensively moni- 
tored their presence and movements in areas adjacent to the 
estuary. Once avian predators entered the estuary they were 
the object of control efforts. 
4) A system of communication between tern monitors 
and ADC personnel was critical. Because only monitors rou- 
tinely entered the tern colony itself, they were most likely to 
observe or suspect predation which would be information 
vital to ADC personnel. However, since both monitors and 
ADC personnel had irregular schedules, face-to-face 
communication was very unpredictable. To facilitate this 
communication, a log book was kept at the estuary for both 
parties to record and receive pertinent information regarding 
predators or nesting activities. 
Results 
The 1988 tern nesting results were outstanding. An esti-
mated 246 pairs fledged an estimated 365-409 birds. This 
was by far the most productive colony in the state in 1988 
(Massey 1988) and the most productive the Santa Margarita 
colony had been in many years (Keane 1989). Virtually no 
nests or birds were lost to predators. The ENRMO attributes 
the successful season primarily to the effective predator con-
trol program (C. Winchell, pers. comm.). 
1989 PROGRAM 
Strategy 
In 1989, the predator control program for the protection 
of the least terns was resumed with the same elements as the 
1988 program with the following exceptions: 
1) DRC-1339 treated hard-boiled chicken eggs were 
again used to control corvids. However, instead of presenting 
the eggs on the ground, the eggs were placed on elevated 
platforms and secured to the platform with a wire. This was 
done to eliminate the caching of eggs by ravens, which was 
observed in 1988, and to reduce the hazard to non-target 
animals. 
2) Raptor control efforts were expanded. Shooting, 
Channing traps and Bal-chatri traps were often effective but 
there were many cases, such as with owls, when they were 
neither effective and/or feasible. Therefore, an adaptation of a 
pole trap was developed and found to be quite effective. It 
involved affixing a small (1 inch) elevated perch to the pan of 
a small (#0 or 1/2) padded jaw steel trap. The trap was 
mounted on a post or sections of aluminum conduit with a 
wire for the trap to slide to the ground when it was sprung. 
The pole trap assembly was built so as to be easily portable 
and was used almost continually once terns began hatching. 
The threat of raptor depredation increased with the rec-
ognition that Northern harriers (Circus cyaneus) were prey-
ing on terns. A harrier, shot while inside the North Beach site, 
revealed upon examination that it had consumed five em-
bryos - three tern and two snowy plover embryos - in ten 
minutes. 
3) Control efforts, in one instance, were begun after pre- 
dation was occurring. As a result of a misunderstanding, con- 
trol of ground squirrels was delayed until significant egg 
predation was occurring at North Beach. Upon discovery of 
this, control of ground squirrels was attempted with shooting, 
gas cartridges, conibear traps and steel traps. It took approxi 
mately 14 days of control until egg losses were halted. By this 
time approximately 50 nests were destroyed. The ground 
squirrel predation, in combination with severe harrier preda- 
tion, seemed to reduce the vigor of the terns defensive mob- 
bing and perhaps increased nest abandonment. 
Results 
The 1989 tern nesting results were significantly reduced 
from 1988. An estimated 163 nests producing only 67 fledg-
lings (Massey 1989). The primary reason for this was consid-
ered to be that ground squirrel and harrier control was done 
reactively rather than proactively. 
1990 AND 1991 PROGRAM 
Strategy 
The predator control program for the protection of the 
least terns was resumed with the elements as in previous 
years with the following modifications: 
1) Reduction in pole trap use. In 1990, in an instance 
that was considered virtually impossible, a tern landed on and 
was caught in a pole trap. The incident was immediately 
reported to the Fish and Wildlife Service and the consensus 
was that the bird should be released on site. To further mini- 
30 
mize the unlikely event of this reoccurring, no pole traps are 
set when monitors enter the colony, which causes significant 
numbers of terns to become airborne. Gradually, pole traps 
were generally limited to nighttime use after owls were ob-
served predating on terns or other shorebirds within the estu-
ary. This was feasible only because ADC personnel and 
contract monitors were able to provide extensive surveillance 
of the estuary. 
2) Increased use of nonlethal methods on harriers. 
Northern harriers, while common elsewhere in California, are 
uncommon breeders in coastal Southern California and are 
becoming a species of special concern. Efforts are now made 
to disrupt harrier nesting efforts, which seems to divert their 
attention from the tern colony, before lethal control of the 
adults is attempted. 
3) Increased use of relocation of target animals. Because 
of the large size of Camp Pendleton, it affords unusual oppor- 
tunities to relocate animals within the property boundaries. 
At the request of ENRMO, this option was occasionally used, 
particularly for barn owls and bobcats. The fate of the relo- 
cated animals is unknown but their return to the tern colony 
has not been documented. 
4) Reduce lethal control of ravens. The Navy has con- 
tracted with the Denver Wildlife Research Center to study 
raven behavior and taste aversives as it pertains to deterring 
predation upon terns. To accommodate the research, ravens 
were only controlled when they foraged in a colony. 
Results - Tern nesting was very successful. In 1990, 293 
pairs fledged and an estimated 275 - 335 birds (Belluomini 
1991a). In 1991, 328 pairs fledged an estimated 377 birds, 
which was the highest ever recorded at Camp Pendleton 
(Belluomini 1991b). While many factors contributed to this 
success, the reduction of predation is considered to be a major 
factor (D. Boyer pers. comm.). 
DISCUSSION 
California ADC personnel had not been involved in any 
operational program protecting endangered birds, such as the 
least tern, until 1988. After four years, we believe we have 
developed a strategy that is effective if thoroughly applied. 
Two principles for effective predator control can be summa-
rized from this case history: 
1) Effective predator control must be of sufficient scale 
and intensity. Buffer zones and the removal of all known and 
potential predators ought to be considered. 
2) Effective predator control must be implemented prior 
to the onset of predation. This was clearly proven in the 
inadvertent delay in the control of ground squirrels in 1989. 
Obviously, there are other factors that pertain to the for- 
mation of a successful predator control program. However, 
wildlife managers unfamiliar with predator control would be 
inclined to violate these principles by underestimating preda-
tor control requirements. Wildlife managers may tend to un-
wisely restrict the scale and intensity of control or delay 
control efforts in order to minimize the number of predators 
removed. Such restrictions or delays reduce the margin of 
safety and increase the risk of predation to the resource, which 
is ill-advised, if the resource is truly deserving of protection. 
ACKNOWLEDGEMENTS 
We are deeply indebted to the U.S. Navy and the Camp 
Pendleton ENRMO who assisted and supported our efforts: 
T. Burr, S. Buck, C. Winchell, D. Boyer, and GySgt M. 
Murphy. We wish to recognize the following ADC personnel 
who have participated significantly and professionally: C. 
Adams, E. Knittle, S. Little, W. Millsap, R. Moschetti, S. 
Moyles.P. O'Neil, R. Scott, and J. Turman. Special thanks to 
G. Ferreira and V. Dice for preparing this manuscript. 
LITERATURE CITED 
BELLUOMINI, L. A. 1991a. The Status of the California 
Least Tern at Camp Pendleton, California, During the 
Breeding Season of 1990. Unpubl. report prepared for 
the Natural Resources Management Branch, Southwest 
Division of Naval Facilities Engineering Command, San 
Diego, California. 77 pp. 
BELLUOMINI, L. A. 1991b. Summary of the Status of the 
California Least Tern at Camp Pendleton During the 
1991 Breeding Season. Agreement Number: N68711-
91-LT-1009.6pp. 
BUTCHKO, P. H. 1990. Predator control for the protection 
of endangered species in California. Proc. Vertebr. Pest 
Conf. 14:237-240. Univ. of Calif., Davis. 
KAUFMAN, N. M. 1988. Field Supervisor, U. S. Fish and 
Wildlife Service, Laguna Field Office, Laguna Niguel, 
California 92656. Letter to Mr. Ron A. Thompson, State 
Director, USDA-APHIS-ADC, dated April 5,1988. 
KEANE, K. 1989. Nesting Efforts of the California Least 
Tem at Camp Pendleton California, 1987. Unpublished 
report prepared for Natural Resources Management 
Branch, Western Division Naval Facilities Engineering 
Command, San Bruno, CA. 66 pp. 
MASSEY, B. W. 1988. California Least Tern Field Study, 
1988 Breeding Season. Final Report to CA Department 
ofFish&Game.22pp. 
MASSEY, B. W. 1989. California Least Tern Field Study, 
1989 Breeding Season. Draft Report to CA Department 
of Fish & Game. 24 pp. 
 
31 


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DEVELOPING A STRATEGY OF PREDATOR CONTROL FOR THE PROTECTION OF THE CALIFORNIA LEAST TERN: A CASE HISTORY

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