ID: 9041; IWPF Grant #05-028W issued November 30, 2005INHS Technical Report prepared for Illinois Department of Natural Resource

Wenny, Daniel G.

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Winter diet of Long-eared Owls at Lost Mound

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HI LLINI SUNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGNPRODUCTION NOTEUniversity of Illinois atUrbana-Champaign LibraryLarge-scale Digitization Project, 2007.Z / HSQLAJF 6GLoos Ci) Winter diet of Long-eared Owls at Lost MoundSubmitted by:Daniel G. WennyIllinois Natural History SurveyLost Mound Field Station3159 Crim Dr.Savanna, IL 61074andCenter for Wildlife and Plant Ecology1816 S. Oak StreetChampaign, IL 61820Illinois Natural History Survey Technical Report 2005 (15)IWPF Grant #05-028WPrepared forEd AndersonNatural Heritage BiologistIllinois Department of Natural Resources3159 Crim DrSavanna, I 61074Nov 30, 2005Introduction:Long-eared owls (Asio otus) formerly nested in northern Illinois. They were removed fromthe Illinois Endangered Species list because the breeding population had been extirpated. Nowlong-eared owls occur in Illinois only in the winter, primarily in the northern part of the state.Protecting and maintaining significant winter roost areas may facilitate the establishment of abreeding population as occurred during the recovery of bald eagles (Haliaeetus leucocephalus) inthe midwest.Long-eared owls occur at the Lost Mound Unit of the Upper Mississippi River NationalWildlife Refuge from November to March. The numbers and locations of owls vary each year.During the winter of 2003-04 a roost containing 30-40 birds was the largest report for the speciesduring the Great Backyard Bird count in Feb. 2004 (so large in fact that staff from Cornellquestioned the numbers sent in!). Although long-eared owls had been reported from this area inprevious years, it appears that the number of owls at Lost Mound in winter has increased sincegrazing by cattle ceased in 1999. A better understanding of the winter habits of these owls mayhelp with recovery efforts.Project Objectives:The overall goal of this project was to evaluate the winter diet of long-eared owls at LostMound. The specific objectives originally included the following:1. Determine relative abundance of prey species in Long-eared owl diet from thewinters of 2002-03 and 2003-04.2. Estimate the number of each rodent species eaten over the course of the winter.3. Measure selected bones to estimate age classes of rodents eaten by the owls(Lyman et al. 2001).4. Compare relative abundance of rodents in owl diet with past studies of smallmammal abundance(Mankowski et al. 1994, Hofmann et al. 2000).5. Compare the results from this study with other studies on owl diets in the Midwest(Cahn and Kemp 1930, Errington 1932, Voight and Glenn-Lewin 1978).6. Involve volunteers with field station research.These objectives turned out to be overly ambitious for a volunteer-run project. Identificationof all skulls to species was too time-consuming and technical for the volunteers to do efficientlyand with confidence. However, this work is on-going and will be reported at a later date.Estimating age classes by measuring certain bones was not possible because most pellets hadmultiple prey items of differing species. The original paper using measurements to estimate ageclass of rodents (Lyman et al. 2001) was apparently a unique situation in which the owl diet wascomposed primarily of two easily distinguished species. We found at least 7 species ofmammals including two genera (Microtus and Peromyscus) each with two very similar species.After recognizing these difficulties I revised the objectives to include:1) Determine relative abundance of prey groups in Long-eared owl diet.22) Estimate the number of each type of prey eaten over the course of the winter.3) Compare relative abundance of mammal groups in owl diet with past studies ofsmall mammal abundance.4) Compare the results from this study with other studies on owl diets in theMidwest.Study SiteLost Mound Unit (LMU) is part of the Upper Mississippi River National Wildlife and FishRefuge, Savanna District. LMU is a 9800 acre portion of the former Savanna Army Depot inCarroll and Jo Daviess counties, northwestern Illinois. The owl roosts were in eastern red cedar(Juniperus virginiana) trees in sheltered locations. These included areas protected by topographyor dense stands of cedars. All roosts were in Jo Daviess county and surrounded by remnant sandprairie of varying quality and plant composition (Robertson et al. 1997).MethodsSeveral thousand pellets were collected from three locations during the winters of 2002-03and 2003-04. Pellets were collected on five occasions and are referred to below as the fivecollections (see also Table 1). Pellets were dried in a drying oven, and stored in a freezer. Eachpellet was weighed and then dissected. Volunteers from Natural Area Guardians and NorthwestIllinois Audubon Society assisted with the pellet analysis. Large bones such as femur, humerus,skull, and mandibles were removed from the pellet and stored in snap-lid vials. Skulls wereidentified as bird, shrew or rodent. Identification of skulls to species was beyond the scope ofthis phase of the project. This work is ongoing and will be reported upon completion.Because pellets may break apart we defined a complete pellet as one with a jaw to skull ratioof 2. Data from the 349 complete pellets were used to calculate the average pellet mass andnumber of prey per gram of pellet. Estimates of total number of prey consumed were based ondata from complete pellets.ResultsFifteen volunteers dissected 822 pellets, 734 from the winter of 2002-2003 and 88 from2003-2004. An additional 573 pellets from 2003-2004 were dissected prior to this project and arenot included here but will be included in future reports. The pellets dissected in this studycontained 970 rodents, 46 shrews, and 16 birds (Table 1). Species identified thus far includeMicrotus ochragaster (prairie vole), M pennsylvanicus (meadow vole), Peromyscus maniculatus(deer mouse), P. leucopus (white-footed mouse), Reithrodontomys megalotis (western harvestmouse), Blarina brevicauda (short-tailed shrew), Sorex cinereus (masked shrew). In addition, 13pellets contained seeds, presumably ingested by the prey species before eaten by the owl.The five collections were generally similar with rodents comprising 91-98% of prey items(Table 1). Shrews were more common than expected in the E-1408 roost and less common in theB-203 collection from April 2003 than expected ( X= 19.8, df= 8, P < 0.05). This differencemay reflect the better-quality prairie in E-area compared to B-area.The number of skulls per pellet ranged from 0 to 6 and the number of mandibles from 0 to12. On average complete pellets (see definition in methods) weighed 2.41 ± 0.96 grams (range0.94 - 7.57g) and contained 1.41 ± 0.72 skulls (range 1-6 skulls) the majority of which (1.34 ±0.7 skulls) were rodents. Assuming an owl regurgitates one pellet each day and assuming 40owls were present from November through March then approximately 6040 pellets wereproduced each winter. Using the average number of skulls per pellet from above then 8516 preyitems were consumed each winter.The prey items in owl pellets reflected the relative abundance of rodents and shrewsestimated in two small mammal surveys (Table 2). Although the 1994 survey found a trend formore shrews than expected, the difference was not statistically significant ( 5= 5.5, df = 2, P >0.05).The results from Lost Mound were similar to data from Iowa, Wisconsin, and Urbana Illinois(Table 3). In each of these sites approximately 95% of prey were rodents. In Tennessee moreshrews (15%) were eaten than in the upper Midwest. Wisconsin had the lowest percentage ofshrews and highest percentage of rodent in owl diets (Table 3).Overall the diet of Long-eared Owls at Lost Mound is not unusual compared to other sites orto the available prey. Thus, the primary goal for land managers at this site is to protect the knownroosts from disturbance (including limiting the number of people visiting the sites to an absoluteminimum). Presumably the owls spread out from the roosts to forage, so habitat in thesurrounding prairie and savanna areas should be protected and enhanced if possible. Adjacenttenants and landowners should be aware that the presence of so many owls in one area is a signof a healthy ecosystem. Owls are an economic asset because they eat so many rodents so poisonbait for rodent control should be avoided.AcknowledgmentsThanks to the many pellet pickers including (listed in descending order of number of pellets) JoeGarrity, Richard Peterson, Pat Wemstrom, Chuck Wemstrom, Barbara Rutherford, Joan Harmet,Carol Mantey, Dick Harmet, Randy Nyboer, Doug Wenny, Betsy Wenny, and Jim Crossley.Thanks to the IDNR Wildlife Preservation Fund for funding. Special thanks to Ed Anderson forincredible patience and Randy Nyboer for reviewing a draft of this report.4ReferencesCahn, A. R., and J. T. Kemp. 1930. On the food of certain owls in east-central Illinois. Auk47:323-328.Errington, P. L. 1932. Food habits of southern Wisconsin raptors. The Condor 34:176-186.Hofmann, J. E., E. J. Heske, and D. G. Wenny. 2000. Small mammal survey in upland habitats atthe Savanna Army Depot, Carroll and Jo Daviess Counties, Illinois. Illinois NaturalHistory Survey Technical Report 2000 (30), Illinois Natural History Survey, Champaign.17 pages.Klippel, W. E., and P. W. Parmalee. 1982. Prey of a wintering Long-eared Owl in the NashvilleBasin, Tennessee. J. Field Ornithol 53:418-420.Lyman, R. L., E. Power, and R. J. Lyman. 2001. Ontogeny of deer mice (Peromyscusmaniculatus) and montane voles (Microtus montanus) as owl prey. American MidlandNaturalist 146:72-79.Mankowski, A., E. A. Anderson, and T. Bittner. 1994. Small mammal survey of the SavannaArmy Depot, Savanna, IL. Division of Natural Heritage, Illinois Depeartment of NaturalResources, Mt. Carroll, Illinois. 12 pages.Robertson, K. R., L. R. Phillippe, G. A. Levin, and M. J. Moore. 1997. Delineation of naturalcommunities, a checklist of vascular plants, and new locations for rare plants at theSavanna Army Depot, Carroll and Jo Daviess Counties, Illinois. Technical Report 1997(2), Illinois Natural History Survey, Champaign. 90 pages.Voight, J., and D. C. Glenn-Lewin. 1978. Prey availability and prey taken by long-eared owls inIowa. The American Midland Naturalist 99:162-171.Table 1. Composition of Long-eared owl pellets from five collections at Lost Mound Unit.Collection site year month pellets mass Skulls per pellet1 B-203 2003 Jan 193 2.13 ± 0.86 1.33 ± 0.912 B-203 2003 Apr 214 2.24 ± 0.86 1.11 ± 0.743 B-203 2004 Mar 88 3.0 + 1.01 1.48 ± 0.924 E-1408 2003 Mar 258 2.03 ± 0.72 1.28± 0.835 F-205 2003 Mar 69 1.71 ± 0.69 1.13 ±1.01Total 822 2.18 ±0.88 1.26 ±0.86Table 1. ContinuedCollection Rodents Shrews Birds total# % # % # %1 237 92.6 12 4.7 7 2.7 2562 234 98.3 3 1.3 1 0.4 2383 124 95.4 3 2.3 3 2.3 1304 304 92.1 23 7.0 3 0.9 3305 71 91.0 5 6.4 2 2.6 78970 94.1 46 4.5 16 1.6 1032Table 2. Comparison of prey abundance (excluding birds) in Long-eared Owl pellets with smallmammal abundance at Lost Mound. Mammal abundance from (Mankowski et al. 1994) and(Hofmann et al. 2000).pellets 1994 mammal survey 2000 mammal survey# % # % # %Rodents 970 95.5 185 92.5 681 96.1Shrews 46 4.5 15 7.5 26 3.7Other 0 0 - - 2 0.2total 1016 __200 _709Table 3. Comparison of long-eared owl pellet composition from Lost Mound Unit withpublished studies. Sources for other studies as follows: Tennessee (Klippel and Parmalee 1982),Iowa (Voight and Glenn-Lewin 1978), Urbana, Illinois (Cahn and Kemp 1930), Wisconsin(Errington 1932).site Lost Mound Tennessee Iowa#owls 30-40 1 13# % # % # %pellets 882 71aRodents 970 94.1 251 78 - 95.9Shrews 46 4.5 47 15 - 3.2Birds 16 1.6 21 7 - 0.9Other 0 0 0 0 0total 1032 319 2112a Additional remains collected from area under owl roost, number of prey from complete pelletsnot given.Table 3. continuedUrbana, IL Wisconsin#owls 5-7# % # %pellets 1201 -PRodents 1140 95.2 3233 98.8Shrews 38 3.2 14 0.4Birds 17 1.4 26 0.8Other 3 0.2 0 0total 1198 32733 includes stomach contents and summer data.

Winter diet of Long-eared Owls at Lost Mound

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