Crop depredation by blackbirds (Icteridae) and Starlings (sturnus vulgaris) in North America has long prompted experimentation with control techniques. These efforts have been centered in the northeast and northcentral United States where concentrated cultivation of vulnerable crops coincides with the location where flocks of blackbirds congregate in the fall prior to their migration south (Stone, et al., 1972; Wiens and Dyer, 1975). In these areas the high cost and logistic impracticality of implementing widespread controls has suggested the need for modifying agricultural practices instead (Wiens and Dyer, 1975).  More recently, attention has been focused farther south, particularly in Kentucky and Tennessee where winter roosts of mixed flocks of blackbirds and Starlings commonly number over several million birds. These highly localized concentrations have prompted concern for both agricultural damage and danger to human health (Department of Army, 1975). A major difference between the problem in the South and that farther north is that in the South the birds present themselves as much more accessible targets for control measures. By virtue of their tremendous concentrations they make the potential for large scale extermination very real. It is quite conceivable that a significant proportion of the entire North American populations of these birds could be eliminated if extermination efforts were maintained for several years at the major winter roosts.  Before such extermination is allowed to proceed, a number of questions must be answered. These can be divided into two areas. The first area deals with whether or not the control measures are actually justified--how much agricultural damage is done by the birds and how real is their threat to human health? The second area of questioning concerns the environmental impact of both the control measures themselves as well as the effect of suddenly removing such large numbers of birds from the ecosystem. This paper presents the results of a preliminary study aimed not at answering these questions so much as providing some insight into the factors which must be considered if accurate answers are to be obtained

Holroyd, Geoffrey L.

Lester, Nigel

Phelan, Frank J. S.

Robertson, Raleigh J.

Weatherhead, Patrick J.

English

UNL | Libraries

University of Nebraska - Lincoln 
DigitalCommons@University of Nebraska - Lincoln 
Bird Control Seminars Proceedings Wildlife Damage Management, Internet Center for 
November 1976 
SPECIES COMPOSITION, FOOD HABITS, AND THE ECONOMIC 
AND ECOLOGIC IMPACT OF WINTER BLACKBIRD FLOCKS 
Raleigh J. Robertson 
Queen’s University, Kingston, Ontario 
Patrick J. Weatherhead 
Queen’s University, Kingston, Ontario 
Frank J. S. Phelan 
Queen’s University, Kingston, Ontario 
Geoffrey L. Holroyd 
Queen’s University, Kingston, Ontario 
Nigel Lester 
Queen’s University, Kingston, Ontario 
Follow this and additional works at: https://digitalcommons.unl.edu/icwdmbirdcontrol 
 Part of the Environmental Sciences Commons 
Robertson, Raleigh J.; Weatherhead, Patrick J.; Phelan, Frank J. S.; Holroyd, Geoffrey L.; and Lester, Nigel, 
"SPECIES COMPOSITION, FOOD HABITS, AND THE ECONOMIC AND ECOLOGIC IMPACT OF WINTER 
BLACKBIRD FLOCKS" (1976). Bird Control Seminars Proceedings. 65. 
https://digitalcommons.unl.edu/icwdmbirdcontrol/65 
This Article is brought to you for free and open access by the Wildlife Damage Management, Internet Center for at 
DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Bird Control Seminars 
Proceedings by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. 
155
SPECIES COMPOSITION, FOOD HABITS, AND THE ECONOMIC
AND ECOLOGIC IMPACT OF WINTER BLACKBIRD FLOCKS
Raleigh J. Robertson, Patrick J. Weatherhead, Frank J.S. Phelan,
Geoffrey L. Holroyd, and Nigel Lester
Biology Department Queen’s University
Kingston, Ontario Canada
Crop depredation by blackbirds (Icteridae) and Starlings (sturnus vulgaris) in North
America has long prompted experimentation with control techniques. These efforts have been
centered in the northeast and northcentral United States where concentrated cultivation of
vulnerable crops coincides with the location where flocks of blackbirds congregate in the
fall prior to their migration south (Stone, et al., 1972; Wiens and Dyer, 1975). In these
areas the high cost and logistic impracticality of implementing widespread controls has
suggested the need for modifying agricultural practices instead (Wiens and Dyer, 1975).
More recently, attention has been focused farther south, particularly in Kentucky and
Tennessee where winter roosts of mixed flocks of blackbirds and Starlings commonly number
over several million birds. These highly localized concentrations have prompted concern
for both agricultural damage and danger to human health (Department of Army, 1975). A
major difference between the problem in the South and that farther north is that in the
South the birds present themselves as much more accessible targets for control measures.
By virtue of their tremendous concentrations they make the potential for large scale exter-
mination very real. It is quite conceivable that a significant proportion of the entire
North American populations of these birds could be eliminated if extermination efforts were
maintained for several years at the major winter roosts.
Before such extermination is allowed to proceed, a number of questions must be answered.
These can be divided into two areas. The first area deals with whether or not the control
measures are actually justified--how much agricultural damage is done by the birds and how
real is their threat to human health? The second area of questioning concerns the environ-
mental impact of both the control measures themselves as well as the effect of suddenly
removing such large numbers of birds from the ecosystem. This paper presents the results
of a preliminary study aimed not at answering these questions so much as providing some
insight into the factors which must be considered if accurate answers are to be obtained.
STUDY SITE AND METHODS
A blackbird roost located on the cantonment area of the U.S. Army base at Fort Camp-
bell, Kentucky was selected for study due to the large size of the flock involved, its
focus of impending control measures, and the availability of background information in the
form of flock size estimates, history of the roost and estimates of impact of the flock on
surrounding agricultural areas. The size of the flock had been estimated to include 4 to
5 million birds during the period December 1974 to mid-January 1975 (Department of Army
1975), By late January the flock size was thought by local land management personnel to
be perhaps only 2/3 to 3/4 of its December size. The roost area was in a pine (pinus taeda)
plantation approximately 11 ha in area. Scattered deciduous trees within and at the edges
of the plantation were used for staging and for roosting of some species. All information
was collected by a five-man team during a three-day period from January 18 to 20, 1975.
Flock composition
Three techniques were employed to determine relative abundance of species in the flock.
The first involved a road census method during the day when the birds were foraging away
from the roost. A random course was driven along roads within 20 km of the roost. With
the aid of binoculars the species composition and approximate size of all blackbird flocks
were recorded. A distance of 64 km was driven.
The second technique involved strip census counts of dead birds within the
roost area. These were birds which had apparently died of natural causes and had been
accumulating since the initiation of roosting activity in the fall. Each strip was 5
metres wide and approximately 50 metres long. An attempt was made to sample all areas of
the roost. Every carcass within the strip was identified to species, sex (where possible),
and state of decomposition. The last characteristic was classified on a scale of 1 to 4
with the latter representing a bird which had recently died with virtually no decomposition,
while individuals classed as 1 were so badly decomposed that carcasses were incomplete and
only plumage remnants allowed identification.
The final technique employed was sampling birds from the roost at night using a shot-
gun. Sampling was carried out on two nights. The first night individuals from all shots
taken were counted together, while on the second night separate counts were kept for each
shot and the shot locations marked on a map of the roost.
156
Diet Information
The crops and gizzards of all birds shot on the two sampling nights were removed and
individually stored in alcohol. The contents of each were examined under a dissecting
microscope and the percent by volume of each identifiable constituent was estimated. The
samples were oven-dried at 50° C for 24 hours and weighed. Weed seeds and insects were
identified by experts at Agriculture Canada.
Non-pest Species
A period of one hour was spent by three observers in the roost and in the mixed deci-
duous woodlot and open field within 200 metres of the roost area. With the aid of binocu-
lars all birds were identified, and the number of each species recorded.
RESULTS
The results of the three sampling techniques used to estimate the species composition
of the flock are presented in Table 1. Also included are the results of two sampling ef-
forts performed at the same roost by the Army, as published in their environmental impact
statements (Department of Army 1974 and 1975). The variability both within and between
techniques is very striking. Of particular interest is the fact that Army sampling failed
to detect the presence of Rusty Blackbirds (Euphagus carolinus) in the flock. A possible
explanation, other than sampling error, could be that this species had begun using this
roost after the Army sampling date. However, a breakdown of the dead bird strip counts
(Table 2) indicates that Rusty Blackbirds occurred in three of the four decomposition clas-
ses, suggesting that they had been present at the roost for some time.
In order to get an indication whether flock composition might change markedly over
the roosting season, dead bird counts from the strip census technique have been arranged
into decomposition classes in Table 2. Although causes of death and specific decomposition
rates are unknown, a general trend appears evident. Assuming roost mortality reflects
the relative abundance of species present, there seems to have been a gradual replacement
of Grackles (Quiscalus quiscula) by Starlings, or at least an increase in the relative
abundance of Starlings, if the flock had been increasing in size. The results at least
suggest that the flock size and/or species composition may be very dynamic over the course
of the roosting season.
In the shotgun sample on January 19, 273 birds were collected with 17 shots. Figure
1 presents the results of the analysis of flock composition by shot, indicating as well
the location of each shot in the roost. This analysis clearly indicates how a sample could
be greatly biased if sampling were confined to a particular area of the roost. This is
further borne out by Table 3 in which the species composition of shots has been determined
by tree type and shot location. Included in the table are the number of birds per shot
for each area, giving some indication of roosting density.
The results of the crop and gizzard analysis are presented in Table 4. The percent by
volume values have been used to separate total dry weights into constituent parts. Of the
identifiable contents, it should be noted that over 30% consisted of weed seeds and animal
matter. Tables 5 and 6, respectively, present the identification of the weed seeds and
animal matter, with the former also being broken down by the bird species in which they were
found. Although the identification of animal matter content has not been quantified on a
species basis, it is notable that much of the animal matter in the bird stomachs was com-
prised of economically important insect pests.
The results of the survey for non-pest species in the roost vicinity are presented in 
Table 7. The fact that 17 species of birds were observed in only three man-hours inside
and within only 200 metres of the roost area is in striking disagreement with the results
of a similar survey by the Army (Department of Army 1975). They report that no non-pest
species of birds were seen in the blackbird roost.
DISCUSSION
During the winter of 1974-75, a flock estimated to consist of approximately 5 million
blackbirds, including Starlings, Red-winged Blackbirds (Agelaius phoeniceus), Grackles,
Rusty Blackbirds, and Cowbirds (Molothrus ater) roosting in the cantonment area of Fort
Campbell, Kentucky became the focus of control measures by the U.S. Army (cf. Department of
Army 1975). Based on the premises that the blackbird roost constituted a health hazard
(due to possible histoplasmosis), an agricultural nuisance, an inconvenience to local resi-
dents, and a possible hazard to aircraft, the U.S. Army sought to control this flock of
blackbirds and accordingly filed an Environmental Impact Statement (EIS) (Department of
Army 1975). The emphasis of this impact statement was on the negative effects of the
blackbird roost in its immediate environment and on the possible impact on the roost and
surrounding environment of the control effort per se, that is, the spraying of Avian
Stressing Agent PA-14 (Tergitol 15-S-9).
157
Little if any consideration was given to the possible long-term consequences of
removing large numbers of blackbirds from their breeding habitats. As was pointed out
by the senior author, and by several others in letters commenting on the Draft EIS, the
beneficial effects of having the birds consume insect pests, weed seeds, and waste grain
may signficantly counter-balance any economic losses attributable to the blackbirds.
Severe reduction of blackbird numbers may be accompanied by increases in populations of
insect pests, such as army worms and corn borers (cf. Table 6) and weeds, as well as
rodent populations which would be released from competition with blackbirds for the waste
grain left by mechanical harvesters. It seems obvious that both sides of the economic
and ecologic ledger must be completed if a valid statement of the impact of any major
environmental alterations is to be obtained.
Even within the context of issues and information considered in the EIS regarding
the control of winter blackbird roosts, the present study has revealed discrepancies
and inadequacies that demand attention before extensive control is allowed to become
routine as threatened by recent legislation (Jackson, 1976). Dietary differences between
species require that accurate information on the type and quantity of food consumed by
each species, and the relative abundance of each species in the flock, be determined
before estimates of economic losses are valid. A single sample per winter season to
determine diet and flock composition is inadequate, because both the size and composition
of a given flock are subject to continual change, as is the availability and consequently
the use of various food types. For example, our study showed that in late January, the
Fort Campbell blackbird roost was composed of between 2 and 9 percent Rusty Blackbirds,
whereas sampling by the army 9 days earlier failed to reveal the presence of this species.
Whether Rusty Blackbirds had very recently begun using this roost (unlikely as mentioned
earlier, cf. Table 2), or the clumped dispersion of species within the roost (Fig. 1) re-
sulted in the failure of the Army sampling to collect this species, it is obvious that
insufficient sampling prior to the control operation led to errors in determining environ-
mental impact of the birds and their control.
Similarly, extrapolation of annual economic losses based on a sample of the birds
diet on a single day in December does not account for the shifting pattern of food avail-
ability and use. Wheat and corn that may have been taken from fields in December was no
longer available in late January, consequently diets had compensated by shifting to other
food sources. Continual monitoring of the diets of each species, and the relative abun-
dance of each species in the flock, is essential to provide a meaningful estimate of the
extent of economic damage attributable to the birds.
The fact that the individuals of all species are not distributed randomly within the
roost points to the need for extensive sampling to determine the species composition of
the flock. Also, if control is deemed necessary, this clumped dispersion of species within
the flock allows the possibility of controlling primarily those species responsible for
the most extensive damage. In our samples, Starlings and Grackles made up 98% of the birds
in the interior part of homogeneous pine stands. If control measures were confined to
these areas, Redwings, Cowbirds, and Rusty Blackbirds, which seem to roost at the margins
of pine stands, would not be so severely affected. Since Grackles consume the preponder-
ance of corn, and Starlings of wheat, that is eaten by birds, such a control practice may
be just as effective and not as deleterious as extermination of an entire roost.
LITERATURE CITED
Department of Army. 1974. Blackbird control on Army installations. Draft Environmental
Impact Statement. Office of the Chief of Engineers, U.S. Army, Washington, D.C.
Department of Army. 1975. Blackbird control on two Army Installations: Fort Campbell,
Kentucky and Milan, Tennessee AAP, Environmental Impact Statement. Office of the
Chief of Engineers, U.S. Army, Washington, D.C.
Jackson, J.A. 1976. Blackbirds, scare tactics, and irresponsible legislation. Editorial.
Wilson Bull. 88:159-160.
Stone, C.P., D.F. Mott, J.F. Besser, and J.W. DeGrazio. 1972. Bird damage to corn in the
United States in 1970. Wilson Bull. 34:101-105.
Wiens, J.A. and M.I. Dyer. 1975. Simulation modelling of red-winged blackbird impact on
grain crops. J. Appl. Ecol. 12:63-82.
158
TABLE 1. Flock composition estimates for the Fort Campbell, KY
Blackbird roost.
TABLE 2. Species Composition in strip counts of dead birds.
TABLE 3. Flock composition in different roost areas.
159
TABLE 4. Stomach contents of birds taken from Fort Campbell Blackbird
roost 18 and 19 of January, 1975.
TABLE 5. Importance (% Species Composition) of weed seeds in the
diet of various blackbird species. Only seeds comprising
> 1% of diet are considered.
160
TABLE 6. Identifiable animal species found in stomach analysis of
Grackles, Cowbirds, Redwings, Rusty Blackbirds, and Starlings
Fort Campbell, Kentucky 18 and 19 January, 1975.
161
TABLE 7. Other Bird Species Sighted in the Roost Area
(<200 m from main roost)
162
FIGURE 1.
Location and Species
Composition of Blackbird
Roost Shoots
Fort Campbell, Ky.


SPECIES COMPOSITION, FOOD HABITS, AND THE ECONOMIC AND ECOLOGIC IMPACT OF WINTER BLACKBIRD FLOCKS

DigitalCommons@University of Nebraska

University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Bird Control Seminars Proceedings Wildlife Damage Management, Internet Center for November 1976 SPECIES COMPOSITION, FOOD HABITS, AND THE ECONOMIC AND ECOLOGIC IMPACT OF WINTER BLACKBIRD FLOCKS Raleigh J. Robertson Queen’s University, Kingston, Ontario Patrick J. Weatherhead Queen’s University, Kingston, Ontario Frank J. S. Phelan Queen’s University, Kingston, Ontario Geoffrey L. Holroyd Queen’s University, Kingston, Ontario Nigel Lester Queen’s University, Kingston, Ontario Follow this and additional works at: https://digitalcommons.unl.edu/icwdmbirdcontrol  Part of the Environmental Sciences Commons Robertson, Raleigh J.; Weatherhead, Patrick J.; Phelan, Frank J. S.; Holroyd, Geoffrey L.; and Lester, Nigel, "SPECIES COMPOSITION, FOOD HABITS, AND THE ECONOMIC AND ECOLOGIC IMPACT OF WINTER BLACKBIRD FLOCKS" (1976). Bird Control Seminars Proceedings. 65. https://digitalcommons.unl.edu/icwdmbirdcontrol/65 This Article is brought to you for free and open access by the Wildlife Damage Management, Internet Center for at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Bird Control Seminars Proceedings by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. 155SPECIES COMPOSITION, FOOD HABITS, AND THE ECONOMICAND ECOLOGIC IMPACT OF WINTER BLACKBIRD FLOCKSRaleigh J. Robertson, Patrick J. Weatherhead, Frank J.S. Phelan,Geoffrey L. Holroyd, and Nigel LesterBiology Department Queen’s UniversityKingston, Ontario CanadaCrop depredation by blackbirds (Icteridae) and Starlings (sturnus vulgaris) in NorthAmerica has long prompted experimentation with control techniques. These efforts have beencentered in the northeast and northcentral United States where concentrated cultivation ofvulnerable crops coincides with the location where flocks of blackbirds congregate in thefall prior to their migration south (Stone, et al., 1972; Wiens and Dyer, 1975). In theseareas the high cost and logistic impracticality of implementing widespread controls hassuggested the need for modifying agricultural practices instead (Wiens and Dyer, 1975).More recently, attention has been focused farther south, particularly in Kentucky andTennessee where winter roosts of mixed flocks of blackbirds and Starlings commonly numberover several million birds. These highly localized concentrations have prompted concernfor both agricultural damage and danger to human health (Department of Army, 1975). Amajor difference between the problem in the South and that farther north is that in theSouth the birds present themselves as much more accessible targets for control measures.By virtue of their tremendous concentrations they make the potential for large scale exter-mination very real. It is quite conceivable that a significant proportion of the entireNorth American populations of these birds could be eliminated if extermination efforts weremaintained for several years at the major winter roosts.Before such extermination is allowed to proceed, a number of questions must be answered.These can be divided into two areas. The first area deals with whether or not the controlmeasures are actually justified--how much agricultural damage is done by the birds and howreal is their threat to human health? The second area of questioning concerns the environ-mental impact of both the control measures themselves as well as the effect of suddenlyremoving such large numbers of birds from the ecosystem. This paper presents the resultsof a preliminary study aimed not at answering these questions so much as providing someinsight into the factors which must be considered if accurate answers are to be obtained.STUDY SITE AND METHODSA blackbird roost located on the cantonment area of the U.S. Army base at Fort Camp-bell, Kentucky was selected for study due to the large size of the flock involved, itsfocus of impending control measures, and the availability of background information in theform of flock size estimates, history of the roost and estimates of impact of the flock onsurrounding agricultural areas. The size of the flock had been estimated to include 4 to5 million birds during the period December 1974 to mid-January 1975 (Department of Army1975), By late January the flock size was thought by local land management personnel tobe perhaps only 2/3 to 3/4 of its December size. The roost area was in a pine (pinus taeda)plantation approximately 11 ha in area. Scattered deciduous trees within and at the edgesof the plantation were used for staging and for roosting of some species. All informationwas collected by a five-man team during a three-day period from January 18 to 20, 1975.Flock compositionThree techniques were employed to determine relative abundance of species in the flock.The first involved a road census method during the day when the birds were foraging awayfrom the roost. A random course was driven along roads within 20 km of the roost. Withthe aid of binoculars the species composition and approximate size of all blackbird flockswere recorded. A distance of 64 km was driven.The second technique involved strip census counts of dead birds within theroost area. These were birds which had apparently died of natural causes and had beenaccumulating since the initiation of roosting activity in the fall. Each strip was 5metres wide and approximately 50 metres long. An attempt was made to sample all areas ofthe roost. Every carcass within the strip was identified to species, sex (where possible),and state of decomposition. The last characteristic was classified on a scale of 1 to 4with the latter representing a bird which had recently died with virtually no decomposition,while individuals classed as 1 were so badly decomposed that carcasses were incomplete andonly plumage remnants allowed identification.The final technique employed was sampling birds from the roost at night using a shot-gun. Sampling was carried out on two nights. The first night individuals from all shotstaken were counted together, while on the second night separate counts were kept for eachshot and the shot locations marked on a map of the roost.156Diet InformationThe crops and gizzards of all birds shot on the two sampling nights were removed andindividually stored in alcohol. The contents of each were examined under a dissectingmicroscope and the percent by volume of each identifiable constituent was estimated. Thesamples were oven-dried at 50° C for 24 hours and weighed. Weed seeds and insects wereidentified by experts at Agriculture Canada.Non-pest SpeciesA period of one hour was spent by three observers in the roost and in the mixed deci-duous woodlot and open field within 200 metres of the roost area. With the aid of binocu-lars all birds were identified, and the number of each species recorded.RESULTSThe results of the three sampling techniques used to estimate the species compositionof the flock are presented in Table 1. Also included are the results of two sampling ef-forts performed at the same roost by the Army, as published in their environmental impactstatements (Department of Army 1974 and 1975). The variability both within and betweentechniques is very striking. Of particular interest is the fact that Army sampling failedto detect the presence of Rusty Blackbirds (Euphagus carolinus) in the flock. A possibleexplanation, other than sampling error, could be that this species had begun using thisroost after the Army sampling date. However, a breakdown of the dead bird strip counts(Table 2) indicates that Rusty Blackbirds occurred in three of the four decomposition clas-ses, suggesting that they had been present at the roost for some time.In order to get an indication whether flock composition might change markedly overthe roosting season, dead bird counts from the strip census technique have been arrangedinto decomposition classes in Table 2. Although causes of death and specific decompositionrates are unknown, a general trend appears evident. Assuming roost mortality reflectsthe relative abundance of species present, there seems to have been a gradual replacementof Grackles (Quiscalus quiscula) by Starlings, or at least an increase in the relativeabundance of Starlings, if the flock had been increasing in size. The results at leastsuggest that the flock size and/or species composition may be very dynamic over the courseof the roosting season.In the shotgun sample on January 19, 273 birds were collected with 17 shots. Figure1 presents the results of the analysis of flock composition by shot, indicating as wellthe location of each shot in the roost. This analysis clearly indicates how a sample couldbe greatly biased if sampling were confined to a particular area of the roost. This isfurther borne out by Table 3 in which the species composition of shots has been determinedby tree type and shot location. Included in the table are the number of birds per shotfor each area, giving some indication of roosting density.The results of the crop and gizzard analysis are presented in Table 4. The percent byvolume values have been used to separate total dry weights into constituent parts. Of theidentifiable contents, it should be noted that over 30% consisted of weed seeds and animalmatter. Tables 5 and 6, respectively, present the identification of the weed seeds andanimal matter, with the former also being broken down by the bird species in which they werefound. Although the identification of animal matter content has not been quantified on aspecies basis, it is notable that much of the animal matter in the bird stomachs was com-prised of economically important insect pests.The results of the survey for non-pest species in the roost vicinity are presented in Table 7. The fact that 17 species of birds were observed in only three man-hours insideand within only 200 metres of the roost area is in striking disagreement with the resultsof a similar survey by the Army (Department of Army 1975). They report that no non-pestspecies of birds were seen in the blackbird roost.DISCUSSIONDuring the winter of 1974-75, a flock estimated to consist of approximately 5 millionblackbirds, including Starlings, Red-winged Blackbirds (Agelaius phoeniceus), Grackles,Rusty Blackbirds, and Cowbirds (Molothrus ater) roosting in the cantonment area of FortCampbell, Kentucky became the focus of control measures by the U.S. Army (cf. Department ofArmy 1975). Based on the premises that the blackbird roost constituted a health hazard(due to possible histoplasmosis), an agricultural nuisance, an inconvenience to local resi-dents, and a possible hazard to aircraft, the U.S. Army sought to control this flock ofblackbirds and accordingly filed an Environmental Impact Statement (EIS) (Department ofArmy 1975). The emphasis of this impact statement was on the negative effects of theblackbird roost in its immediate environment and on the possible impact on the roost andsurrounding environment of the control effort per se, that is, the spraying of AvianStressing Agent PA-14 (Tergitol 15-S-9).157Little if any consideration was given to the possible long-term consequences ofremoving large numbers of blackbirds from their breeding habitats. As was pointed outby the senior author, and by several others in letters commenting on the Draft EIS, thebeneficial effects of having the birds consume insect pests, weed seeds, and waste grainmay signficantly counter-balance any economic losses attributable to the blackbirds.Severe reduction of blackbird numbers may be accompanied by increases in populations ofinsect pests, such as army worms and corn borers (cf. Table 6) and weeds, as well asrodent populations which would be released from competition with blackbirds for the wastegrain left by mechanical harvesters. It seems obvious that both sides of the economicand ecologic ledger must be completed if a valid statement of the impact of any majorenvironmental alterations is to be obtained.Even within the context of issues and information considered in the EIS regardingthe control of winter blackbird roosts, the present study has revealed discrepanciesand inadequacies that demand attention before extensive control is allowed to becomeroutine as threatened by recent legislation (Jackson, 1976). Dietary differences betweenspecies require that accurate information on the type and quantity of food consumed byeach species, and the relative abundance of each species in the flock, be determinedbefore estimates of economic losses are valid. A single sample per winter season todetermine diet and flock composition is inadequate, because both the size and compositionof a given flock are subject to continual change, as is the availability and consequentlythe use of various food types. For example, our study showed that in late January, theFort Campbell blackbird roost was composed of between 2 and 9 percent Rusty Blackbirds,whereas sampling by the army 9 days earlier failed to reveal the presence of this species.Whether Rusty Blackbirds had very recently begun using this roost (unlikely as mentionedearlier, cf. Table 2), or the clumped dispersion of species within the roost (Fig. 1) re-sulted in the failure of the Army sampling to collect this species, it is obvious thatinsufficient sampling prior to the control operation led to errors in determining environ-mental impact of the birds and their control.Similarly, extrapolation of annual economic losses based on a sample of the birdsdiet on a single day in December does not account for the shifting pattern of food avail-ability and use. Wheat and corn that may have been taken from fields in December was nolonger available in late January, consequently diets had compensated by shifting to otherfood sources. Continual monitoring of the diets of each species, and the relative abun-dance of each species in the flock, is essential to provide a meaningful estimate of theextent of economic damage attributable to the birds.The fact that the individuals of all species are not distributed randomly within theroost points to the need for extensive sampling to determine the species composition ofthe flock. Also, if control is deemed necessary, this clumped dispersion of species withinthe flock allows the possibility of controlling primarily those species responsible forthe most extensive damage. In our samples, Starlings and Grackles made up 98% of the birdsin the interior part of homogeneous pine stands. If control measures were confined tothese areas, Redwings, Cowbirds, and Rusty Blackbirds, which seem to roost at the marginsof pine stands, would not be so severely affected. Since Grackles consume the preponder-ance of corn, and Starlings of wheat, that is eaten by birds, such a control practice maybe just as effective and not as deleterious as extermination of an entire roost.LITERATURE CITEDDepartment of Army. 1974. Blackbird control on Army installations. Draft EnvironmentalImpact Statement. Office of the Chief of Engineers, U.S. Army, Washington, D.C.Department of Army. 1975. Blackbird control on two Army Installations: Fort Campbell,Kentucky and Milan, Tennessee AAP, Environmental Impact Statement. Office of theChief of Engineers, U.S. Army, Washington, D.C.Jackson, J.A. 1976. Blackbirds, scare tactics, and irresponsible legislation. Editorial.Wilson Bull. 88:159-160.Stone, C.P., D.F. Mott, J.F. Besser, and J.W. DeGrazio. 1972. Bird damage to corn in theUnited States in 1970. Wilson Bull. 34:101-105.Wiens, J.A. and M.I. Dyer. 1975. Simulation modelling of red-winged blackbird impact ongrain crops. J. Appl. Ecol. 12:63-82.158TABLE 1. Flock composition estimates for the Fort Campbell, KYBlackbird roost.TABLE 2. Species Composition in strip counts of dead birds.TABLE 3. Flock composition in different roost areas.159TABLE 4. Stomach contents of birds taken from Fort Campbell Blackbirdroost 18 and 19 of January, 1975.TABLE 5. Importance (% Species Composition) of weed seeds in thediet of various blackbird species. Only seeds comprising> 1% of diet are considered.160TABLE 6. Identifiable animal species found in stomach analysis ofGrackles, Cowbirds, Redwings, Rusty Blackbirds, and StarlingsFort Campbell, Kentucky 18 and 19 January, 1975.161TABLE 7. Other Bird Species Sighted in the Roost Area(<200 m from main roost)162FIGURE 1.Location and SpeciesComposition of BlackbirdRoost ShootsFort Campbell, Ky.

https://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1064&amp;context=icwdmbirdcontrol

SPECIES COMPOSITION, FOOD HABITS, AND THE ECONOMIC AND ECOLOGIC IMPACT OF WINTER BLACKBIRD FLOCKS

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