BROWN-HEADED COWBIRDS: From BuffaloBirds To Modern Scourge

What is a Cowbird? nests parasitized (usually of at least several Cowbirdstend to parasitize birds smaller in size differentspecies) per female in one season. than themselves. For example, by the time they 1l1e Brown-headed Cowbird (Molothms ater) is Typically, female cowbirds find potential host are ready to leave the nest (fledge), most warbler a broodparasite that lays its eggs in the nests of nests by perchingduring early morning hoursin a or vireo nestlingsweigh lessthan half as much as other birds. These "foster parents", called hosts, locationwith high visibility such as adead tree in cowbird nestlings. In fact, a fledgling cowbird is usually raise cowbird young at the expense of a forest opening, observing behavior of host larger than the adult warblers and vireos! The theirown eggs or young. females that are building nests, and followingthe monstrous cowbird nestling not only can cause unsuspectingdirectly birds to their nests. Once host young to starve by monopolizing the food Cowbirds earned their common name from the the nest location is known, the cowbird returns supply brouglit by parent birds but also literally habit of following herds of buffalo(and cattle) in during the egg-laying period of the host female may crowd host young right out of the nest. search of the insects that were flushed up by the and depositsher own egg in the nest. large grazing mammals. Plumage of the male The combination of these parasitic habits can Brown-headed Cowbird is mostly glossy black By parasitizingduring the host egg-laying period, severely affect the host family. hnpacts of with a contrasting dark-brown head; females are the female cowbirdimproves her chancesthat the parasitismare different for differenthost species, dull grayish brown. The dull plumage of the host female will be away from the nest often but usually result in theloss of at least one, and female is thouglit to be advantageous to the (allowing the cowbird access without being often all, of the 3-5 host young. Althougli each parasite, as it potentially makes them less detected), and that the cowbird egg will be cowbird female only lays one egg in a nest, a, conspicuousto host species. acceptedby the host and receivefull incubation. high abundance of cowbirds in an area often will lead to many cowbirds parasitizing the same Althouglicowbird parasitismoccurs in almost all limited supply of host nests. The presence of habitat types, cowbirds tend to select overgrown more than one cowbird in a nest almost fields and forest edges more than other habitats inevitably leads to death of all host young. In for reproduction, and species that build open-cup addition, raising even one cowbird can nests in shmbs in these habitats are most potentially reduce the survival of host parents commonly parasitized. Because of their affinity ' becauseof the increasedphysical effortneeded to for forest edges, increases in forest edge due to care forthe ravenous intruder. forest fragmentation can increase parasitism frequencyfor many forest bird species, including Why do Hosts Tolerate the Imposters? Neotropical migrants. Some species of forest­ dwelling Neotropical migrants have been found In about60- 70% of nests parasitized,the cowbird Host species arerejecters or accepters of cowbird to sufferhigher rates of nest parasitism in small, removes one of tl1e host eggs before laying her parasitism. Rejecters recognize and remove isolated forest tracts than in large w1broken own egg. 11lis is possiblyto ensure that the host cowbird eggs from their nests immediately after forests. In addition, parasitism levels are often does not "notice" a different number of eggs in they are laid. Accepters rarely eject parasite higher in regions with highly fragmented forests the nest, or to ensure that the cowbird egg eggs, incubating cowbird eggs and feedinghuge, than in largelyforested landscapes. receives full incubation (an extra egg in the nest noisy cowbird young as if they were their own. may not allow the female to incubate all eggs Only 9 host species have beendetermined to be Portrait of a Parasite evenly). rejecters and most are birds with beaks large enougli to remove unwantedeggs easily. Cowbirds have been called songbird chickens Cowbird eggs require an incubation period of because they can lay more eggs than any other only I 1-12 days, whereas most host species wild bird. A single female is capable of laying require 12-14 days, and some asmany as17 days nearly one egg per day at the peak of the of incubation. Consequently, cowbird young breeding season, and produces a total of 30-40 hatch earlier than tl;e host young, affording the eggs over the 2-3 month breeding period (May­ parasite a distinct advantage in competing for July). Because female cowbirds usually lay only foodwith its yotmger foster siblings. one egg in a host nest, this translates into 30-40

F a c t S h e e t N o. 3 Smithsonian MigratoryBird Center Fact Sheet No. 3

Why somebirds acceptparasitism is W1clear,but and habitat use. Preferred foraging sites are FurtlierReading: it may be because they simply do not recognize livestockareas that provide bothinsect and grain parasite eggs, orattempts at removing the thick­ foods,but cowbirds alsoexploit bird feedersand shelledcowbird egg could result in breakage and refuse from campgroWlds, picnic areas, etc. lossof theirown eggs, or raising the cowbirdis a Because no parental care is given to offspring, Brittingham, M.C. and SA. Temple. 1983. better optionthan the risk of not nestingat all. cowbird foraging areas can be separate from Have cowbirds caused forest songbirds to Perhaps someaccepter species, particularly those reproductiveareas, allowing female cowbirds to decline? 33:31-35. inhabiting forests, only encoW1tered cowbird exploit a varietyof habitatsover greatdistances. parasitism in thelast 100years andso have not Cowbirds have been foW1d to commute up to 7 Friedmann,BioScienceH. 1929. evolvedto recognizeand reject the parasitism. In kmdaily between foraging and egg-laying areas. C. Thomas, essence, they are easy targets for the invading Springfield, IL. The Cowbirds:A Studyin cowbirds. the Biology of Social Parasitism. Ke11y, S.T. andM.E. Decapita. 1982. Cowbird Luckily, some accepters are able to escape How Can We Control Cowbirds and control and its effect on Kirtland's Warbler parasitism to an extent by nestingearlier or later Parasitism? reproductivesuccess. 94:363-365. than the cowbird breeding season (generally ear.lyMay to earlyJuly), or by nestingmore than Because cowbirds thrive in a human-altered WilsonBull. once during a season (at least one nest attempt landscape, there will be no easy solutions to the Mayfield, H.F. 1977. Brown-headed cowbird: probablywill not beparasitized). problem. However, some alternatives for Agent of extermination? 31:107-1 13. contro11ingcowbirds exist. Am. Birds Human-Made Problem Cowbird removal through trapping or shooting Robinson, S.K., J.A Grzybowski, S.I. Rothstein, has proven useful in the conservation of M.C. Brittingham, L.J. Petit, and F.R. Almost everything humans do in manipulatin endangered species such as the Kirtland's Thompson. 1993. Management implications of our environment is beneficial to cowbirds Warbler and Black-capped and Least BeU's cowbird parasitism on Neotropical migrant Removing or cutting into the forest for roads vireos. For these species, intensive cowbird songbirds. Pages 93-102 in D.M. Finch and settlements, or timber harvesting, for example control has drastica11y reduced the parasitism P.W. Stangel, eds.. and canimprove thehabitat for cowbirds by creatin levels that threatened to drive their populations USDA Forest grassy foraging areas, open perch sites fo to rapid extinction. However, this controversial Service, GeneralTechnical StatusReport ManagementRM-2 29. of surveyinghosts, and more access to host specie practice probably wi11 only be effective in Neotropical Migratory Birds. in edge or open forest habitats. Agricultura extreme cases such as these where host practices that leave waste grain behind durin populations are small and localized. Cowbird winter provide superior winter feeding sites fo trapping would be too costly and labor-intensive Rothstein, S.I. 1975. An experimental and cowbirds, improvingtheir annualsurvival. to be effective in the control of cowbird teleonomic investigation of avian brood parasitism on a regional basis. parasitism. 77:250-271. Historically, the range of the cowbird prior to ' ,, European settlementwas restricted to the open TI1e most effective way to control cowbird Condor short-grass prairies of the Midwest. Presumably numbers and the impact of cowbirds on as a result of large-scale deforestation, Neotropical migrants is to control the features of introduction of cattle, and other activities the environment on which they thrive. associated with European settlement, the Landscape-level measures that maintain large cowbird's range expanded rapidly eastward and forest tracts with minimal edge-to-interior ratios the species was widespread throughout the (i.e., compact versus long, linear shapes) would eastern U.S. by the late l 800's. A similar be most effective. In addition, openings in expansion occurred in the west during the forests for gas wens, picnic areas,etc., should be l 900's, resulting in their distribution being minimized and concentrated into a few areas spread throughout southern Canada and the rather than dotted throughout the forest. Such lower 48 states. Cowbird populations have long-term and large-scale habitat planning is continuedto increasedramatically throughout the ultimately necessary for conservation of breeding range since 1900, possibly due to Neotropical migratory bird species, not only to greater overwinter survival from increases in . control cowbird populations but also to control food supply (residual agricultural grain) and in populations of nest predators (raccoons, blue reproductive success from exposure to new, jays, crows, squirrels) that also increase in vulnerable host species. fragmented forests. Only through sotmd habitat preservation, planning, and management can we Cowbirds are capable of exploiting htunan­ stem the expansion of cowbird populations and alteredlandscapes particularly well becausethey conserveour migratory birds. are very generalized in their food requirements