Brown-headed Cowbird: agcnt of cxtcrmination? by Harold Mayfield

Hooded Warblerfeeding a youngcowbird. Photo by Alvin E. Staffan.

HEN A BEGINNINGbirdwatcher finds a maudlinsuperficiality. cowbird egg or large voraciousyoung in The customary reassurances have much the nest of a small songbird, he invariably truth in them. But theyare not the wholetruth. reactswith indignation, if not violence.But the Relationshipsin nature are often complex-- more sophisticatednaturalist reassureshim, sometimesso complex we do not claim to pointing out that there should be no causefor understandthem fully -- but everythingis not alarm, that nature's ways are sometimes goingwell with everyliving creature.In nature inscrutable but these birds would not be here if there are losersas well as winners.At any theycouldn't live together. momentwe are likely to be looking at a select group of survivors.What about those that fell The prevailing mood among naturalists is by the wayside? that wild creatures are usually secure among themselves. The obvious dangers are often Extinction also is a reality of nature. inconsequential.and apparent enemies may Naturalists are acutely aware of the intrusion actually be friends in disguise.This senseof of man, and theseare oftenso gross as to make dynamicequilibrium is embodiedin the famil- us forgetfulof changingstress buried deeperin iar phrase "balance of nature." Also serious the fabric of nature. Changeis inevitable.Some studentsof biology have acquired a deep dis- changes are abrupt and dramatic. Those trust of sentimentality.and the prevailingview wrought by man are almost instantaneousin may have an element of backlash against nature's scale of time, but so are those caused

Volume 31, Number 2 107 by naturalcataclysms --volcanoes, forest fires, How the cowbird operates tidal waves, and hurricanes. Others proceed with almostimperceptible slowness -- climatic HE COWBIRDIS uniqueamong "pred- shifts, the advanceand retreat of glaciers,ris- atory" agentsin America. It is a socialpar- ing and falling of oceanlevels, movements of asite,building no nestof its ownbut usingthe the earth'scrust, and adaptations (evolution) in nestsof otherbirds, usuallysmaller species,for plantsand animals.All of theseshifts, fast or the depositof eggs,their incubation,and the slow, upset old equilibriumsand bring new care of its young.Many hostsare unawarethey deals, with new winners and new losers. One havea strangerin their midst, and theyrear the paleontologistsays no animalspecies has lived cowbird as their own until it achieves completelythrough a geologicalperiod. Every independenceand joins its own kind. specieshas evolved into another or it hasdied. The host suffersat every stage of this pro- The schoolchild knows about dinosaurs. He cess.First, the cowbird removes about as many knows they flourishedin a different climate eggsfrom nestsas it lays,usually leaving the from that of today.Presumably they could not total clutchsize unchanged. This tendsto mask copewith thecooling of theirenvironment, but the entryof the new eggand to keepthe host the exact mechanisms that forced them out are from abandoningthe nest as it might do if it not known.Perhaps their greatestproblem was noticed an abrupt change in the contents. food supply,but alsoperhaps they were too These acts normally take place during the slowin adaptingto matchthe up-and-coming host'sseveral days of egglaying, when the nest classof mammals.Man had nothingto do with is left unattendednearly all the time. it He wasn't here. Next, the hostsuffers from reducedhatching successof its owneggs. Where the cowbirdegg is larger,it getsmore than its shareof the heat mainlandSLANDS invariablyTHAT GET lose cut species off fromof birds,the from the breast of the host; and when the cow- evenwithout significantchanges in climate or bird egghatches first, as it usuallydoes, the vegetation.Yet most of the declineswe have hosttends to slackoff incubatingeven though witnessedwith our own eyeshave come about its owneggs are still unhatched. directlyfrom the hand of man. Market hunting And finally, the cowbird nestling, arriving exterminatedthe PassengerPigeon and almost first and being larger than the host nestlings finished the bison. The gun has eliminated when they appear, tramples them and gets large mammalsfrom the vicinity of most pop- more than its share of the food brought by ulated areas.More often, however,the damage adults. The cowbird hatches after about twelve by man has been indirect, and other agents days of incubationand thus getsone to four havebeen involved.When rats, dogs,cats, and days'headstart over most nestmates.Hence, a pigs have been introducedon remote islands, youngwarbler weighing less than two grams the effecthas oftenbeen disastrous. The taking often arrives in a nest already occupiedby a over of the northern Great Plains for agricul- cowbirdweighing more than ten grams.In th•s ture has pushedthe WhoopingCrane from the unequalstruggle, the warbler often doesnot heart of its nestingrange to the extreme per- last throughthe first day. iphery.The preemptionof the deepestsoils on Damage doneby the cowbird Bermuda for men's purposes expelled the Cahow to the stony terrain of islets where it couldnot competefor burrowswith the White- able,HEHARM but theTO ultimatethe nest harm invaded to theis hostundeni- spe- tailed Tropicbird. cies may be negligible. If only a few nestsare Probablythe most sweepingchange in bird bothered,the lossesmay be easilysupportable life in recent historyoccurred in New Zealand, by the reproductivecapacity of the species. where specieswere eliminatedwholesale when Somebirds are much lessinjured than others man introduced the first land mammals there. Large birds suffer less than small ones. Some Evensuch gentle creatures as deerand rabbits birds have effective defenses. The classic had a disastrous effect on some native birds. defense is desertion of the nest; many, par- Theseexamples are remote from most of us, ticularly the ground-nesting sparrows that but we may have an exampleclose to home in have had long experiencewith the cowbirdin the Brown-headed Cowbird. It, too, is the the Midwest, usually abandon a nest when an beneficiaryof changesbrought by man. It, too, egg is removedor added by any other agent. has the potential for extirpation of small Then they renestin anotherlocation, repeating songbirdsit has reachedthrough recent expan- this again and again if necessary,until sion of its range in United Statesand Canada. unmolested. The Yellow Warbler is famous for The case is best documented for the Kirtland's a variant of desertion; it sometimes builds a Warbler, but similar damagemay be occurring new floor over a dutch containing a foreign in otherspecies, at leastin localpopulations. egg, and lays a new set in the same cup. The

108 American Birds, March, 1977 Gray Catbird, whosedeep-green egg contrasts sharply with the pale speckled egg of the cowbird, promptly throws out the offending object. If a hostspecies has no adequatedefense of its own, the harm it suffersdepends on the number of cowbirdspresent. The more cow- birds, the more nests that will be entered, and the more nests that will receive two or more eggs each. This last factor is particularly significant,because one cowbird nestlingmay do only moderate damage while two cowbird nestlingsmay be fatal to all other youngin the nest.So the harm risesmore sharplythan does the densityof the cowbirds. With one host or with a group of receptive hostsin an area. the probabilitythat a nestwill get cowbirdattention and the probabilityit will get one, two, three, or more cowbirdeggs can be predictedfrom just two facts:the numberof cowbird eggs laid and the number of nests available.The cowbirddistributes its eggswith the impartialityof a roulettewheel. In her classicstudy of the Song Sparrowin Ohio, Margaret Nice found, with lessthan half Above: cowbird eRRin Gray Catbird nest. Photo by Dave Norris. tielow: juvenile cowbird. Photo by l, eonard Lee Rue. II1. both from Photo Researchers, Inc. its nestparasltlzed, the sparrowsuffered about rarestof them. The verylast nestof a vamshlng 30 per-centloss of productionof young,but her speciesls just ashkely to be usedas one among alarm was allayed by the fact thather sample many. was not typical, being somewhatmore heavily In the early 1940s I helped in a Breeding parasltizedthan othersreported. The damage Bird Census on the best of Kirtland's Warbler from each cowbird egg was higher in the habitat. We censused the same tract thor- Eastern Phoebestudied by Erwin E. Klaas in oughly in three consecutiveyears. Here Kirt- Kansas. He found that one cowbird egg land's Warblers comprisedjust 10 per cent of doomedthe otheroccupants of the nestsince it potential cowbird hosts, and the number of hatched ordinarily three or four days ahead of female cowbirds approximatelymatched the the phoebes.Yet the phoebekeeps its produc- numbers of pairs of warblers. These would be tion up through the stratagemof raising two sufficientto provideeggs for all the available broods routinely, the second coming late Kirtland's nests at least twelve times over, if enoughto escapemost of the cowbirdinter- cowbird femaleslay twelveeggs per season,a ference. With about one-fourth of all phoebe very conservativeestimate. So obviously the nestsentered, the total lossto the phoebeswas cowbirdis visitingmany other kinds of nests. nearlyone-fourth of their potentialyield. If any cowbirdsspecialize on onehost, as the On the other hand, in the Kirtland's War- European Cuckoo does,we have not discovered bler I found the cowbirds exacted 40 per cent it. A female cowbirdtrying to lay all her eggsin losswith 50 per cent of nestsparasitized, and Kirtland's Warbler nestswould surelyhave an many years ago expressedconcern about the impossibletask keeping a large enough area warbler's ability to sustainthis. A 40 per cent under surveillanceto find enoughnests at the loss might not seem prohibitive since 75 per right stageat the right time. centof youngare destinedto be lostin the first year of life anyway. But it becomesominous when it comesfrom a singleunremitting cause Recent spread of the cowbird at the outset, and all the usual causesof death are superimposedupon it. It is hard to find F ALL OURsmall songbirds had coexisted otherexamples in naturewhere so large a toll is with the cowbird from time immemorial, we taken regularlyby one agent. could probably rest our fears. But this is not true. The cowbird has vastly expanded its range and numbers in modern times, and The cowbird's insidious effect this processit has gained accessto many hosts that are new. Originally the cowbird was native to the reflectedUTWORSE conditionsWASto mainly come. inMy theanalys•is 1940s grasslands of the midcontinent. There and 1950s.Subsequently, the work of Lawrence followed the bison and other grazing animals, Walklnshaw, Nicholas Cuthbert, and Bruce eating the insects stirred up in their wake. It Radabaugh in the 1960s and 1970s showed preferred in nesting seasonnot expanses of parasitismrates climbing to about 70 per cent, completelyopen plains, but regionswhere trees with alarmingly high losses.In one disastrous rose above the grassesand provided elevated year 83 per cent of nestswere parasitized,and perches. The human settler created exactly 29 nestsin the study sample yielded only two these conditions. He made openings in the warbler fledglings.No short-livedbird could forest and he planted trees in the plains. endure many yearslike this one. livestocktook the place of the bison, and the The cowbird's effect on a vulnerable host is bird got its name from its habit of frequenting particularly insidiousbecause it is unrelenting cowpens. even though the host may be vanishing.Many The cowbird seems to have been missing natural hazards ease up when the threatened from eastern North America when the colonists creature becomesscarce. Competition within arrived. It was not listed in the definitive tenth speciesfor food, shelter, and space relaxes edition of Linnaeus' "Systema Naturae" in when the population declines. Similarly, the 1758, although the other commonblackbirds, classicpattern of predation is densitydepend- the Red-wing and Common Grackle, were ent; that is, predatorsconcentrate on a certain included. Peter Kalm, who visited New York, prey when it is abundant and turn elsewhere Pennsylvania,and Ontario in 1747-50, gave a when it becomesscarce. Every trout fisherman detailed account of mixed flocks of blackbirds uses this fact when he selects his lure to match in grain fields, but did not mention the the kind of food the trout are seekingthat day. cowbird. The cowbird is not deterred by the scarcityof Already much of the eastern seaboard had onehost. It may be dependenton the totality of been under cultivationfor a century, and even its hosts, but it is not steered away from the earlierthere had beenextensive clearings in the

11o American Birds, March, 1977 cattle range throughout the 1700s. Herds beat wide pathsthrough the tall grassesof the valley floors,and the herdsmenimproved the grazing with fire and ax. Gatewaysthrough the forest were first opened in the southern Appala- chians. Mark Catesbymet a "cowpen bird" between 1722 and 1725 in the Carolinas, but the fact that he pictured a female rather than the more striking male and the fact that no oneelse men- tioned it for many years thereafter, suggestsit was just beginning to cross the mountains. However, by 1790, it was common as far north as Pennsylvaniaand New York. Heavily for- ested states west of the mountains, Ohio and Michigan, did not get cowbirds until about 1850, and the bird did not reach the pinelands of northern Michigan until about 1880, after the regionsto the south had been cleared and the northwoodshad been opened by the lum- bermanand the marginal farmer who supplied him with hay and vegetables. The early progressof the cowbird into the East has been chronicled only by scattered milestones,but the creeping advance of the bird is now beingfollowed minutely. It is prob- ably encouragedboth by changedland useand the spilling outward of the cowbird's own populationexplosion. Throughout the last cen- tury observers in nearly every part of the Northeast -- Ontario, Quebec, northern Michigan and western Pennsylvania-- have Cowbird eggs in rebuilt Yellow Warbler nest. Photo commented on the "recent increase" of the by L Jelkin. cowbird in their areas. eastern forest -- variously called "meadows," If Kirtland's Warblers were unprotected "prairies," and "swales" -- maintained by flooding or by fires set deliberately by the HATHAS BEEN the effect of the cowbird Indians. Some of this land was doubtless in new regionsit hasentered? suitableto the cowbird, but a continuouspath Again the best evidenceis available for the had not yet been openedfor the bird from its Kirtland's Warbler. Between 1961 and 1971 heartland. This was probably providedfirst by the entire populationof this bird declinedfrom the herdsmenwho precededthe farmers in the about 500 pairs to 200 pairs -- a 60 per cent tide of Europeans westward. In the occupation drop. A dangerouslylow productionof young of America, the hunters, trappers, and traders in this period had already been noted, and the came first and left their mark in trading posts. cowbird had been identified as the principal Next came the herdsmen who are almost for- culprit. This diagnosiswas confirmed in 1972 gotten by history because they left so few and thereafter, when control of cowbirds was traces.Moving far ahead of the permanentset- instituted and the warbler respondedby pro- tlers, they wandered the free range and pro- ducing more fledglings per pair than ever duced the only crop that could walk itself to before reported for a member of this family. market from the far frontier. When protectedfrom cowbirds,the Kirtland's The free range in Virginia in the late 1600s Warbler has producedmore than four young lay at the outskirts of the tidewater com- per pair of adults each year. This laid to rest munities. Soon it was on the Piedmont, and by any doubts about the fecundity of the nesting 1750 officers of Braddock's army noted the birds. "cowpen men" beyond the Cumberland gap. Walkinshaw's recent studies of protected Shortlythereafter, drovesof swine, sheep,and nestsindicate that the previousdamage from cattle were moving through Kentucky. The cowbirdswas even greater than we thought. southernAppalachians remained largely free Protected warblers lay more eggs, produce

Volume 31, Number 2 111 more secondbroods, and bring off more young Less than 300 miles away at almost the same than pairs supposedlyunmolested in previous latitude but in northern Ontario, Louise de years We had assumedthat a nest without Kiriline Lawrencehad just completeda study cowbird eggs was untouched,but now we of the Red-eyedVireo and found no cowbird believe the cowbird removessome eggsfrom eggs at all in the nests. Her region is not nests it does not use subsequently.Also we unbrokenwilderness, but the approachessouth suspectthat warblersrearing cowbirdsare less of it are more heavilyforested than Michigan, likely to nest a second time in the season, and the cowbird, although present, was still perhapsbecause of the prolongedburden of scarce. From areas like this we would expect feedingmore voracious fledglings. surplusesthat could replenishlocalities where It is rare for men to witnessthe vanishingof the bird is sufferingreal damage. a speciesat closerange. Usually the realization How much pressurefrom the cowbirdcan a comesonly after the event. But here it almost small host stand? Obviouslythis will vary by happened.How closeit wasis shownby a pro- species, and any answer at present would be jectlon from observed former nesting losses speculative.The most vulnerablepresumably and observedpresent mortality betweennest- would be thosealready stressedby marginal lng seasons.These calculationslead to the habitat and barely holding on. Here a small jolting conclusionthat the presentpopulation increasein mortality at the nest could tip the of Klrtland's Warblers would be down to about scales. Populationsof small birds that nor- 20 pairsnow finsteadof nearly200) if the pro- mally bring off only one brood a year are likely duction of young had not been quadrupled to be delicatelypoised, and thesemight include throughprotection. particularly some of the warblers, vireos, and flycatchersin our own neighborhoods.For someof theseI would becomeuneasy if the rate What other birds are prime targets? of parasitismwere above 30 per cent. Yet I would not want to offer any one magic E HAVE NO conclusiveevidence of major figure. For each situationthere is probably a damage to other songbirdspecies by the uniquethreshold, above which the statusof the cowbird. But there is enough circumstantial population deterioratesrapidly. At 50 per cent evidenceto alert us. When populationsdecline, of nestsparasitized, I believedthe Kirtland's the exact facts are usually unclear and the Warbler was close to the brink. But at a similar reasonsunknown. Some kind of habitat change rate, the Ovenbird in southernMichigan was can usuallybe found. In thesedays pesticides not in dangerin the opinionof Harry W. Hann, are often suspected.If the cowbird were a who studiedthat bird. Yet these,the Kirtland's prime agent, it would not be proved unless and the Ovenbird,are amongthe largestof the someonewere doing an intensive,long-range warblers, and we would expect smaller mem- studyof the hostat a crucialtime. The factsare bersof the family to suffermore damageat the elusiveand they are often obscuredby people samerates of parasitism. whoremove cowbird eggs from neststhey find. Indeed, something•s happening to many Localdamage can be ignoredif the lossesare small birds around us. In my area of north- made up eachyear by recruitmentfrom other western Ohio the Yellow-throated Warbler areas where the same speciesis highly suc- nested fairly commonly along the larger cessful An examplewas provided in a studyof streamsamong the sycamoresand cottonwoods the Red-eyedVireo in northern Michigan by (remember the Sycamore Warbler?) until William E. Southern. He studied the vireo over about 1900. I havealways been intrigued by the a three-yearperiod at the Universityof Mich- disappearanceof this warbler, becauseof all igan BiologicalStation on DouglasLake. Out the foresttypes in this area, the leastdisturbed of 104 nests, 75 (72%) received one or more has been that of the flood plains and stream cowbird eggs.These vireosfledged less than banks, where the sycamoreand cottonwood oneyoung per pair peryear. This is not enough still growto their greatestsize. Also I haveread to sustain the population in the face of any with interest that the Cerulean Warbler was the reasonable estimate of mortality for a small most abundant member of this family next to migratorybird. Southernconcluded the vireo the Yellow Warbler before the turn of the cen- was the prime target of cowbirdsthere, and tury. And I remember groups of Cerulean said, "The cowbirdplays a critical role in the Warblers in areasof the countyparks 25 years nestingsuccess of the Red-eyedVireo in the ago from which they have vanished today DouglasLake region." But he wassaved from Why? The glib answeris "lack of habitat," but undue alarm because there were plenty of one would be hard put to describe major vireoseach year nevertheless. changesin wildernessparks. Perhapsa general This circumstanceis readilyexplained by the reduction in woodland elsewhere must be con- undisturbedproduction of vireos elsewhere. sidered.But I am alsosuggesting the cowbird

112 Amencan B•rds, March, 1977 be considered. It was on the scene of the crime pesticides,herbicides, and a myriad of human -- and increasingsteadily in the sameperiod. disturbances. I point out that an agencycapable of damag- ing suchbirds was presentand increasingin CampbellN NORTHWESTERN has kept meticulousOHIO. recordsLouis W. of the sameperiod of time. birdsin the area for morethan SOyears. He has Other birds that might be vulnerableare the taken a particular interest in the birds of the small flycatchers,particularly the Traill's and Oak OpeningsPark, a tract of more than 4000 Acadian. Walkinshaw'sstudies in Michigan acres.where there has been no cutting, drain- showedthat the entireclutches of thesespecies ing, or burning, although inevitable changes are wiped out by the presenceof one cowbird have occurred through the maturing of the egg. In his samplethe parasitismrate for the forestand the encroachmentof treesinto open Acadian was24 per cent and the lossrate about spaces.He has recordedconsistent and severe the same.This is damagethe bird may be able declinesin a numberof speciesthat nestedhere to survive,but lossesmuch higher than this regularlyin some numbersforty or fifty years wouldbe alarming. ago. These include the Yellow-throatedand It may be that I take too gloomya viewof the Warbling Vireos and the followingwarblers: cowbird effects -- except in the case of the Black-and-white, Golden-winged, Cerulean, Kirtland's Warbler where the proof is con- Common Yellowthroat, Yellow-breasted Chat. clusive.It may be that other specieshave and American Redstart. In speculatingabout defensesthat we do not yet appreciateor there declinesin this varied group, he wondered if are limitationson cowbirddensity that prevent the local changeswere a part of a continental it from exceedingcritical numbers almost deterioration in conditions resulting from everywhere.But lackingsuch assurance, I can- not escape the conclusion that the cowbird in Kirtland• Warbier.Photo by MichaelBolton. some places can become a menace to some small nestingsongbirds.

Volume 31, Number 2 113 114 American Birds, March, 1977