Brown-headed Cowbird: agcnt of cxtcrmination? by Harold Mayfield Hooded Warblerfeeding a youngcowbird. Photo by Alvin E. S taffan. 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 they couldn'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 other birds, 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 the coolingof 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