4 PRAIRIE FIRE July 2010

Swallows from page 2

guarding that bad things do happen. Often one birdwill steal grass from its neighbor, only to have thatneighbor return the favor at a later time; I oncewatched what looked like the same grass stem change“hands” several times as it was repeatedly stolen froma nest and then stolen back.

Probably the most unique way that cliff swallowsexploit others is by foisting some of their eggs off ontotheir neighbors. Mary and I discovered relatively earlyin our research that these birds are like other species inthat they sometimes lay eggs in the nests of othercolony members. Typically, a female finds a neighbor-ing nest unattended, slips in and lays an egg, and theunsuspecting neighbor is none the wiser. This enablesthe parasitic bird to then lay an additional egg in herown nest, and in that way increase her reproductiveoutput. It also “spreads the risk” by not putting all hereggs in the same nest, should her own nest be depre-

dated or be washed away by a storm. Up to 20 percentof swallow nests have parasitic eggs laid in them, andthe odds of both finding a nest to parasitize and havingyour own nest victimized increase in larger colonies.

But cliff swallows have evolved an even neater trick toparasitize their neighbors. If you lay a parasitic egg, youhave to time the laying to coincide with when yourneighbor lays her eggs. Lay too early and the neighborwon’t be at the appropriate stage to care for (incubate)the eggs. Lay too late and the neighbor’s own eggs willhave a head start in development and hatch beforeyours. This means parasitic laying is only possible dur-ing a fairly narrow window of time (three to four days).Yet, if you carry an egg already laid in your own nest intoa neighbor’s, you have a longer window of time (theentire 16-day incubation period) to find your neighbor’snest left unguarded. Cliff swallows do just that. Theyphysically transfer eggs in their beaks from their ownnests into ones nearby. They even seem to be able to pre-dict (how we don’t know) which nests in the colony areless likely to have lots of swallow bugs, and eggs aretransferred into those nests at a higher frequency thaninto the more infested nests. In this way, females seem to

insure that at least some of their offspring are reared innests with fewer bugs and are thus more likely to survive.

Why colonies vary in size

One big reason that the cliff swallows of the PlatteValley have proven to be such marvelous research sub-jects is that they live in colonies that vary so much insize. A few birds are found each year that live by them-selves, and others form colonies containing up to6,000 nests on a single bridge. Colonies spanning thisentire range are found every year, with most birds liv-ing in groups of several hundred nests. This variation,which I was unaware of when I selected Keith Countyas a research site, has allowed us to study not only howthe costs and benefits of social life vary with groupsize but also what ecological conditions might causecolonies to differ in size in the first place. This is a fun-damental question about animal behavior but, sur-prisingly, one that has not been widely addressed.

One factor that determines, in part, the number ofcliff swallows at a given colony site (e.g., on a particularbridge) is the nature of the habitat surrounding it.Feeding on small, swarming insects, the swallows preferareas that support large numbers of swarms. Insectabundance and distribution is influenced by the type ofvegetation, land use and amount of water within thebirds’ foraging range. Sites that have a moderate amountof water (ponds, rivers, canals) relatively close and alsohave diversity in other kinds of habitat arethe ones that support the largest swallowcolonies. Sites surrounded by all one kind ofhabitat (e.g., a cornfield) tend not to regular-ly attract as many birds, probably becausethere isn’t enough insect food in the vicinity.The overall land use in the Platte Valley, withmoderate amounts of standing or flowingwater plus a mixture of cropland, prairie,riparian woodland and town-like areas,probably accounts for the high abundance ofcliff swallows in this area, simply becauseconditions are so good for insects.

Perhaps the most surprising single dis-covery about cliff swallows in the last 28years is the realization that colony size is,in part, based on genetics. It is well knownthat some animals produce increased lev-els of stress hormones in social situations,and some 15 years ago I guessed that cliffswallows might “perform” better (i.e., bemore successful in raising young) when ingroups of particular sizes. Few colleagues,however, thought that an individual’schoice of what size group it would live incould be determined by genes.

To determine if there were innate pref-erences for groups of particular sizes, weperformed an experiment in which somenestlings born in a colony of one size wereraised in a colony of a very different size.We had monitored when nestling cliffswallows hatched, and as soon as theywere old enough (three to four days), webanded them for permanent identification.We then switched some babies born inlarge colonies to nests in small colonies tobe raised by foster parents; we did thereverse with additional hatchlings.

If the babies’ subsequent choice of whereto live was based largely on genetic tenden-cies, the youngsters should later choosecolonies that matched in size where theywere born (where their parents chose tolive), not where they were reared. On theother hand, if early experience as a fledglingin a particular social environment dictatedlater choices, the exchanged birds shouldchoose colonies that matched the size of thesites where they were reared. The next sum-mer we caught breeding birds, looking to see

where our experimental birds returned and what sizecolonies they chose as first-time breeders. Yearling cliffswallows chose breeding colonies that matched in sizewith where they were born and rarely occupied sites sim-ilar to where they were reared. Thus, cliff swallows seemgenetically programmed to use colonies of a particularsize, and this accounts in part for the diversity of colonysizes seen in the population along the Platte River.

Although unappreciated by most people and cer-tainly ignored by the many tourists cruising acrossInterstate 80 each summer, the cliff swallows of west-ern Nebraska have provided insights that havereshaped the way scientists think about group living inanimals. They are a fascinating treasure that everyonewho lives in the state should protect and take theopportunity to enjoy.

Charles R. Brown, professor of biological sciences at the University

of Tulsa, has studied swallows since age 11. Over the last 28 years in

western Nebraska, he and a research team have banded more than

200,000 cliff swallows. Receiving degrees from Austin College

(B.A.) and Princeton University (Ph.D.), Brown published his first

scientific paper at age 15, and his articles have appeared in Nature,

Science, Evolution, Ecology and many other publications. His book,

“Swallow Summer,” was published by the University of Nebraska

Press in 1998. In 2009, Brown and collaborator Mary Bomberger

Brown of the University of Nebraska-Lincoln received the Elliot

Coues Award from the American Ornithologists’ Union in recogni-

tion of their outstanding research on cliff swallows.

PRAIRIE FIRE July 2010 21

The cost of being parasitized by swallow bugs. Thenestling cliff swallow on the right is from a nest wherebugs were removed by fumigating with an insecticide;the nestling on the left is from a nest in the samecolony with natural numbers of bugs. Both nestlingswere the same age (10 days old). (Mary BombergerBrown)

Perhaps the most surprising single

discovery about cliffswallows in the last 28

years … colony size is …based on genetics.

BY SAMUEL TRUAX

JUST like human society, avian societyhas many individual personalities.There are those raucous types, likecrows and jays, loudly quarrelingamong themselves as they accomplishtheir duties of survival and procreation.There are those like doves, that quietlygo about doing their prescribed duties.With their lovely melodies, varioussongbirds bring enjoyment to humansas they accomplish their tasks. And thenthere are those who seem to be com-pletely irresponsible in accomplishingtheir life duties, without the redeemingqualities of lovely feathers or beautifulsongs, like the brown-headed cowbird.

How has the brown-headed cowbirdestablished such a reputation for irre-sponsibility? The book “Birding” statesthat “The brown-headed cowbird’smost famous, or infamous, attribute is

that it is a brood parasite.” That is, “thefemale lays eggs in the nests of otherbirds and takes no part in the rearing ofher young.” And the female cowbird isnot even concerned about who raisesher offspring. “Brown-headed cowbirdeggs have been recorded in the nests of220 species, ranging from ducks tohummingbirds.” Even more extreme,studies find that “they remove the eggsof the host bird on some occasions.”

The cowbird developed these seem-ingly irresponsible habits by evolving anomadic lifestyle. Cowbirds followedthe once massive buffalo herds throughtheir migration cycles, feeding oninsects and seeds stirred up by the con-stant movement of the buffalo herds,and so were never in one place longenough to build and tend to their ownnest. In fact their name comes from thebuffalo cows they associated with.

From their position in the grass-lands, the female cowbird could easilyspot nests for her eggs in the shrubberyand undergrowth that occur where theforest edge abuts the grassland. Nor-mally the nests deeper into the forestswere not used. The female cowbirdwould deposit her eggs in the selectednest and then move on with the herd.Though a seemingly irresponsible traitmuch criticized by humans, this was abehavior well adapted to the conditionsof life in which the cowbirds evolved.The species survived, and, though someharm was done to other affected birdspecies, that harm was typically mini-mal due to the constant migration of thecowbirds as they followed the bison.

However, the habitat changes thatoccurred with the arrival of Europeansettlement in America, such as thereplacement of migrating buffalo with

confined cattle, ex -panded the conditionssuitable for the brown-headed cowbird, at theexpense of the birdswhose nests they used.The European settlersalso cleared trees forcrop fields and pas-tures, creating moreand more forest edgesuitable for host nestselection by the femalecowbird. The cowbirdpopulation increasedvery substantially and,according to somestudies, continues toincrease, if not in actu-al numbers, then cer-tainly in range. Astudy by the AudubonSociety concluded thateven as the cowbird’srange continues toexpand, their actualpopulation has de -clined 1 percent from1966 to 1996. Regard-less of whether thecowbird population isstable or increasing,its expanding rangeallows parasitism ofmany more species,some that are poorlyadapted to avoid suchbehavior.

While cowbird eggshave been recorded int h e n e s t s o f 2 2 0

species of birds, and cowbirds havebeen fledged by 150 different species,they primarily target the nests of vireos,warblers and some sparrow species.These birds have a tendency to nest inbrushy undergrowth typical of the for-est edge.

The cowbird eggs have a shorter ges-tation period than their hosts, so theyoften hatch before the host species.Cowbird hatchlings are usually largerand more aggressive during feeding bythe host parent, and they are oftenaggressive toward the young of the hostspecies. This can cause the death of theyoung of the host species, and whencombined with occurrences when thefemale cowbird actually removes theeggs of the host from the nest, thehabits of the cowbird are causing a greatdeal of stress on the populations ofsome species. Interestingly, some birdspecies recognize and remove cowbirdeggs from their nests. In some cases,engaging in what is called “mafia behav-ior,” the cowbird will return to the nestand destroy all of the host species eggs,sometimes even tearing the nest apart.

The black-capped vireo is on theEndangered Species List. Studies indi-cate that the primary cause of theendangerment of the species is nest par-asitism. The second most importantcause of the endangerment, after habi-tat destruction, of the Kirtland’s war-bler, is nest parasitism. Recovery plansfor the endangered golden-cheekedwarbler include a cowbird-trappingprogram and the control of cowbirdpopulations. Such control methods arecontroversial.

Studies of Bell’s vireo in South Dako-ta concluded that they “are very com-mon victims of brown-headed cowbirdparasitism, and are in decline in manyregions.” In New Mexico it was foundthat Bell’s vireo “suffers from low pro-ductivity due to brood parasitism bybrown-headed cowbirds.” In someyears, nest failures have exceeded 90percent in the Carlsbad population.

However, the Audubon study doesnot indicate that these conditions arequite as serious as other scientistsbelieve. The study did find the black-capped vireo experienced a 90 percentnest parasitism in Texas. A study ofBell’s vireo and the yellow-breastedchat showed they had 80 percent to 90percent of their nests parasitized. Andthe lazuli bunting had 50 percent oftheir nests parasitized in a Montana

CONTINUED ON PAGE 16

‘Irresponsible’cowbirds and the declineof songbirds

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Lower Platte River Corridor Alliance

for the Lower Platte River

Kayak TourTues., July 20th, 2010, 8:30am - 1pm

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Trip goes from the Pla�e River Landing to the Two Rivers Recreation area. Approximately 2 – 2 1/2 hours. Light snacks and water will be available andlunch will be provided. Registra-tion at 8:00 am at the Two RiversRecreation area. Shu�les willtake you to the put-in area.

A male cowbird on a metal post.(Paul A. Johnsgard)