Brood Parasites or Never an Empty Nest?

Many bird species have developed what is called brood parasitism-an effective way to have your young raised without much effort on your part. Cowbirds, for example, are blackbirds in the genus Molothrus which specialize as brood parasites-they lay their eggs in other bird’s nests, do not construct their own nests, and do not raise their own young. The genus consists of five species, three of which are found in North America. The three species choose from the nests of many species in which to lay their eggs, and all have been increasing in population and distribution. The genus appears to have originated in South America and doesn’t occur in the Old World. However there are other brood parasites in Europe, Asia and Africa. In fact, five families, include birds that lay eggs in other bird’s nests. Many nest parasites are found in the cuckoo family (Cuculidae) which includes at least 10 genera of brood parasites, most occurring in the tropics.

Perhaps the best known member of the cuckoo family (Cuculidae), is the Common Cuckoo (Cuculus canorus-formerly known as the European Cuckoo) which parasitizes nests of songbirds in Europe and Asia, and winters in Africa. In North America there occur three species called cuckoos- the Mangrove Cuckoo,  the Yellow-billed Cuckoo and the Black-billed Cuckoo which, despite sharing the common name ”cuckoo”  with C. canorus, are really quite different. Besides belonging to a different genus (American cuckoos are in the genus Coccyzus) and sub-family within the Cuculidae, the American cuckoos build their own (simple) nests and generally do not lay eggs in other bird’s nests. In addition, American cuckoo egg coloration is monotonous and unrelated to host egg color while C. canorus eggs occur in several different base colors and spotting patterns that match those of the eggs of selected host species. The relationship is so predictable that some workers have subdivided C. canorus into genetically fixed, host-specific lineages defined by the host, e.g., the reed-warbler cuckoo, the redstart cuckoo, etc. The photo below illustrates how well this egg mimicry has developed. But how does the female cuckoo “know” which nest to lay its egg in? Scientists have no answer yet. However this egg mimicry does not occur in Coccyzus or in Molothrus.

Egg mimicry in the Common Cuckoo. Pictured are eggs of host species and paired "races" of the cuckoo. Figure from

Egg mimicry in the Common Cuckoo. Pictured are eggs of host species and  to the right  of each, those of paired “races” of the cuckoo. Figure from Mary Caswell Stoddard,Cambridge University

    In another form of mimicry, some cuckoo young of other genera (Clamator, Eudynamis) raised in nests of crows or magpies, have blackish plumage like the hosts’ nestlings thus lessening the chances that the typically aggressive corvid hosts will detect and kill the interlopers. Much more can be written about the cuckoos. But I note here only that the familiar vocalization “coo-coo” in birds or clocks (!) is that of the Common Cuckoo and not at all similar to calls of the North American members of the cuckoo family.

The Brown-headed Cowbird (Molothrus ater) is the common brood parasite of North America and was the only species present there until the mid-20th century. Presently its range extends well into Canada from British Columbia to Nova Scotia and reaches to the Gulf and Pacific Coasts. As late as the 1940’s, its distribution in the eastern U.S. extended no further south than central Virginia and Tennessee (Pough 1946). However, expansion into the remaining suitable habitat in the rest of the country, including Florida, was soon to follow. The first Florida nesting was recorded in 1956 in Pensacola. In the 1980s M. ater  expanded “explosively” in Florida and today is resident over most of the state. Some range maps widely available on the Internet still depict M. ater as non-breeding in most of Florida.

Adult male Browned-headed Cowbird

Adult male Browned-headed Cowbird

Prior to the impact of European man, M. ater was dependent on grasslands, especially the Great Plains where bison grazed, but expanded eastward following the replacement of forests by range and cropland. The brood parasite strategy may have evolved because it allowed cowbirds to keep pace with the mobile bison while arranging for their young to be raised by devoted foster parents. Today the cowbirds follow livestock as they did bison, feeding on insects disturbed by grazing.

Considering the abundance of brown-headed cowbirds today, it seems reasonable to conclude that  for the cowbird, opening up of extensive forests and prairie, and  expansion of cattle grazing and agriculture  compensated for the near extinction  of the bison herds in the 19th century. Cowbirds avoid extensive forest and grassland except along their edges (the thickness of the “edge” varies with habitat type and other factors but may be 100 meters or more) while small landscape fragments of natural vegetation like woodlots or parks, are accessible to predators or parasites throughout most or all of their extent.

Most hosts are smaller than the cowbird and studies show that the parasitic birds tend to select nests with eggs smaller than theirs. Cowbird eggs develop faster than many host’s eggs presumably due to higher eggshell porosity which allows for greater exchange of respiratory gases by the developing embryo (Jaeckle, et al.  2012). Because of these developmental inequities, host adults are sometimes observed feeding a cowbird chick as big as itself while several of its own eggs remain unhatched. In most cases, however, the larger cowbird nestling will have damaged the host’s eggs and or killed the young leaving the host to feed it until it is independent.

Nest of a Phoebe with a single M. ater egg. U Cal Berkeley News

Nest of a Phoebe with a single M. ater egg. U Cal Berkeley News

If food is adequate, M. ater females can lay eggs almost daily throughout the nesting season with a single female producing up to 40 eggs. Although over 90% of cowbird eggs fail to reach adulthood, reproduction obviously is adequate to support vigorous populations.

Over 220 bird species are known to host M. ater eggs and nestlings providing a sizable population of hosts to choose from although the hosts do not all accept foreign eggs. Spreading out parasite loads across many species, however, reduces impacts on individual species thus reducing the selective pressures on host species that would favor counter-adaptive traits (Wiley 1985) and lower cowbird success rates. This strategy is quite different than that of the parasitic cuckoos which specialize in one or a few host species and employ mimicry to fool the host into raising its young.

Red-gaped cowbird nestling with smaller Phoebe young.

Red-gaped cowbird nestling with smaller Phoebe young.

Species such as robins, catbirds, thrashers, and jays resist nest parasitism by puncturing cowbird eggs and ejecting them from their nests. Yellow Warblers are known to respond to cowbird eggs by roofing-over their invaded nest with a new one and Yellow-breasted Chats desert their nests that contain a cowbird egg. Chickadees, woodpeckers and other hole-nesters are protected from depredations of M. ater because this cowbird species doesn’t lay eggs in tree holes or cavities. Hole-nesters thus may continue to succeed in small woodlots as long as suitable cavities are available and the predator load is not too great.

Carolina Wren feeding M. ater fledgling. Photo by Dave Kinneer

Carolina Wren feeding M. ater fledgling. Photo by Dave Kinneer

Most small birds that construct open nests, however, tolerate foreign eggs. The most vulnerable species include warblers, vireos, thrushes, sparrows and flycatchers. In some areas native birds may be subject to ecologically significant reduction in numbers due to cowbird parasitism. For example, numbers of Wood Thrushes and likely other species, have been greatly reduced in smaller forest fragments and woodlots which cowbirds fully penetrate in search of nests. In fact, studies of small woodlots in Illinois have shown that cowbirds are capable of eliminating Wood Thrushes from these sites, although if the woodlots are not too far away, colonization from large forested areas may prevent this from actually happening. Nevertheless Robinson (1990) showed how damaging cowbird predation can be- 55% of the 450 songbird nests found in his central Illinois study area contained at least one cowbird egg including 90% of the 92 Wood Thrush nests. Some of the parasitized thrush nests were situated over 400 m from the forest edge. Similar results have been found in other studies, although parasitism rates vary. Elsewhere, three taxa at risk of extinction, the Kirkland’s Warbler, Golden-cheeked Warbler, Black-capped Vireo, and the California subspecies of the Bell’s Vireo, are particularly vulnerable and Brown-headed Cowbirds are probably contributing to their endangered status. However, separating out habitat loss and deterioration from cowbird inroads is not easily done and studies of the Kirkland’s Warbler found that reducing cowbird parasitism was insufficient to increase warbler population size without also upgrading warbler habitat by controlled burning.

References  cited or reviewed:

DeMarsico et al

Fraga, R M, 1984. Bay-winged Cowbirds (Molothrus badius) remove ectoparasites from their brood parasites, the Screaming Cowbird (M. rufoaxillaris) Biotropica 16:223-336.

Jaeckle, W. B., et al. 2012. Comparison of eggshell porosity and estimated gas flux between the Brown-headed Cowbird and two related hosts. Journal of Avian Biology 43:486-490.

Massoni, V. and J. C. Reboreda 2002. A neglected cost of brood parasitism: egg punctures by shiny cowbirds during inspection of potential host nests. The Condor 104:407-412.

Perez-Rivera, R A, 1986.  Parasitism by the Shiny Cowbird in the interior parts of Puerto Rico. Journal of Field Ornithology 57:99-104.

Pough , R. H. 1946. Audubon Land Bird Guide. Small land birds of eastern and central North America from southern Texas to central Greenland. National Audubon Society. Doubleday & Co., Garden City New York.

Pranty, B. and A. Nelson 2010. First breeding record of the Bronzed Cowbird (Molothrus aeneus) in Florida. Florida Field Naturalist 38:1-42. Robertson, W.R.,Jr. and G.Woolfenden 2000. Florida Bird Species An Annotated List. Florida Ornith. Soc. Spec. Pub.#6.

Robinson, S. K. 1990. Effects of Forest Fragmentation on Nesting Songbirds. The Illinois Natural History Survey Reports. No. 296. Texas Breeding Bird Atlas

Van Tyne, J. and A. J. Berger 1959. Fundamentals of Ornithology, John Wiley & Sons, Inc. New York. Wiley, J. W. Shiny Cowbird Parasitism in two avian communities in Puerto Rico. The Condor 87:165-176 1985.

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