Cowbird chicks need nest mates, but two is better than four

Nest parasites, such as cuckoos, are notorious for being the ultimate free-loaders in nature. Females lay their eggs in the nests of other bird species and then leave the unsuspecting host parents to raise a chick that is not their own. Different species of cuckoos have chicks that behave differently once hatching has occurred. Some cuckoo chicks kick out the host nestlings and become the sole survivor in the nest. Others share the nest and food with the host nestlings, and yet others may actually enhance the survival of host chicks by exuding a bad smell that deters predators. 

Brown-headed cowbirds (Molothrus ater) are also obligate brood parasites but differ from cuckoos in that they are generalists, laying their eggs in the nests of any other bird they can find. In fact, observers have recorded brown-headed cowbird eggs being laid in the nests of more than 200 different species. Not all the chicks are able to survive – it seems that cowbirds sometimes pick hosts that feed their chicks a different diet (i.e. seeds) to what a growing cowbird needs (insects). Cuckoos tend to be specialists, laying their eggs in the nests of specific host species that will be equipped to raise the cuckoo chick.

In a recent study, experts investigated the behavior of cowbird chicks in the nests of prothonotary warblers (Protonotaria citreas). The researchers aimed to understand the strategies used by cowbird chicks to survive in prothonotary warbler nests when the total number of warbler chicks differs. Cowbird chicks may hatch in a nest with as many as six other nestlings, or sometimes as few as zero, and the researchers wondered how cowbird chick behavior would differ under these conditions. 

“Brood parasites like brown-headed cowbirds lay their eggs in the nests of so many different species, we wanted to know about one really important aspect of how they make a go of it in a world that, when they hatch, could be any one of 200 different scenarios,” said Nicholas Antonson, a Ph.D. candidate at the University of Illinois Urbana-Champaign who led the study with Mark Hauber, a U. of I. professor of evolution, ecology and behavior. “We focused on what happens when a cowbird hatches into a nest with different numbers of host nestlings.”

To study cowbird chick success, the researchers deployed nest boxes that were attractive to warblers and cowbirds in a southern Illinois swamp forest. The boxes were designed to exclude ground predators like snakes and raccoons, and aerial predation by hawks and owls. By changing out the bedding in the boxes four days after the eggs hatched, the team also reduced the threat from invertebrate parasites like blowflies.

The team followed nests with and without cowbird parasites and manipulated the number of host chicks in the nests by moving warbler eggs or chicks between nests. All the nests parasitized by cowbirds had either zero, two or four warbler nest mates. Control nests containing only warbler chicks were matched for the total number of young.

The results of the study, published in the Proceedings of the Royal Society B Biological Sciences, showed that cowbird chicks survive best when they are in a nest with two other nestlings. Their survival is not as good when four host chicks are present, and is also reduced when no other nestlings are present. In general, cowbirds hatch earlier than their nest mates, they tend to be bigger and more aggressive than warbler chicks, and they extract more than their fair share of food from the host parents, Antonson said. Cowbirds need more food because they grow two-to-three times larger in the nest than the warbler chicks.

Interestingly, the researchers noted that when cowbird chicks hatched in nests with four warbler chicks present, the cowbirds seemed to cause the number of host chicks to decrease down to two. The cowbirds did not do this by ejecting or killing warbler chicks, however, as is commonly seen with cuckoos.

“Cowbirds have adapted to live with host nest mates,” Hauber said. “They outcompete them, they raise their heads higher, and they beg more loudly and for longer periods of time, but they do not actively displace the host nest mates.”

“One of the unique aspects of our study is that we also quantified how many cowbirds survived to fledging in each of these different scenarios,” Antonson said. “We found that cowbirds survive best in the nests where they hatch with two host nestlings. That’s better than when they hatch with four host nestlings or with zero host nestlings. Cowbirds raised with more than that optimal number of two host nestlings appear to reduce the host brood sizes down to two.”

This reduction in host nestling numbers when cowbirds are present didn’t occur in the control warbler nests that had no cowbirds, the researchers found.

“We think that the cowbirds are manipulating the host numbers to reduce them to the optimal so that they can grow best,” Hauber said. Stealing food meant for the other nestlings is the likely means by which they do that. Further studies will be needed to confirm that hypothesis.

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By Alison Bosman, Earth.com Staff Writer