Cowbird Chicks Must Adopt ‘The Goldilocks Principle’ To Boost Survival

Cowbirds need to grow up alongside two host nestlings — no more and no less — to maximize their own survival

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Brown-headed cowbirds, Molothrus ater, are fascinating because they have a peculiar child-rearing strategy. Female cowbirds lay their eggs into the nests of other songbirds and trick them into raising their chicks to independence. This reproductive strategy is known as brood parasitism. Some brood parasites, like cowbirds, are known as generalist brood parasites, because they don’t specialize on parasitizing just one avian host species. Instead, cowbird chicks hatch into the nest of one of several hundreds of different potential host species, each with their own style of chick care and level of competition between chicks. Thus, cowbird chicks must quickly figure out how to survive and thrive despite the unpredictable social environment they find themselves in.

“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 lead author of the study, behavioral ecologist and physiologist Nicholas Antonson, a Ph.D. candidate at the University of Illinois Urbana-Champaign.

Cowbirds are not cuckoos

The cowbird’s strategy contrasts with some brood parasites, such as the parasitic cuckoos, which reduce unpredictability faced by their chicks by eliminating all host offspring to avoid the burdens of sharing the nest and parental care. But cowbirds are different: they actually do share with their foster parents’ chicks. Which raises the question: Why don’t cowbirds kill all the host’s chicks?

According to a previous study, this murderous policy does not work for cowbird chicks hatched in the nests of another of their host species, the Eastern phoebe, Sayornis phoebe (ref). That 2004 study found that nestling cowbirds were fed less by their host parents when they had no host nest mates, whereas a cowbird growing up with two host chicks attracted the most parental care, even though they are rivals for host resources. However, it was never clear how cowbird chicks cope when faced with more nest mates than are optimal for their own survival. A recently published follow-up study suggests that cowbird chicks rely upon niche construction strategy and reduce larger, more competitive host broods to a manageable number, thereby promoting its selfish interests in exploiting host parents to maximize their own survival to fledging.

In short, cowbirds have adapted to live alongside a few host nest mates, although they do out-compete them for parental attention and food: cowbirds are physically bigger and more aggressive than their foster siblings, and they increase the likelihood they’re preferentially fed by raising their heads higher when begging, and beggoing more loudly and for longer periods of time — but they apparently do not throw the hosts’ chicks out of the nest.

How do cowbirds end up with just two host nest mates?

To investigate this, the researchers looked to one of the songbirds commonly parasitized by cowbirds: prothonotary warblers, Protonotaria citrea. Prothonotary warblers typically lay five eggs per nesting cycle and their incubation period ranges from 11-12 days. Cowbird incubation times are similar (10-11 days) although in a naturally parasitized nest, cowbirds typically hatch one day earlier than the hosts’ chicks, thereby giving them a headstart in growth and food procurement. Prothonotary warblers are acceptors of cowbird eggs and nestlings and apparently lack any adaptive defenses against parasitism.

Prothonotary warblers are a neotropical migratory songbird and obligate secondary cavity nester that breeds in central and eastern North America. This species preferentially nests in natural or artificial cavities above standing water in habitats such as bottomland and swamp forests.

The goal of this study was to create environments where only internal nest dynamics between nestlings and parent–offspring interactions could determine survival differences for both cowbird and warbler nestlings. Rates of brood parasitism by cowbirds during these 3 breeding seasons (2019-2021) were as high as 60% annually in the artificial nest-boxes.

Prior to the 2019 breeding season, Mr Antonson and his collaborators set up 200 artificial nest cavities (cleaned out milk cartons, painted black) for prothonotary warblers across a 150 hectare (1.5 kilometer) site of connected wetland forest. Prior to the 2020 breeding season, they added another 100 nest-boxes to the system. Prothonotary warblers readily breed in nest-boxes, often preferring them over natural cavities.

The boxes were designed to exclude ground predators like snakes and raccoons, and aerial predation by hawks and owls. By changing the bedding in the nest boxes four days after the eggs hatched, the team also reduced the threat from invertebrate parasites, particularly blowflies.

“We focused on what happens when a cowbird hatches into a nest with different numbers of host nestlings”, Mr Antonson told me in email.

Nests of prothonotary warblers were experimentally manipulated during the 2019–2021 breeding seasons within the Cypress Creek National Wildlife Refuge and Cache River State Natural Area in the Cache River watershed of southern Illinois.

For each nest, Mr Antonson and his collaborators used one of three experimental treatments: one cowbird with (1) no nest mates; (2) two warbler nest mates; or (3) four warbler nest mates. When the study nests contained between 3 and 4 warbler eggs, Mr Antonson and his collaborators experimentally parasitized the nests by adding a newly-laid cowbird egg from a different nest.

Prothonotary warblers are protected by the Migratory Bird Treaty Act: what happened to the warbler chicks that were removed from the experimental nests?

“Luckily we have a large enough study system that we were able to foster out any warbler chicks we removed to other nest boxes that had chicks that had hatched within a day of the nests we were manipulating, which kept them well cared for”, Mr Antonson told me in email.

Cowbirds versus warblers

To provide a comparison between the effects of one cowbird chick in the nest versus the effects of overall warbler brood size, Mr Antonson and his collaborators also compared chick survivals in cowbird-free prothonotary warbler nests containing either one, three or five chicks.

“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”, Mr Antonson explained. “We found that cowbirds survive best in the nests where they hatch with two host nestlings.”

The data show that when a cowbird was raised alone or alongside more than two nestlings, it either died or reduced the number of host nestlings down to the “just right” brood size of two (Figure 1).

Mr Antonson and his collaborators found that brood reduction rarely occurred in warbler nests without any cowbird chicks present (Figure 1). Further, they found that cowbird nestlings do not eject host eggs or nestlings from the nest — a scenario portrayed in wildlife documentaries featuring another nest parasite, common cuckoos, Cuculus canorus, a species not found in The New World.

This goes against what many birders and naturalists think they know about cowbirds. Mr Antonson said there may be some rare instances where a host chick falls out of the nest when there is some really intense jostling as they compete for food, “but it’s actually pretty rare.”

“This actually gets into another really interesting idea too, which is that most birders and the general public think that a cowbird egg spells doom for the entire host nest”, Mr Antonson elaborated in email. “This is why so many people remove cowbird eggs, which is illegal under the Migratory Bird Act.”

Cowbirds manipulate their environment to meet their needs

This nest sharing strategy is consistent with a type of niche construction known as the Goldilocks Principle. Niche construction is an evolutionary strategy where a species modifies its environment to maximize its own survival. The Goldilocks Principle is a specific sort of niche construction, named for the children’s story “Goldilocks and The Three Bears”, in which a young girl named Goldilocks tastes three different bowls of porridge and finds she prefers porridge that is neither too hot nor too cold, but is “just right”.

“What is really neat about our results is that we show that cowbird nestlings NEED host nestlings in order to survive”, Mr Antonson elaborated in email. “Without host nestlings, cowbirds only survive about 25% of the time, and we think […] this is because a single cowbird in the nest by itself is not good enough on its own at stimulating the warbler parents to bring food to it.”

Thus, two host nest mates appear to be “just right”: a balance between the cowbird’s need for maximal food provisioning accompanied by tolerable amounts of competition for food.

“As a generalist, it is unlikely that a cowbird ‘knows’ which species it is being raised by and competing with”, Mr Antonson pointed out to me in email. “Rather, its begging and growth are likely driven by its circumstance.”

As this and other studies have established, young cowbirds are not known for direct acts of physical aggression nor for evicting host nestlings, so instead, cowbirds must be relying on more subtle and indirect methods to manipulate their environment for their advantage. What might those subtle and indirect methods be? Mr Antonson and his collaborators suspect that cowbirds are stealing food meant for other nestlings, but further studies are necessary to confirm this hypothesis.

“I think that this study really illustrates nicely that cowbird eggs, nestlings, and adults have a really rough time of it! It’s hard out there for a cowbird! They have no control over the conditions they are raised in, and without the help of hosts they will likely die”, Mr Antonson told me in email. “I hope this study helps more people understand that cowbirds don’t kill all the host nestlings in the nest and appreciate that they are a protected species with a really cool parental care strategy.”

Source:

Nicholas D. Antonson, Wendy M. Schelsky, Deryk Tolman, Rebecca M. Kilner, and Mark E. Hauber (2022). Niche construction through a Goldilocks principle maximizes fitness for a nest-sharing brood parasite, Proceedings of the Royal Society B, 20221223 | doi:10.1098/rspb.2022.1223

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