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Cowbirds Raised By Surrogate Bird Parents Maintain Their Identity in Unique Ways, New Study Shows

Nov 03, 2015 02:02 PM EST
After tracking young cowbirds, one of which is pictured here, researchers shed light on how adopted nestlings avoid imprinting and grow independently from their host parents. (Photo : Mike Ward)

Cowbirds are known for giving their young up for adoption by laying their eggs in a neighboring nest for other birds to raise them. In a recent study, researchers from the University of Illinois at Urbana-Champaign examined how young cowbirds grow up knowing they are different than their step sisters and brothers.

Juvenile cowbirds leave their host parents' nest at dusk to spend nights in nearby fields, returning just after daybreak. The time apart enables them to maintain their identities apart from the warblers, thrush or sparrows they're growing up with, according to the researchers' news release

Imprinting is basically the process of passing down behavioral traits from parents to offspring. Through imprinting, young birds learn to recognize their parents and other animals they can trust. Imprinting is widespread among birds and other animals; however, cowbirds seem to have developed a unique Resistance to imprinting.

"If I took a chickadee and I put it in a titmouse nest, the chickadee would start learning the song of the titmouse and it would actually learn the titmouse behaviors," Matthew Louder, one of the study's researchers, explained in the release. "And then, when it was old enough, the chickadee would prefer to mate with the titmouse, which would be an evolutionary dead end."

For their study, researchers examined prothonotary warblers, which are common cowbird hosts. These birds have their own habits and habitats, living in forests where they feed on insects and caterpillars. To the contrary, cowbirds spend most of their adult lives in open fields where they eat seeds. For this reason, it is important for cowbirds to learn early on what to eat, so that they can survive.  

To get a better idea of how the cowbirds avoid imprinting on their host parents, researchers theorized that the cowbird moms were the ones that led their offspring out of the forest. This idea stems from another recent study that concluded that female cowbirds don't abandon their young completely, and do check in occasionally to see how their young are doing and assess which nests are most successful for depositing their eggs in the future. When cowbirds near the vicinity of their young they may even respond to their nestling's begging calls, Wendy Schelsky, of the Illinois Natural History Survey (INHS) and a member of the recent study, explained in the release.

To match the juvenile cowbirds to their biological mothers, researches took blood samples and performed a genetic analysis of cowbird nestlings and adult females found in the forest of a host nest. They also used an automated telemetry system to track the birds.

"We put up three radio towers, each with six antennas on it, so you have 360-degree directional coverage. All three towers track one individual cowbird at a time and then move to the next individual," Louder said. "We were able to watch the juveniles and see if they left the forest at the same time as a female and, if so, whether that female was their mom."

Their hypothesis was disproven, however, since the juveniles did not follow the female adult cowbirds. Instead, they left on their own after dark and returned the following morning – sneaking while their parents are asleep. When Louder examined one individual juvenile, he noted that the cowbird spent the night alone, cozied up near a rosebush in an adjacent field.

"As soon as the sun came up, the juvenile flew back into the forest and to the warbler's territory," Louder added in a statement. "Without the automated radio telemetry, I would have assumed that it had stayed in the forest all night."

Further research is needed to determine how juvenile cowbirds find their way back to cowbird flocks, how they individualize and learn social and survival skills, and how they identify a fellow cowbird during mate. However, this recent study, published in the journal Animal Behaviour, is shedding light on cowbird maturation.

"Clearly, there's a lot more to these birds than people would have thought," Jeff Hoover, an avian ecologist with the INHS, added in the release. "We still have more layers to peel away from this onion that is the cowbird."

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