These birds entrust their young to other species for a better chance of survival.

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These birds entrust their young to other species for a better chance of survival.

brown-headed cowbirds are parasitic birds. Ithey lay their eggs in the nest of other birds and let them raise their young. In a recent study, researchers wanted to capture survival strategy chicks in a nest of warblers. Surprisingly, the results show that cowbirds do better with two nestmates than with four or zero.

cowherd chicks

A sneaky war for survival

The researchers said they quantified the number of cowbirds that survived to fledging under different scenarios.

When a cowbird is in a nest with four warbler chicks, two scenarios can generally occur. Sor the cowherd dies, or the number of host chicks decreases. Moreover, a previous study shows that cowherds receive less food when there are no host chicks, than when there were two. This case was studied in a host nest of an Eastern Phoebe.

The results show that cowherds survive best in a nest where they hatch with two host chicks. Beyond this number, the number of host chicks seems to be reduced to two.

“Cowbirds have adapted to living with host nestmates. They compete with them they raise their heads higher, and they beg louder and longer, but they do not actively move host nestmates.”

It would therefore seem that the cowherds manipulate the number of hosts to reduce it indirectly. And this is how they manage to develop in optimal conditions. But this hypothesis remains to be proven.

Researchers deployed attractive nesting boxes

Testing interactions between parasitic cowbirds and prothonotary warblers is not easy. The team of Nicholas Antonson, author of the study, has had to deploy attractive nesting boxes.

Indeed, for a better result, the nesting boxes have been designed to exclude the many predators air and land. In addition, they renewed the box litter four days after egg hatch to reduce the threat of invertebrate pests.

In a swamp forest in southern Illinois, researchers manipulate the number of eggs and chicks. In doing so, they ensure that all nests parasitized by cowbirds had either zero or two or four warbler chicks.

The particularly subtle survival strategy

Generally, the cowbird eggs hatch earlier than those of their companions. Gradually they become two to three times larger than warbler chicks. It is quite natural that they take more food, to the detriment of the host chicks.

cowbird chicks are not hostile to other chicks. However, a struggle for food is necessary to survive.

“Cowbird chicks never eject eggs from host nests and never directly kill young hosts. This differentiates them from another brood parasite, the common cuckoo, which kills all other nestlings in the nest and demands all the food the foster parents can provide. »



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