Yellow warblers where i live will build a new layer of nest on top of the first one once they have identified the cowbird eggs presence in the nest. They will move their eggs to the new top layer of nest and leave the cowbird egg in the lower tier so it wont be incubated properly and it doesnt hatch.
Can yellow warblers successfully fledge their young to independence and the cowbird chick as well? Or do the warblers always lose their chicks as the cowbird grows larger?
David Andrew It’s possible to go either way. Mortality rates for all songbird chicks are very high. So statistically speaking, all of the chicks (including the cowbird) probably won’t make it to adulthood. That said, it’s also possible for mixed broods to fledge. One or more of the warblers might survive alongside the cowbird. Or the cowbird might be removed from the nest by a predator because it’s clearly the biggest target and most nutritive meal. Because of adult cowbirds’ tendency to destroy the nests of birds that eject cowbird eggs or chicks, the warblers might have a better shot at making it to adulthood if the cowbird stays alive and successfully fledges. It’s impossible to say any particular outcome “always” happens. Because there are so many factors at play.
Cowbirds are the least ruthless of the brood parasites, (with honeyguides being the MOST brutally ruthless), cuckoo are actually in the middle. A long term Finnish study of Cuckoos in the nests of birds that prefer tree cavities or wooden boxes proved that most parents most years are able to feed both their own young as well as the cuckoo. This suggests the primary reason for killing their nestmates in open air nests is that they’re simply too small to accommodate everyone. Their combined weight might collapse the nest, dooming them all. But in a box, or many kinds of tree hollows, the cuckoo is unable to push them out and doesn’t need to. The boxes have room enough for everyone. Cowbirds don’t get as big as cuckoo, however, which gives them more options.
cowbirds are not native to the eastern US - they are native to the US but so what - they are present in the east solely due to actions of humans - just like starlings and weaver finches (sparrows) are not native but are here by the hand of man - why on earth are these cowbirds protected?
