Winter provides an opportunity to get a close look at last year’s bird nests to see who might have been nesting under our very noses without divulging their presence (Peterson’s Field Guide to Bird Nests is a great resource). A walk near wetlands in winter often reveals a yellow warbler nest. It is quite easy to recognize as it is lined with downy plant fibers and is fairly thick-walled. Yellow warblers are often victims of brown-headed cowbirds, which lay their eggs in other birds’ nests and therefore avoid the labor of raising their own chicks. Many birds don’t recognize a cowbird’s egg, and incubate it and raise the young cowbird chick as their own. Yellow warblers, however, can distinguish between their eggs and a cowbird’s. Upon returning to her nest and finding a cowbird egg (often laid before the host bird begins laying her eggs), the female yellow warbler simply builds another nest right on top of the nest containing the cowbird egg, and begins anew. As many as six stories of nests have been found with cowbird eggs buried in each layer.