Why is this adult Yellow-rumped Warbler feeding a juvinile Brown-headed Cowbird that is twice its size. Well, the cowbird does not build its own nest, instead it lays eggs in the other bird nests. Where the cowbird egg will hatch faster than that of the host species, giving the cowbird a head start getting the food from its host parent. As well the cowbird will develop faster and sometimes push out the eggs or the young nestlings or just smother them at the bottom of the nest. The host parent does know any better and ends up raising a Brown-headed Cowbird.