These New World warblers seem to mob predators only rarely. An exception are cowbirds, which are significant brood parasites. The yellow warbler is a regular host of the brown-headed cowbird ("Molothrus ater"), with about 40% of all nests suffering attempted or successful parasitism. By contrast, the tropical populations are less frequent hosts to the shiny cowbird ("M. bonariensis"), with only 10% of nests affected. This may be due to the slightly larger size of shiny cowbirds, which are less likely to survive being feed by the much smaller warbler, compared to brown-headed cowbirds. The yellow warbler is one of the few passerine proven to be able to recognize the presence of cowbird eggs in its nest. Upon recognizing one the warbler will often smother it with a new layer of nesting material. It will usually not try to save any of its own eggs that have already been laid, but produce a replacement clutch. Sometimes, the parents desert a parasitized nest altogether and build a new one. Unlike some cuckoos, cowbird nestlings will not actively kill the nestlings of the host bird; mixed broods of "Setophaga" and "Molothrus" may fledge successfully. However, success of fledging in yellow warbler nests is usually decreased by the parasitism of cowbirds due to the pressures of raising a much larger bird.