The American Yellow WarblerSetophaga petechia, formerly Dendroica petechia, is a New World warbler species. They make up the most widespread species in the diverse Setophaga genus, breeding in almost the whole of North America and down to northern South America.
Other than in male breeding plumage and body size, all subspecies are very similar. Winter, female and immature birds all have similarly greenish-yellow uppersides and are a duller yellow below. Young males soon acquire breast and, where appropriate, head coloration. Females are somewhat duller, most notably on the head. In all, the remiges and rectrices are blackish olive with yellow edges, sometimes appearing as an indistinct wing-band on the former. The eyes and the short thin beak are dark, while the feet are lighter or darker olive-buff.
The 35 subspecies of D. petechia can be divided into three main groups according to the males' head color in the breeding season. Each of these groups is sometimes considered a separate species, or the aestiva group (Yellow Warbler) is considered a species different from D. petechia (Mangrove Warbler, including Golden Warbler); the latter option is the one currently accepted by the International Ornithological Congress World Bird List.
The summer males of this species are generally the yellowest "warblers" wherever they occur. They are brilliant yellow below and golden-green above. There are usually a few wide washed-out rusty-red streaks on the breast and flanks. The various subspecies in this group mostly in brightness and size as per Bergmann's and Gloger's Rule.
The song is a musical strophe that can be rendered sweet sweet sweet, I'm so sweet, although it varies considerably between populations.
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 host to the Shiny Cowbird (M. bonariensis), but less than one nest in 10 is 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 a cowbird egg in its nest, 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 Dendroica 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.
To humans, these birds are quite beneficial. For one thing, in particular the young devour many pest insects during the breeding season. For another, the plumage and song of the breeding males have been described as "lovely" and "musical", and they can help to generate revenue from ecotourism. No significant negative effects of American Yellow and Mangrove Warblers on humans have been recorded.