Yellow warblers are small widespread songbirds found in the Americas. The summer males of this species are generally the yellowest warblers wherever they occur. They are brilliant yellow below and greenish-golden above. Winter females 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.
Yellow warblers breed in almost the whole of North America, the Caribbean, and down to northern South America. They winter to the south of their breeding range, from southern California to the Amazon region, Bolivia and Peru. The breeding habitat of Yellow warblers is typically riparian or otherwise moist land with ample growth of small trees, in particular willows. The other groups, as well as wintering birds, inhabit mangrove swamps and similar dense woody growth. Less preferred habitats are shrubland, farmlands, and forest edges. Yellow warblers can also be found in suburban or less densely settled areas, orchards, and parks, and may well breed there. On the wintering grounds, these birds inhabit mangrove forests, marshes, tropical moist forests, and shrubland.
Outside the breeding season, Yellow warblers usually spend time in small groups, but while breeding they are fiercely territorial and will try to chase away any conspecific intruder that comes along. Males perch near the tops of the bushes or trees and sing in order to claim their territory; they also establish their territories by performing a “circle flight” display. Yellow warblers are active during the day and find their prey by gleaning in shrubs and on tree branches; they will also hawk prey that tries to fly away. These birds communicate with postures, perhaps with touch and vocally. Their song is a musical strophe that can be rendered 'sweet sweet sweet, I'm so sweet', although it varies considerably between populations. Their call is a soft or harder 'chip' or 'ship' which is frequently given by females after a male has finished his song. In territorial defense, Yellow warblers give hissing calls, while 'seet' seems to be a kind of specialized alert sound. Other calls are given in communication between pair-members, neighbors, or by young begging for food.
Yellow warblers are generally serially monogamous and form pairs that remain together only during a single breeding season. Some males, however, may be polygynous and mate with more than one female. Yellow warblers start breeding in May/June. They nest in trees, building a small but very sturdy cup nest. The female lays 3-6 eggs and incubates them alone for 11-14 days. The chicks are born helpless and weigh 1.3 g (0.046 oz) on average; they are brooded for 8-9 days after hatching and leave the nest the following day or the one thereafter. Around 3 to 4 weeks after hatching, the young are fully independent of their parents. They become reproductively mature at one year of age and attempt to breed right away.
Being generally common and occurring over a wide range, Yellow warblers are not considered a threatened species. Some local decline in numbers has been found in areas, mainly due to habitat destruction and pollution. The chief causes are land clearance, the agricultural overuse of and herbicide and pesticide, and sometimes overgrazing.
According to Partners in Flight resource the total breeding population size of the Yellow warbler is 97,000,000 birds. Currently, this species is classified as Least Concern (LC) on the IUCN Red List and its numbers today are stable.
Due to their insectivorous diet, Yellow warblers are important predators of a wide range of insect species, especially those that are considered pests in their ecosystem. For example, they reduce the number of coffee berry borer beetles in Costa Rica coffee plantations by 50%. These small birds also help to disperse seeds of fruits that they regularly consume in their wintering grounds.