A bird of scrubby second-growth areas and forest edges, the Chestnut-sided Warbler is one of the few neotropical migrants that has benefited dramatically from human activities on the North American continent. Virtually unreported in the time of Audubon and other early American naturalists, this species has increased greatly in numbers with the clearing of primeval forests and the subsequent growth of scrubby habitats. A bird of early successional habitats (e.g., abandoned farmlands and regenerating clear-cut areas), it now can be among the most abundant breeding warblers in second-growth deciduous woodlands. Its populations have greatly expanded since the early 1800s. Despite some declines since the 1960s, this species appears to maintain healthy populations, and management does not seem warranted.
Distinctive in breeding and winter plumage, this warbler is a specialized forager, eating mainly insects, with some fruit on its wintering grounds. Highly mobile, it searches the undersides of leaves, often moving with its tail cocked.
Males use two song classes. The well-known song, generally described as Please, please, pleased to meetcha, belongs to the accented-ending class of songs and is used before the arrival of females, and early in the nesting cycle; it is believed to be used to attract females. Unaccented-ending songs, a second class, are used as the nesting cycle progresses and in aggressive encounters against other males. The two song classes are learned separately. Birds require visual contact with tutor males to fully develop their repertoires.
During the breeding season the Chestnut-sided Warbler is strongly territorial, with a monogamous mating system, although male bigamy has been noted. Pairs raise at least one brood per year, with some renesting.
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