If you've been birding for a while, you might be puzzling over the name. Aren't they just Yellow Warblers?
It turns out this one of the most widespread and abundant of all New World wood warblers was hiding a secret in plain sight. Since the late 1950s, all Yellow Warblers in the Western Hemisphere had been considered a single, widely distributed species, from the fully chestnut-headed birds in Latin American mangrove swamps to the rusty-crowned birds of the Caribbean and coastal Mexico, to the familiar yellow-headed migrants that breed across an enormous swath of North America. Recent analysis found that the migratory and non-migratory populations carry significant genetic and vocal differences, enough that some experts now recognize two species: Northern Yellow Warbler and Mangrove Yellow Warbler.
At a time when many migratory songbirds are in steep decline, Northern Yellow Warblers are holding on reasonably well. Population estimates suggest roughly a 20 percent decline since 1966, not good, but far better than many neotropical migrants have fared.
Northern Yellow Warblers are thicket specialists, preferring wet, shrubby habitat. At the northern edges of their range, climate change may actually be expanding suitable habitat as woody vegetation pushes into previously treeless areas at high elevations and northern latitudes. On their wintering grounds across southern Mexico, Central America, and northern South America, they use a wide variety of habitats, including hedgerows, gardens, towns, and shade coffee farms, giving them more flexibility around human-altered landscapes than many of their relatives.
Highly insectivorous like most songbirds, Northern Yellow Warblers depend on seasonal flushes of caterpillars to feed their chicks. But scientists barcoding the DNA in warbler droppings to identify prey have found something unexpected: during the nonbreeding season, these birds may be specifically targeting crop pests in edge habitat near agricultural land, including the coffee berry borer beetle on coffee plantations.