Setophaga pensylvanica: The ‘Pennsylvania Moth Eater’

The chestnut-sided warbler (Setophaga pensylvanica) is a diminutive but beautiful neotropical migrant songbird that breeds throughout the forests of the Chesapeake Bay Watershed. Sporting a yellow cap, slate mottling, and a chestnut ‘side,’ this species is truly unique and especially striking. One of my personal favorite warblers, this bird has always fascinated me. Not just because of its unique coloration, but because its specific epithet contains a reference to Pennsylvania. Setophaga roughly translates to ‘moth eating’ which refers to their preference for feeding on moth caterpillars! The specific epithet ‘pensylvanica’ is likely a reference to the location where this bird may first have been described to science. While these birds certainly do breed in Pennsylvania, we share them with many other eastern states and Central American countries.

A chestnut-sided warbler (Setophaga pensylvanica) perches on a highbush blueberry (Vaccinium corymbosum) during mid-summer (Photo: Jim Kauffman)

Neotropical migrants like chestnut-sided warblers are birds that spend their winters in the American tropics then travel north to breed and raise young in northern latitudes. Each spring, the chestnut-sided warbler makes its annual return to our eastern forests from its wintering range in Central America. As spring progresses, this bird begins to return to eastern deciduous forests to nest and raise young. By splitting their time between habitats, neotropical migrants utilize a strategy known as resource partitioning. These birds exploit the abundance of food resources in deciduous woodlands during the growing season when food sources are most abundant, then return to equatorial habitats where resources are available throughout the winter. In addition to moth caterpillars, chestnut-sided warblers also feed on other insects, insect larvae, small arthropods, and occasionally seeds or berries.

In its breeding range, it is a species that occupies edges, early-successional habitats, and shrublands. Forests that contain these habitat components are important for the breeding success of chestnut-sided warblers, and benefit many other species that have evolved to exploit these habitat types. By applying forest management practices to second-growth woodlands, forest managers help to create the young forest habitat that is vital for this species. Young forest habitat is also important for species like ruffed grouse (Bonasa umbellus) and woodcock (Scolopax minor).

A chestnut-sided warbler singing during the spring breeding season. Note the ‘chestnut side’ (Photo credit: allaboutbirds.org).

The best way to observe a chestnut-sided warbler is to visit suitable habitat during early morning hours in the spring. Woodlands with shrubs, early-successional habitat, relatively recent timber harvests, and scrub-shrub riparian habitats are good places to start. In their thick, shrubby habitat, they are easier heard than seen. So you will have to listen for a singing male, then look for movement and use your binoculars. The chestnut-sided warbler song has been described as “please, please, pleased to meet cha,” but honestly I don’t hear it! Chestnut-sided warblers are occasionally mistaken for yellow warblers (Setophaga petechia), and vice versa, as their songs can sound quite similar. Here’s a helpful link for telling them apart!

As spring progresses, chestnut-sided warblers and many other neotropical migrants continue to return to our forests to breed. The diversity of their songs, calls, feeding behavior, and coloration make for an exciting opportunity to head afield and try to observe these crown jewels of the eastern forest.

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