Meet the Chestnut-Sided Warbler
by Susan Shea
RANDOLPH – While planting the vegetable garden last May, I heard a repeated bird song emanating from the adjacent raspberry patch: “Pleased, pleased, pleased to MEETCHA.” Finally, the small songster perched near the tip of a raspberry cane, its tail cocked. The bird’s yellow crown, black mask, olive back with black streaks, and white breast with rusty side patches were clearly visible: the striking markings of a male chestnut-sided warbler. Breeding females are similarly-colored but lack the black eye mask.
Chestnut-sided warblers migrate north each spring and nest throughout the Northeast, north to Atlantic Canada, and west across the Great Lakes states and southern Canada to North Dakota and Saskatchewan. Their breeding range extends south in the Appalachians as far as Georgia.
Unlike many warblers, which prefer mature forest, chestnut-sided warblers thrive in early-successional habitat in rural areas: overgrown fields, regenerating deciduous forest, and woodland edges. They benefit from forestry practices that encourage diverse vegetation and shrub growth.
The song of the chestnut-sided warbler is more complex than that of most warblers. While there are two basic song patterns, there are several different variations within each category. The male warbler in my raspberry patch was singing to attract a female. Once nesting begins, males do not sing as often and switch from the “MEETCHA” song to a whistled, unaccented song used to defend territory and in aggressive encounters with other males.
In “The Singing Life of Birds,” author and ornithologist Donald Kroodsma describes listening to a dawn chorus of chestnut-sided warblers at a power line cut in a Berkshires woodland. Although the birds were all singing the “MEETCHA” song, he was able to differentiate individual males by their songs and to draw a rough map of their territories. His auditory observations were confirmed by the different patterns on sonograms made from his sound recordings. Kroodsma described this early morning singing as a massive competition for females among neighboring males.
In addition to his song, a male chestnut-sided warbler courts a potential mate by spreading his tail and wings and raising, lowering, and vibrating his crown and flight feathers. He then guards the female as she builds the nest, and he will chase other males that enter their territory. This is with good reason; DNA analysis has shown that about half the young of a nest are typically fathered by other males, often neighbors.
The female builds the nest between one and six feet from the ground in a crotch of a small tree or shrub such as blackberry or alder. She constructs the nest from strips of cedar bark, grapevine, or other plant fibers and lines the cup with fine grasses and hair. She lays three to five cream or greenish-white eggs speckled with brown or purple, and incubates them for 11 to 12 days. The nestlings hatch sparsely covered with down and helpless. Both parents feed the young regurgitated insects and caterpillars many times each day, and the babies develop rapidly. When a little older, the parents offer their young small insects. Ten to twelve days after hatching, the nestlings are ready to embark on their first flight. The parents continue to feed the fledglings until they are a month old.
Ninety percent of the chestnut-sided warbler’s diet year-round is insects, caterpillars, fly larvae, and spiders, with seeds and fruit comprising the remaining portion. In fall, chestnut-sided warblers migrate by night to the second-growth forests, thickets, and shade-grown coffee plantations of Central America, where they can continue to find insect prey, sometimes joining mixed-species flocks of resident birds.
As farm fields in the Northeast and beyond were abandoned and reverted to second-growth woodland during the early 20th century, the chestnut-sided warbler population grew. The species is now one of our most common warblers. Still, following the trend for most songbird species, this population has declined in recent decades, by 45 percent between 1966 and 2015, according to the North American Breeding Bird Survey. In addition to the usual stressors of habitat loss, building strikes during migration, pesticide use, and climate change, this warbler has likely been affected by the maturation of northeastern forests.
But in our many young forests, the distinctive song of the chestnut-sided warbler prevails. If you follow the song and watch closely, you may catch a glimpse of this colorful little bird.
[Susan Shea is a naturalist, writer, and conservationist based in Randolph. Illustration by Adelaide Murphy Tyrol.]