Yellow-rumped Warbler

Colloquially known as the “Butterbutt,” the Yellow-rumped Warbler is widespread across northern and western North America and has two distinct subspecies. “Audubon’s” Warbler is found in the west, while the “Myrtle” occurs from Alaska to the northeastern United States. The latter gets its name from its frequent association with bayberry (also known as a wax myrtle) during fall migration and winter. Unlike most other birds, Yellow-rumped Warblers can digest the waxy fruits of this shrub, and thus survive the winter farther north than any other warbler (but see Pine Warbler). They can be common in coastal thickets as far north as Cape Cod. As berries are depleted over the winter, warblers can shift south in search of more reliable food supplies, a phenomenon known as facultative migration.

Yellow-rumped Warblers also exhibit another interesting twist to migration: a general tendency for males to winter farther north than females. Known as “differential migration,” this is not uncommon in short distance migrants, and is believed a result of indirect male-male competition. Males that spend the winter closer to the breeding grounds can return sooner and claim better territories, but their northern limit is still set by the potential for harsh weather. Females, on the other hand, are less constrained by this need to arrive early and go farther south where their overwinter survival is less tenuous.

Across most of the northeastern United States the Yellow-rumped Warbler has shown an unusual population trajectory. From the 1960s through 1990s it increased strongly, a pattern that coincided with a southward range expansion and which is sometimes attributed a combination of reforestation and the species adapting to nest in pine plantations. Around the year 2000 this trend reversed quite suddenly, and by 2020 numbers were back to where they were in the 1960s in most of the region. Farther north and west the populations have shown much more variation of this same time frame. The Yellow-rump is not the only common species to show this shift around the turn of the century, and investigating possible causes is a worthy area for further research.