Laura Erickson's For the Birds

Every now and then in November, you run into a tiny warbler shivering in the cold. These incredibly tiny mites weigh in at about a third of an ounce. That’s pretty much the same as a chickadee, except the warbler measures smaller because the chickadee wears thick down underwear. Warblers are primarily tropical species that come to the northern forest to raise their young, but the yellow-rump spends its winters further north than any other warbler species.

Yellow-rumped Warblers are not so much hardier than their relatives as more versatile in their food habits. Yellow-rumps can successfully forage near the trunks of trees, which keeps them protected from wind, snow, and rain. Other warblers must spend their foraging time on the outer branches of a tree, where they are most exposed to the elements. And even more importantly, most warblers depend almost exclusively on insects for their food, and cannot survive if the weather is too cool for insects to move about. But yellow-rumps eat bayberries and the fruits of wax myrtle in winter, allowing them to remain as far north as the central states. There are dozens of records of yellow-rumps overwintering in Michigan, Minnesota, and Wisconsin, a few even in the northern parts of the states.

Surviving a midwestern winter is tricky for any tiny bird, but the hazards of migration, the competition for food in the tropics, and the simple expenditure of energy for travel encourage yellow-rumps to minimize their migration. Even so, many of them travel as far south as Panama each winter. Even a yellow-rump will die if the weather is too awful and the food is hard to find.

Most birds lose the urge to migrate pretty early in winter, and then stay put until a hormonal surge sends them north in spring. But yellow-rumps remain in condition to migrate well into the season. While other species are sleeping through a January night, and will freeze to death before it would occur to them to migrate further south, yellow-rumps wake up feeling restless after an hour or so of sleep. If the weather has been bad, or the food resources are diminishing, the yellow-rump will migrate even in the dead of winter. Then, as the northern weather begins to improve at the end of February, Yellow-rumped Warblers grow restless again, and head north much earlier than other warblers.

Yellow-rumped Warblers are probably the most abundant warblers of the spruce forest. Their loud chip and bright yellow rump patch, which they expose often when flitting about or flycatching, make them more conspicuous than other warblers, but on the warbler social dominance scale, they are near the bottom of the hierarchy. If a yellow-rump lives in the same area as a Black-throated Green Warbler, the yellow-rump must settle for a second-best territory. But because their feeding habits are more general, being able to take flying insects and fruits in addition to the usual leaf-eating bugs, they manage just fine. And yellow-rumps do especially well on their wintering grounds. Many warblers spend their winters in the tropical rain forest, but the yellow-rump prefers slashes and second growth–habitat becoming increasingly common both in the United States and in Mexico and Central America.

Late autumn is hardly the time to go out warbler watching in the Northland, but there are little yellow-rumps out there, bringing good fortune to their finders like tiny avian four-leaf clovers.