These birds flock in large numbers, migrating along the eastern seaboard where there are plenty of wax myrtle berries. The Yellow-Rumped Warbler is the only warbler capable of digesting the waxes found in bayberries and wax myrtles. They have a subdued color palette during winter, but in the springtime their plumage transforms into an impressive display of bright yellow, charcoal gray, black, and bold white.
The Yellow-Rumped Warblers are the most versatile foragers of all the warblers. Unlike other warblers, as the Cornell Lab of Ornithology notes, in spring the Yellow-Rumped Warblers can be seen fluttering out from trees to catch a flying insect, and they can quickly adjust to eating berries in the fall. These birds have also been seen foraging for insects on washed-up seaweed along a beach, “skimming from the surface of rivers and the ocean, picking them out of spiderwebs, and grabbing them off piles of manure.”
Audubon tells us: “While most of its relatives migrate to the tropics in fall, the Yellow-rump, able to live on berries, commonly remains as far north as New England and Seattle; the Yellow-rump is the main winter warbler in North America.” It can winter farther north than most warblers because it can digest the wax in berry coatings.
Both Yellow-Rumped Warbler parents feed their nestlings. The young leave the nest after 10-12 days, and they can fly short distances as soon as 2-3 days later.
The nest is usually on a horizontal branch away from the trunk or sometimes in a fork where a branch meets the trunk of a deciduous tree. The nest is built by the female. It is an open cup made of bark fibers, weeds, twigs, and roots; lined with hair, feathers, and moss in such a way as to curve over and partly cover the eggs. It has been discovered that the Yellow-Rumped Warbler has also used moose, horse, or deer hair to line its nest.
During courtship, the male accompanies the female everywhere. He fluffs his side feathers, raises his wings and his colorful crown feathers, then calls and flutters.