By Lee Franks
The Yellow-rumped Warbler (Dendroica coronata) is the Mercedes-Benz of birds—it has all the extras: wing bars, attractive crown, breast streaks, yellow throat, colored rump, etc. The name warbler refers to “singing with trills.” Until recently, the Yellow-rumped Warbler was considered to be two separate but closely related species: Myrtle Warbler (which have white throats) of eastern habitats, and Audubon’s Warbler (which have yellow throats) of the west. Ironically, although the western race bore the name of one of the greatest ornithologists, it was one of the few birds that John James Audubon failed to meet. Although it no longer officially holds the Audubon title, many western birders continue to refer to this spry bird by its former name, affirming its western roots.
Yellow-rumped Warblers are relatively large warblers (5.5″), distinguished by conspicuous yellow patches on the rump, throat, crown, and sides of breast. The upperparts are gray, and the underparts are white. The breast and flanks are solid black, and the tail shows white spots in flight. Their bill is very thin.
Yellow-rumps display a wider range of foraging strategies for hunting insects than most other warblers. They may be seen sallying from treetops after flying insects (mosquitoes, gnats, flying ants), or searching for scale insects on tree trunks. They also like to glean berries from shrubs like poison oak during the winter months when insects are scarce. This winter diet of berries enables Yellow-rumps to winter as far north as Seattle.
The Yellow-rumps we see in the Park during the winter months begin departing from their breeding grounds in British Columbia in early September, arriving here in late September or early October. Look for flights within or between trees.
They leave the Park in late March or early April. Yellow-rumps are nocturnal migrants that can travel about 300 km per day. They orient themselves at dusk, using polarized light. Their migratory movements are associated with declines in food abundance and the passage of major cold fronts.
Only the male sings, and they have at least 2 song types which are heard in the early morning hours during the breeding season. In general, their songs often lack distinctive patterns, and can best be described as erratic and flat shivering trills with a thin, bell-like quality, usually rising at the end. Some individuals sing songs with stronger patterns, sometimes closely resembling those of other warblers.
Since the Yellow-rumps do not breed in the Park, we hear only their calls, which are sharp, emphatic, and nonmusical. The calls are rendered as either a psit or chek, while foraging.
Pairs form shortly after females arrive on the breeding grounds, usually in early-to mid-May, when food sources become available. Nests are built in mixed forest habitats in pine or fir trees. They place their nests well out on the central stem of a horizontal branch, as high as 15 m. The female is usually the sole nest builder. The male occasionally brings material and may sing near and follow the building female. Nests may be loosely or compactly built cup-shaped constructions of twigs, pine needles, and grasses interwoven with rootlets, animal hair, and lichens. The cups are lined with finer hair and feathers. Feathers are woven into the nest with vanes curving up into the nest, thereby covering eggs from above to protect them from sun and insulate them from the cold.
Typically the hen will lay 4 to 5 eggs, the first egg usually laid within a day after nest completion. One egg is then laid per day until the clutch is complete. Incubation lasts 12 to 13 days from laying of the last egg. The male often sings in the vicinity of the nest during incubation. Brooding is done by the female, and both parents bring food for the young, consisting of soft insects for the first few days, and then shifting to an adult diet thereafter.
The young depart the nest 10 to 14 days after hatching, at which time they are able to fly weakly for short distances, usually in the direction of a singing parent. The young split-up at this time, but remain within hearing range of the parents. Once able to fly well, they follow parents who will continue to feed them for a couple more weeks until they reach the independent stage, Both adults and the fully independent young join flocks in late summer on their breeding grounds to our north. The flocks we see in the Park during the winter months could very well include this mix of adults and young which formed in the summer and then migrated together to Edgewood, where the oak woodlands provide plenty of food and essential shelter from foul weather and predators.
The Birds of North America, No 376, 1998, P.D.Hunt and David J.Flaspohler.