Published Date Written by David EastmanOne of the commonest warblers to experience and easy to observe is the widespread and abundant yellow warbler. We have all seen pictures of the colorful, golden yellow male with its light, rusty-reddish streaking on his fine chest, and then remarked about this stunning appearance upon encountering one. Because the yellow warbler pair nests within shrubbery clumps and willows close to our homes, we are very likely to have first hand knowledge of this little bird that looks like a ray of sunshine.
Many of the woods warblers we love to see during migration nest in the boreal forest far to the north of us, but this tiny neighbor is ours to enjoy. Yellow warblers have an extraordinarily broad distribution for a warbler species, and also show a considerable geographical variation. More than 40 recognized subspecies form three general groups that range from the northern limits of shrubby habitat in Canada to northern South America. The yellow warbler is found all across the northern parts of the USA during breeding season.
In the eastern United States, yellow warblers use a variety of moist, early successional habitats including old fields, overgrown pasture, woodland edges, roadside hedgerows, the borders of swamps, ponds, and bogs, and streamsides — where willows and elderberry grow in bottomland alder thickets. It needs patchy areas of scattered, small trees and shrubs growing together, perhaps where you might commonly walk or visit. The breeding process begins with a fairly elaborate courtship performed by the male, who may sing up to 3,240 songs in a day to attract his mate.
Eventually the less brightly colored female makes a deeply cupped nest here out of downy plant material--such as last year's milkweed pods, cattail spikes, the stems of the cinnamon fern, and other plant fibers. She weaves this exquisitely compact nest with the silk from tent caterpillar infestations, while eating a few of their larvae, too. You might watch her olive-green shape frequently gathering beakfuls of the white fluff from cherry trees, and then follow her to the nest site, which is usually at eye-level in the upright fork of a shrub or small tree.
Yellow warblers can appear quite tame to the human presence and you could observe them throughout their entire reproductive process during late May and past June, without bothering them very much. The male will constantly sing, "Sweet! Sweet! Sweet! I'm so sweet" to resist other male invaders of his 2/3 acre territory. He frequently feeds her while she is on the nest. This species is very beneficial to have around during this time, because 97 percent of their diet is gypsy moth caterpillars, weevils, aphids, and grasshoppers on their list. The female lays four to five eggs, and her incubation lasts 10 to 14 days. The nestling period lasts from eight to 12 days, and parental feeding may extend to two weeks after the young leave the nest, sometimes longer.
The neatest thing about this warbler species which reproduces in open lands is how it handles the brown-headed cowbird. Both evolved in the same edge habitats originally, near the buffalo, so it knows how to take care of this parasitizing species' egg laying in its petite warbler nest. The yellow warbler often builds a new nest directly on top of the parasitized one, sometimes resulting in multi-storied nests with up to six tiers. Each time the cowbird's egg is isolated, and therefore cannot be incubated successfully. Other bird species do not recognize the cowbird egg and get suckered into raising this truant offspring member of the blackbird family, to the detriment of their own nestlings.
A prowling female cowbird skulks through shrubbery, awaiting the brief departure of the female parent she intends to victimize. Then she moves in and lays her egg before that one returns to her own nest. Songbirds that originated in the Eastern forests did not know the cowbird would come this way, with agrarian land-clearing, and have not adjusted well to the cowbird's displacement habits. Wood thrushes are noticeably vulnerable, and in trouble as a species because of this nefarious bird.
Southward migration actually begins early in the summer months. Yellow warblers begin departing the breeding areas as soon as their young can fend for themselves--around late July in New England. This is why I state that mid-summer becomes somewhat of an anti-climax for me watching the birds' reproduction work. So much of the action is finished by this time in their breeding activities, especially for the warblers. A highly migratory bird, our Dendroica petechia winters in southern California, southern Florida, Mexico, and south through the Brazilian Amazon, Bolivia, and Peru. Imagine winging all the way here to feast on our abundant insects, and then returning--after perhaps a minimum of 45 days--upon successfully finishing reproduction in our local environs.