While kingbirds are known to nest out in the open, redstarts are usually more secretive, building their nests in the fork of a tree or a shrub at least two metres above the ground. The tightly woven open cup is typically made of grasses, bark strips, hair, leaves, twigs, or mosses, all glued together with spider silk. Male American Redstarts do not attain full breeding plumage until their second year and, while they may sing and defend small territories in their first year, they typically do not find a mate. Once they have attained breeding plumage, some males will mate with two females at the same time. These males hold two separate territories up to 500 metres (1,640 ft) apart. Once his first mate has finished laying all her eggs has begun incubating them, the male proceeds to attract a second mate in his other territory.
This nest was visible from the trail, with no vegetation to keep her safe from prying eyes. Although both parents will mob intruders that approach the nest too closely and often perform conspicuous distraction displays, I didn’t see the male redstart around (perhaps he was off mating with another female). This is a potentially dangerous spot for the nest given its visibility, its closeness to the trail, and that it is low enough for children to be able to reach it. Fortunately, American Redstarts will renest if the first nest is lost.
Eastern Kingbirds, on the other hand, build bulky, poorly concealed nests – sometimes in an exposed spot out on tree branches or in rotting tree stumps in the middle of a swamp – and perch conspicuously nearby. However, they are very aggressive during the breeding season and will attack larger predators such as hawks, crows, ravens and squirrels…even if they are just passing by. This aggression has been shown to increase their breeding success. In the case of kingbirds, offense really is the best defense!
It takes the female one or two weeks to build the nest. Although the nest is usually about seven inches wide and six inches deep, the inside is only two or three inches wide and one or two inches deep. Kingbirds build very sturdy nests in order to withstand the elements. While small twigs, coarse roots, dry weed stems, strips of bark, and sometimes bits of trash including cigarette butts, plastic, and twine make up the exterior, the interior is lined with softer materials such as fine rootlets, willow catkins, cottonwood fluff, cattail down, and horsehair.
I found this nest in a dead tree or shrub when I spotted the bird sitting on a branch nearby. It was in an open spot, set far back enough from the trail that most casual walkers would probably miss it. The Kingbird was sitting peacefully on the branch the first time I saw her, and when I went back eight days later she appeared to be incubating.
You can see the blue twine she used to construct the nest and how, despite the size of the nest, she barely seems to fit inside. Eastern Kingbirds lay two to five eggs and only raise one brood per year, though they may re-nest two or three times if their first nest fails. The incubation period lasts 14-17 days and the nestling period lasts 16-17 days. Kingbirds usually feed the fledglings near the nest for two to five weeks after fledging. As this nest is much closer to me than the redstart nest, I should be able to go back fairly often and see the adults feed their young as they progress from nestling to fledgling.
In complete contrast, Brown-headed Cowbirds do not build nests or raise young. I started thinking about this when, on another occasion, I found a pair of Brown-headed Cowbirds feeding along a roadside together. It isn’t often that I see a male and female together, and indeed it is difficult to understand the relationship between a mated pair when they have nothing to do with the raising of their own young. We do know, however, from genetic research that male and female Brown-headed Cowbirds are not monogamous, and that both males and females have several different mates within a single season.
Cowbirds are brood parasites; that is, the female lays her eggs in the nests of other species. Cowbirds are not particular about which nests they will use: some lay their eggs in Red-winged Blackbird nests attached to vertical shoots of marsh vegetation; in Ovenbird nests on the forest floor; in cup nests in trees and shrubs; and in tree cavities. They lay their eggs in songbirds’ nests, ducks’ nests, hawks’ nests, gulls’ nests, shorebirds’ nests, and woodpeckers’ nests. There is also a single known case where a cowbird laid an egg in a Ruby-throated Hummingbird nest, though the egg entirely filled the nest and failed to hatch.
A total of 227 species are known to have had cowbird eggs laid in their nests, although not all hosts successfully raise the cowbird chicks. In fact, only 151 species out of the 227 have been documented rearing young cowbirds, including tiny kinglets and endangered Kirtland’s Warblers and Black-capped Vireos. Other species that are frequently parasitized by cowbirds include meadowlarks, Yellow Warblers, Song Sparrows, Chipping Sparrows, Red-eyed Vireos, Eastern Towhees, and Spotted Towhees. Species that have not been observed rearing young cowbirds include Blue-winged Teal, Ferruginous Hawk, Virginia Rail, Killdeer, Spotted Sandpiper, Upland Sandpiper, Wilson’s Phalarope, California Gull, Common Tern, the above-mentioned Ruby-throated Hummingbird, and Red-headed Woodpecker.
Recent studies estimate that only 3% of Brown-headed Cowbird eggs develop into adult birds. However, populations remain relatively stable (though it is declining slightly in Canada according to The Atlas of the Breeding Birds of Ontario) because female Brown-headed Cowbirds have a long reproductive period with an very short interval between clutches. An average female lays about 80 eggs, or 40 per year for two years! These numbers more than compensate for the large number of eggs and young that do not survive.
Even though migration is over and many birders see the month of June as the beginning of the “summer doldrums”, the breeding season can still be fun. The frantic season of migration has ended, giving us more time to study and learn about the birds that stick around for the summer. With so much new life burgeoning all around us, it’s hard not to be enchanted with adult birds bringing food to their nestlings or watching fledglings following their parents around.