Brown-headed Cowbird: Brood Parasite
One has to admire a creature who has managed to eliminate the laboriousness of raising its offspring. Brown-headed Cowbirds, renowned brood parasites, have done just that. These birds do not build nests; females lay up to 40 eggs a summer in the nests of more than 220 species of birds which raise their young for them. Cowbird eggs are generally larger than the host bird’s and hatch in fewer days, thereby putting Cowbird chicks at a distinct advantage over the host’s chicks when it comes to parental attention.
In this photo a Brown-headed Cowbird has deposited three eggs in the nest of an Eastern Phoebe (which has constructed its nest inside an abandoned American Robin nest). Unlike some songbirds, Phoebes do not recognize and remove the Cowbird’s eggs. Neither do they build a new nest on top of the old one, as some smaller songbirds (i.e. Yellow Warblers) are known to do.
Cowbird chicks develop faster than the chicks of the host bird, thereby often getting the first crack at the food parents bring to the nestlings. Not only are the host species’ chicks often at a disadvantage when it comes to parental care, but they are at the mercy of the Cowbird chicks which often remove both the eggs and chicks of the host. (Thanks to friends in Thetford, VT for the use of their photograph of this parasitized Eastern Phoebe nest. The three larger, speckled eggs are Brown-headed Cowbird eggs; the four smaller white eggs are Eastern Phoebe eggs.)
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Chestnut-sided Warbler Sitting On Eggs
Different species of birds have different numbers of broods (sets of eggs/young). Eastern Bluebirds can have up to four broods per breeding season, American Robins up to three and Eastern Phoebes often two. Chestnut-sided Warblers typically only raise one family in a summer. If weather or predation destroys their first attempt, however, they will re-nest, which is just what the pictured female Chestnut-sided Warbler is doing.
By August, a majority of birds have raised their young, but there are birds that nest late in the season, some naturally (American Goldfinches) and some, such as this Chestnut-sided Warbler, by necessity. Where birds nest, geographically, affects the number of broods they have. Birds nesting at higher latitudes tend to produce fewer broods per year. Because it gets colder earlier than further south, there is less time to raise their young. In warmer regions, birds often raise two or even three broods per year. (Thanks to Dean and Susan Greenberg for photo op.)
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Spring Has Sprung!
There’s no denying the arrival of spring, even when the snow is still up to your knees in the woods, if Eastern Phoebes are back! This member of the flycatcher family is one of our earliest returning and nesting migrants, arriving on its breeding grounds in late March and early April. One might wonder what this insect-eating bird subsists on at this time of year. Wasps, bees, beetles and butterflies are not in great supply. Fortunately, there are some insects around, including stoneflies – aquatic insects, some of which mature and emerge from streams in the winter and early spring. When insects aren’t plentiful (in fall, winter and early spring) phoebes will eat small fruits, but they only make up about 11% of their diet.
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