After I returned home from Mud Lake I spent some time packing for my upcoming trip to southern Ontario to visit my Dad. As is my habit, I kept a close eye on the avian activity in my yard throughout the morning. A Mourning Dove visited briefly to eat the seeds beneath the feeder, while a House Finch and Cedar Waxwing were noted flying over. Two Chipping Sparrows visited later, followed by a large group of House Sparrows. The most I ever saw at one time was eight, but there might have been more. Then I noticed this bird:
I don’t often see Brown-headed Cowbirds in the neighbourhood; they sometimes visit my feeder during migration, and indeed the last time I saw a cowbird at my feeder was at the end of April (a female). However, I’d seen and heard a male in the Scissons Park area from May through July, so this young bird could possibly be his offspring. I’m not sure how it came to be in the company of the House Sparrows; Brown-headed Cowbirds very rarely parasitize House Sparrow nests since they prefer laying eggs in open cup nests rather than in cavities or enclosed nests.
While Brown-headed Cowbirds have been recorded laying eggs in the nests of more than 220 species of birds, the female cowbird must be careful in choosing the right nest in which to lay her eggs. Some birds, such as Mourning Doves and kingfishers, feed their young food entirely unsuitable for a member of the blackbird family. A swallow will only feed the young cowbird while in the nest, but not after it fledges, as young swallows are taught to feed on the wing. Similarly, a cowbird that hatches in a Killdeer’s nest would be deserted; young shorebirds are precocial and leave the nest as soon as their feathers dry. An adult would not return to the nest to feed the nestling cowbird, leaving it to die of starvation or exposure. The bird families most often parasitized by cowbirds are those that will feed the young cowbirds even after they leave the nest, until they can fend for themselves. These families include the flycatchers, the finches, the vireos, the warblers, and the sparrows. Common hosts include the Yellow Warbler, Song and Chipping sparrows, Red-eyed Vireo, Eastern and Spotted Towhees, and Red-winged Blackbird.
This bird was clearly a juvenile, as it has the scaly back, heavily streaked breast, and the hint of a gape which all differentiate it from a female cowbird. It remained on the fence, instead of on the ground below where the sparrows were feeding on the fallen seed, and I didn’t witness any adult sparrows flying up to feed the young cowbird. Still, when the House Sparrows flew off, it flew off with them, suggesting the cowbird was part of the flock.
Many people dislike cowbirds for the adverse effect they have on the native species whose nests they usurp; young cowbirds are often raised at the expense of the host’s own young, which can be disastrous for species such as the Kirtland’s Warbler which has highly specific breeding requirements, and whose population is struggling. Every egg or nestling lost is detrimental to the species’ recovery. It is for this reason that Brown-headed Cowbirds are carefully managed in Michigan where the Kirtland’s Warblers breed.
Still, cowbirds are part of the ecosystem, and if anyone is at fault for their dramatic increase in range and population in the 20th century, it is us humans for turning the forest into farmland and providing them with exactly the open habitat and food they need to survive. I don’t mind seeing them at the feeder or while out birding, for I don’t believe that any species is inherently bad for evolving the necessary traits and behaviours it needs to survive and propagate its species. Evolution and natural selection have led it this far in a world increasingly altered by humans, and although the Brown-headed Cowbird is doing well now, we can’t take any one species’ status for granted, as what is thriving and abundant today may be on the brink of extinction tomorrow.