I recall my first encounter with the Brown-headed Cowbird. It was a gorgeous sunny day and I saw this strange small blackbird with a totally brown head, like it had been dipped in ink, sitting atop a tree. The most wonderful gurgling song came out of its mouth. I was hooked! This was actually one of the first birds I ever identified and it started a passion which has lasted almost six decades.
As I learned more about birds, the breeding behaviour of the cowbirds caused me concern, they parasitize other birds’ nests! How could that be? Originally a prairie species, they followed the bison herds across the Great Plains. Developing beside these shaggy nomadic giants, they really couldn’t settle down and raise young, so circumstances led to a unique lifestyle. They simply didn’t build their own nests; they used other birds to raise their young. As the forests were cleared, they moved slowly eastward arriving in Ontario in the late 1800’s and finally reaching Nova Scotia in the 1930’s, so they are a very recent part of our avifauna.
In May, when most other birds are nesting, we see the cowbirds in twos and threes as they sit high in a tree watching other birds. They are master birdwatchers which can separate feeding and breeding behaviour in their target nesting species. The female always seems to be ready to lay an egg and as soon as she spots a nest she’s ready. When the host parent is absent to feed or rest, she swoops in and lays her egg in the nest and then is gone, after only a few seconds have elapsed.
It wouldn’t be so bad if the baby cowbird just hatched and the host species raised both it and her own offspring, but usually something more insidious happens. The female cowbird may oust the host eggs from the nest herself, or the baby cowbird may do likewise once it hatches. Cowbirds have a short incubation period so usually are the first to emerge from their eggs, giving themselves a strong advantage. In some cases the baby cowbird kills the baby host offspring once they hatch!
The outcome for many species of birds is devastating, as the baby host rarely survives, which means the entire nesting season may be lost unless a re-nesting takes place. Raising babies is a very difficult and energy consuming task, so many species only try once, regardless of the outcome.
Over 220 species of birds have been parasitized by cowbirds, with the top five species most frequently impacted being: Yellow Warbler, Song Sparrow, Red-eyed Vireo, Chipping Sparrow and Eastern Phoebe.
Sometimes, the impacts are so severe, a species is actually threatened with extinction. The Kirtland’s Warbler is one such species; its traditional range in North America is linked to Jack Pines which are of a very specific height and density. This unique habitat is limited, and historically their only known stronghold in North America was in the Upper Peninsula of Michigan. The cowbirds in that part if Michigan were relentless in choosing the Kirtland’s Warbler as their host species, such that the population crashed to only about 200 pairs in the 1970s! Cowbird control methods started in 1972, by the US government, removing them from the landscape. Today over 2000 pairs of Kirtland’s are known; this is still a small number but a 10-fold increase over the populations in the recent past.
Can birds identify the cowbirds or their eggs? Simple answer is, yes many can, but not all. Birds can react by ejecting the cowbird eggs (e.g. American Robins), burying cowbird eggs under a new nest lining (e.g. Yellow Warblers), or deserting the nest once cowbird eggs are laid (e.g Eastern Phoebes). Some species are very good at this and reject over 88 percent of cowbird eggs (e.g. Eastern Kingbirds, Blue Jays, American Robins and Gray Catbirds). Most other hosts reject less than 20 percent of the time, which means their nest will likely fail if predated by the cowbirds. Despite their unsavoury behaviours they are an entrenched part of the landscape and nature will prevail!
Geoff Carpentier is a published author, expedition guide and environmental consultant. Visit Geoff on LinkedIn, Instagram and Facebook.
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