When discussing invasive species, we often think of species that evolved far away on other continents. Here in the United States, thousands of non-native species have been brought here since the dawn of colonization; while not all became established, a significant number have since become aggressively integrated into local habitats, much to the detriment of native species they displace or otherwise harm. But sometimes a species will simply encroach upon an ecosystem adjacent to its native home, and then spreads from there, having the same negative impact as other relatively new arrivals. Which begs the question: are birds on the move like barred owls (Strix varia) and cowbirds invasive?
I want to especially look at the brown-headed cowbird (Molothrus ater). Like the barred owl, these chunky songbirds have expanded their range in North America in recent decades due to the extensive damage we’ve done to habitats across the continent. Both were particularly affected by the destruction of the Great Plains. Settlers tore up the ancient grasslands with plows, suppressed fires that had kept forests from encroaching, and started a trend of afforestation–planting trees where they aren’t supposed to be–that continues today. In the latter half of the 19th century, the plains bison (Bison bison bison) was slaughtered almost to extinction to make money off the sale of their remains, and to disempower indigenous communities throughout the region who were reliant on this keystone species for their very existence.
The owls hopscotched across the growing number of tree plantations that dotted first settlements, then towns, then sprawling suburbs and cities, and thus were able to reach all the way to the west coast, where they have put serious competitive pressure on the northern spotted owl. The cowbirds, on the other hand, became refugees as all but the last few hundred bison disappeared from the landscape. They instead turned to domestic livestock like cattle for their survival.
Same Habits, Different Hosts
Brown-headed cowbirds are unusual in that they evolved to migrate with the bison, rather than waiting for new herds to arrive. The birds feed on insects stirred up by the herd’s hooves, along with seeds of grasses and other plants along the way. Cowbirds perching on the backs of these enormous mammals would have been a common sight prior to the bison’s near-extermination, and today they may still be seen watching for prey from on top of cattle, horses and other domestic livestock.
Because cowbirds have spent thousands of years migrating with their bison hosts, they couldn’t afford to settle in one place for months at a time to raise a nest of chicks, particularly as spring and summer would be prime time for finding insects to eat. It might be weeks or even months before a herd would return to a given area, and without the help of the bison the cowbirds would have had a difficult time finding enough insects on their own, let alone for a hungry nest of babies.
So the cowbirds evolved a rather controversial strategy: brood parasitism. Instead of building their own nest, they find someone else’s and leave a few eggs there for the unwitting owners to raise. Some people anthropomorphize the cowbirds as being “bad”, “lazy”, “evil”, and so forth. And, of course, it’s easy to call cowbirds invasive, too, since they literally invade the nests of other animals.
But nature is amoral; there are no inherently good or evil animals. While we may project our distaste for the cowbirds’ practices onto their value as a species, brood parasitism is simply one of a plethora of strategies animals have evolved in order to pass their genes on to the next generation. If it gets the job done, then it is an evolutionary success. Like the mate cannibalism of some mantids and spiders or animal infanticide practiced by male lions, baboons, and dolphins, brood parasitism may be vicious by our standards, but it is a legitimate survival tactic in the sometimes-cutthroat world of nature.
Are Cowbirds Invasive?
We can certainly call cowbirds invasive when they hop into a nest they didn’t build just to drop off an egg. But are they invasive on a more widespread level? Arguably yes. Not all invasive species were physically transported by humans, but the impact is the same: they have a deleterious effect on one or more other species in their habitat. And unlike coyotes, which only spread to new horizons when their competitors were extirpated by human hunters, brown-headed cowbirds are not filling a niche that was previously taken by another species. they are, instead, an often-unwanted addition to local ecology.
In addition to tearing up the Great Plains and then planting trees there, we also cut down massive numbers of trees in historically forested areas across the continent, leaving patches of fields in which cattle and other livestock graze. This has led to the spread of the cowbird beyond its normal range in the prairies. Other bird species that evolved alongside the cowbirds have developed ways to respond to brood parasitism, from throwing cowbird eggs out of the nest, to building a new nest entirely.
But birds in the cowbird’s expanded territory aren’t always so savvy. That includes species that have seen declining numbers in recent decades due to habitat loss, lead and other toxins, and predation by another invasive species, the domestic cat (Felis catus). Since cowbirds and their chicks will both remove their hosts’ eggs from a parasitized nest, and cowbird chicks may toss their host’s young out or steal all their food, this means fewer numbers of the host species being successfully hatched and fledged. All of which means cowbirds are becoming a serious conservation concern.
That being said, we shouldn’t be too quick to dismiss an entire species by calling all cowbirds invasive. Within their native range in the Great Plains, they are an important part of local ecology. And–unlike certain members of our own species–they are not intentionally cruel animals that want to see other living beings suffer. They are simply doing what their ancestors have done for thousands upon thousands of years, and unlike humans they have no capacity to consider the impact on their hosts.
One last note: if you are tempted to remove cowbird eggs from a nest, please don’t. First, it’s an activity best carried out by professionals who have a better sense of what nests should be attended to and when. Moreover, egg removal can not only cause the host birds to abandon their nest and their own eggs, but cowbirds are more likely to attack hosts who remove the offending eggs, and you could be setting the nesting pair up for retaliation from the cowbirds. And brown-headed cowbirds, like almost all native birds in the United States, are protected by the Migratory Bird Treaty Act, which makes it a federal offense to disturb or harm the birds themselves, as well as their eggs or nests. If you are in an area where cowbirds are considered to be invasive, and you are concerned about another species’ nests, contact your state wildlife department or the closest Audubon Society (some of these societies have changed their names in recent years, but fulfills the same roles as before.)
