A comment I occasionally hear, especially while conducting my research in neighborhoods is, “Ugh, I hate the crows. All of a sudden we have tons of crows and they’ve scared off all our songbirds!” This comment always pains me, but I understand that for most people it arises from a genuine concern for songbird abundance and conservation. First off, as a reminder crows are songbirds themselves; ravens are our biggest songbird. Semantics aside, I understand that there are many, many bird lovers who just can’t get on the crow bandwagon and when they talk about wanting songbirds at their feeders they mean chickadees, juncos, grosbeaks, etc. They feel that since the “arrival” of the crows their observations of these other birds have diminished. So is there anything to this? Do crows indeed drive down populations of small, “desirable” backyard birds?
I came across this grizzly scene while conducting research in Bellevue. An adult robin calling frantically while a crow munched on one of its young. Later that same week I would watch of pair of adult crows chase hopelessly after a cooper’s hawk that had taken one of their offspring.
The short answer is: not usually. Now, let’s be clear, crows will absolutely kill and eat eggs, nestlings and even adult birds if they can get their hands on one. I once saw a crow take down an adult house sparrow in an attack so quick and dexterous I only realized what had happened after the crow had already started eating its meal. It’s important to keep in mind, however, that crows are one of many, many animals that are eating the young and adults of other bird species. Raccoons, squirrels, foxes, hawks, owls, bullfrogs, rats, mice, and of course cats will all gladly eat birds, especially eggs and nestlings. The vulnerability of young birds is in fact why the breeding strategy of many birds is to have multiple clutches over the course of the breeding season. Crows themselves are subject to these same predators and very few of their young will make it to adulthood.
Why do we think that crows aren’t responsible for the any observed decrease in feeder birds? Predator removal studies. These studies are straightforward and essentially create two populations, a control population that has been unmodified and a second where the predator in question has been actively removed. Prey abundance or productivity is monitored and compared at the end of the trial.
Recently, Madden et al. published a comprehensive literature review of 42 studies across 9 countries that looked at the impacts of corvid removal on a variety of avian groups including gamebirds, passerines, waders and other ground nesting birds. They found that in 81% of cases corvid removal made no impact on prey abundance or productivity. They also found that impacts of corvids on prey species was similar, and no one group was particularly more sensitive than any others. Of the corvids studied, magpies consistently had the smallest impact on prey productivity, but no difference was found if the study was looking at prey abundance. So if corvids are such conspicuous avian predators, why doesn’t their removal seem to matter in most cases?
This is explained by idea of compensatory mortality, which is essentially that removing one predator just means that the other predators will account for its absence by eating the prey it otherwise would have. Kevin McGowan provided a great description for this idea on his site “I like to use the analogy of handicapped parking spaces at the mall You drive up to the mall, looking for a parking space in a crowded lot. You can’t find a parking space, but there are four near the entrance that are reserved for handicapped permits only. You complain and think that if only those handicapped restrictions weren’t there, you could park in those spots (common sense). In truth, of course, if those spaces were not reserved they would have been taken long ago, just like all the other spaces in the lot.” Indeed, Madden et al. found this to be true. When they looked at studies that only conducted corvid removal, they found that only 16% of cases saw a difference in prey productivity. Whereas if all predators were removed the researchers reported that 60% of studies found a significant difference in prey productivity.
What this means for those of us trying to improve the bad reputations of crows and other corvids is that the data is on our side, crows are not usually the problem predators they’re often made out to be (though in a small number of instances they are, and it’s important to acknowledge when that’s the case). In fact, in 6% of cases the researchers found that corvid presence actually benefited other birds. So what I suspect is happening when residents ask me why they see fewer birds and if crows are to blame is that crows often follow urban development and it’s possible that what these residents are experiencing is a change in species diversity as habitats are disrupted and modified to make way for new human settlements. Though it’s also possible they simply don’t know where to look. It wouldn’t be the first time I’ve listened to folks complain about crows driving away their birds in the presence of yellow-rumped warblers, white crowned sparrows, juncos and chickadees. Indeed, suburbia is often a great place to enjoy both crows and other smaller songbirds.
John Marzluff’s newest book which describes the awesome power of suburbia to become a heaven for a huge diversity of birds. Illustrations by my friend and colleague, Jack DeLap.
John Marzluff’s new book Welcome to Subirdia, highlights that vast species diversity that can come with suburban development, showing that these types of habitat modifications aren’t doomed to be low diversity. With a bit of thought on our part, we can create habitats that attract a variety of birds. Namely, by limiting lawn space, increasing snags, native plants and bushes and keeping our cats indoors, we can expect to see a great variety of birds visiting our feeders, crows included!
Madden, C.F., Beatriz A., Amar, A. (2015) A review of the impacts of corvids on bird productivity and abundance. Ibis: 157, 1-16.
corvidresearch.blog 