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 theyve 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 cant 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?
The short answer is: not usually. Now, lets 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. Its 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 arent 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 doesnt 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 cant 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 werent 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 theyre often made out to be (though in a small number of instances they are, and its important to acknowledge when thats 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 its 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 its also possible they simply dont know where to look. It wouldnt be the first time Ive 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 Marzluffs new book Welcome to Subirdia, highlights that vast species diversity that can come with suburban development, showing that these types of habitat modifications arent 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.
Crows are known to crowd around larger predators, like owls and hawks, because they know that there is power in numbers. They frequently call in dozens of crows to join the attack. Additionally, when a predator approaches, they send out sentinel crows to alert the rest of the flock.
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These recently painted tern decoys are being used by Richardson Bay Audubon Center to attract breeding pairs of Caspian Terns, a tactic that has proven effective in past tern relocation attempts.
Studies have shown that crows and other corvids are particularly intelligent, possibly more intelligent than some primates. This may have something to do with having unusually large brains relative to their body size.
Crows can recognize human faces and remember whether that face presented a threat or a benefit. Crows will even seek revenge on specific humans that have harmed them in the past. Crows will communicate with other crows about dangerous humans or animals.
Recently, Madden et al. released a thorough analysis of the literature that examined the effects of corvid removal on a range of avian groups, including gamebirds, passerines, waders, and other ground nesting birds. The review covered 42 studies conducted in 9 different countries. They discovered that in 2081% of cases, the removal of corvids had no effect on the quantity or productivity of prey. Additionally, they discovered that corvid impacts on prey species were similar and that no particular group was significantly more sensitive than any other. Magpies consistently had the least effect on prey productivity of all the corvids studied; however, this did not change if prey abundance was the focus of the investigation. Given how noticeable corvids are as avian predators, why does it seem that most of the time their removal doesn’t have an impact?
The short answer is: not usually. To be clear, crows will kill and consume any birdeggs, nestlings, or even adult birdsthat they manage to get their hands on. Once, I witnessed a crow dispatch an adult house sparrow in a move so swift and skillful that I didn’t realize it until the crow had begun to devour its prey. But it’s crucial to remember that crows are just one of countless creatures that consume the young and adults of various bird species. Among the animals that will gladly consume birds are raccoons, squirrels, foxes, hawks, owls, bullfrogs, rats, mice, and, of course, cats. Eggs and nestlings in particular Many birds use multiple clutches during the breeding season as a breeding strategy because of their vulnerability as young birds. These same predators also affect crows, and very few of their young will survive to adulthood.
John Marzluffs new book Welcome to Subirdia, highlights that vast species diversity that can come with suburban development, showing that these types of habitat modifications arent 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!
This means that, for those of us working to change the negative perceptions of crows and other corvids, the data is on our side and that, despite what is sometimes said, crows are not typically the dangerous predators that are painted as such (though it’s important to recognize the rare cases in which they are). Indeed, in 6% of cases, the researchers discovered that the presence of corvids actually helped other birds. Crows frequently follow urban development, so 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. This is what I suspect is happening when residents ask me why they see fewer birds and if crows are to blame. Alternatively, perhaps they just don’t know where to look. It wouldn’t be the first time I’ve heard people lament that when they see juncos, chickadees, white-crowned sparrows, and yellow-rumped warblers, crows chase away their birds. Suburban areas are, in fact, frequently excellent places to observe crows and other smaller songbirds.
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 cant 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 werent 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.
FAQ
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