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.
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.
I occasionally hear people say things like, “Ugh, I hate the crows,” especially when I’m doing research in neighborhoods. The comment “All of a sudden we have tons of crows and they’ve scared off all our songbirds!” always hurts me, but I recognize that most people are genuinely concerned about songbird conservation and abundance. To begin with, just to refresh your memory, crows are songbirds in and of themselves; ravens are our largest songbird. Putting semantics aside, I recognize that a great number of bird enthusiasts simply cannot embrace the crow lifestyle, and when they express a desire for songbirds at their feeders, they are referring to other species such as chickadees, juncos, grosbeaks, etc. They believe that since the crows’ “arrival,” they have seen fewer of these other birds. Is there any truth to this? Do crows really reduce the number of tiny, “desirable” backyard birds?
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?
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.
I saw what appeared to be a very clear chase today between two Crows and two fledgling Greenfinches. The Greenfinches were being chased by the Crows through bushes and trees, causing one of them to break off and flee, while the other was chased into a window. I gathered up the one that had struck the window, one Crow squawking loudly from a distance of no more than four meters, and the other in a nearby tree doing the same. After the finch regained some consciousness, I took it and carefully placed it in a tree that was out of the way. When I got back to where the chase had ended, the crows were hopping around below the window, trying to find the Finch. This would seem like active hunting. Are crows really that kind of hunter, and has anyone else seen anything similar?
FAQ
Will crows eat other birds?
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