Backyard bird feeders bring nature close to people, increase appreciation for the natural world, and, in winter, help birds survive. But research also documents some adverse effects, such as the potential to spread avian diseases and to inflate the population of nest predators. There’s a lot of attention on the former, but the latter issue probably does a lot more harm.
Close to half of all bird nests fail due to predation. Corvids (ravens, crows, jays, and magpies) play a big part in that; they are among the most voracious nest predators, taking other birds’ eggs and chicks. Squirrels and rats are also major nest predators. Open cup nesters are especially vulnerable.
Corvids are part of the natural ecosystem, but the problem goes beyond “natural” because many corvid populations are artificially higher due to anthropogenic food subsidies – human garbage, bird feeders, scraps, etc. In short, corvids are often what biologists call human-subsidized predators. Millions of restoration dollars have been spent trying to protect declining species from corvids (e.g., Marbled Murrelet, Least Tern, Snowy Plover, Piping Plover, and Desert Tortoise). Additionally, local populations of otherwise common species are known to be at risk from corvids (e.g. Common Murre, Red-capped Plover). And research shows they can depress bird populations in residential neighborhoods.
By providing food, humans have a remarkable ability to inflate corvid populations. Brunk et al (2021) examined efforts to reduce Steller’s Jay densities at campgrounds in Big Basin State Park in California in order to protect endangered Marbled Murrelets. Jay densities within the campgrounds were nine times higher than in the surrounding forest. Previous research documented that the Steller’s Jay juvenile survival rate in the campgrounds was over 90%, possibly the highest figure ever recorded for a bird; 50% is more typical. After the installation of new garbage cans, food lockers, as well as extensive camper education, the jays disbursed and densities fell to natural levels. I’m proud to have been a part of that project.
There is a long list of papers that show the depressing impact that high numbers of jays and crows can have on the productivity and populations of other birds. Here are a few examples focusing on songbirds:
Jokimäki et al (2020) looked at nest predation rates in nine European cities, finding that cats and corvids had a significant impact on other birds’ nest success in urban and suburban areas.
Hanmer et al (2017) experimented with artificial thrush nests with quail eggs placed in a natural context but in the vicinity of bird feeders that were offering peanuts. Nests near active feeders were far more likely to be depredated than nests that were far away from peanut feeders. The predators were jays, magpies and squirrels.
Malpass et al (2017) conducted an experiment in seven neighborhoods in Ohio, adding bird feeders during the breeding season to some areas. The neighborhoods with the most feeders had triple the numbers of American Crows and Brown-headed Cowbirds compared to the neighborhoods with the least feeders. American Robin nest success fell to just 1% in the high-feeder neighborhoods, compared to 34% in the areas with the fewest feeders. Northern Cardinals, perhaps because their nests are generally more hidden, managed to avoid these impacts.
Jokimäki and Huhta (2000) looked at bird assemblages across urban, suburban, and natural areas in Finland. They found a dramatically higher nest predation rate in the managed urban parks (53 to 92%) compared to more wild suburban parks (22 to 67%). Additionally, most ground nesting species simply avoided the urban areas. Crows and jays were listed among the primary nest predators.
Stoate and Szczur (1994) reported that corvid removal led to dramatic improvement in the hatching success of thrushes and Chaffinch. Some ground-nesting and thicket-nesting species (Dunnock, Yellowhammer, etc) were unaffected by corvids.
Slagsvold (1980) found that Fieldfare populations (the Scandinavian counterpart to the American Robin) doubled after the removal of Carrion Crows (the counterpart to the American Crow). Smaller songbirds also sought protection and nested near Fieldfares. Many songbirds based nest site decisions on the presence or absence of crows.
There are several meta-analyses, studies of studies, that try to summarize the big picture. Most of these include a wide array of avian and mammalian predators—and most focus on ground-nesting shorebirds and gamebirds, where conservation and hunting interests fuel research funding.
Cote and Sutherland (1997) is typical. It reviews 29 studies, focusing on avian (crows, gulls, grackles) and mammalian (foxes, skunk, marten) nest predation on gamebirds (pheasants, quail, etc) and some songbirds. Removing predators had a large positive effect on the prey species’ productivity and post-breeding population.
It is important to add that not all studies have found a link between corvids and reduced densities of other birds. This seems to be truer in rural contexts. Furthermore, impacts seem to be species-specific. I can imagine, for example, that chickadees and nuthatches, which nest in small cavities, are relatively protected from jay and crow nest predation.
Finally, there is Madden et al (2015), a meta-analysis focusing exclusively on corvids, which caused some confusion. It focused mostly on impacts to gamebirds and did not include jays. It is curious because its summaries and conclusions do not match the data they present. Based on their data, Madden could have concluded with this:
“The presence of crows had a negative impact on other bird species’ productivity in 66% of cases. In 10% of cases, a decline in abundance was also detected. These results suggest that, in certain contexts and for many species, large crow populations may create population sinks or actual declines in the populations of other birds.”
That would have been consistent with other studies, but the paper didn’t end that way. Instead, it summarized the data in ways to bury important results, failed to include some well-known cases involving endangered species, and concluded with this head-scratching statement:
“Our review shows that although there is no consistent pattern with regard to corvid impacts on other bird species, the most commonly reported effect is that corvids have no negative impact on prey species abundance or productivity. When combining experimental and correlative studies (326 cases), most cases (81%, n=264) showed no negative influence of corvids on either abundance or productivity of birds…”
“Most commonly” meant more than 50%. Because they lumped magpies with crows, and lumped abundance with productivity, they were able to say this (barely). I am astonished this paper passed thru peer review with this sweeping, and deceptive, summary intact. Some backyard birders, in defense of laying out a smorgasbord of peanuts on their back patio, have cited this paper on social media.
There’s a lot more I can say about this paper, but suffice it to say their own numbers (see Table 4) imply that two-thirds of studies involving crows found significant impacts to other species’ productivity.
Conclusion: No peanuts, and use caution in the breeding season
Make no mistake—bird-feeders in winter are associated with increased bird survival. There are lots of papers about that. I feed birds in winter (but no peanuts!). In the spring and summer, however, when my juncos and sparrows have departed, those same feeders disproportionately attract jays, crows, and squirrels, as well as House Sparrows, Eurasian Starlings, and Brown-headed Cowbirds. It’s no wonder that open cup nesters have a tough time. In the summer, I maintain hummingbird feeders and have a little pond and fountain that attracts bathing warblers.
Bird feeding in spring and summer, and especially offering peanuts which disproportionately attract crows and jays, likely depresses the populations of other songbirds in the vicinity. This is probably a big reason why many suburban neighborhoods can be largely devoid of cup nesters (e.g. warblers, vireos, flycatchers) while still having cavity and thicket nesters (e.g. chickadees, nuthatches, wrens, towhees).
Benmazouz et al. 2021. Corvids in urban environments: A systematic global literature review. Animals.
Brunk et al. 2021. Reducing anthropogenic subsidies can curb density of overabundant predators in protected areas. Biological Conservation.
Cote and Sutherland. 1997. The effectiveness of removing predators to protect bird populations. Conservation Biology.
Hanmer et al. 2017. Provision of supplementary food for wild birds may increase the risk of local nest predation. Ibis.
Jokimäki et al. 2020. Land-sharing vs. land-sparing urban development modulate predator–prey interactions in Europe. Ecological Applications.
Jokimäki and Huhta. 2000. Artificial nest predation and abundance of birds along an urban gradient. The Condor.
Madden et al. 2015. A review of the impact of corvids on bird productivity and abundance. Ibis.
Malpass et al. 2017. Species-dependent effects of bird feeders on nest predators and nest survival of urban American Robins and Northern Cardinals. The Condor.
Shutt and Lees. 2021. Killing with kindness: Does widespread generalised provisioning of wildlife help or hinder biodiversity conservation efforts? Biological Conservation.
Slagsvold. 1980. Habitat selection in birds: on the presence of other birds species with special regard to Turdus pilaris. Journal of Animal Ecology.
Stoate and Szczur. 1994. Game management and songbirds. The Game Conservancy Review of 1993.