The Unintended Consequences of Backyard Bird Feeders
Food availability may be the most significant determinant of the size and distribution of animal populations. In seasonal climates, winter is always a time of food scarcity for terrestrial birds, Yard, and garden feeders in much of Europe and North America have changed this equation by enhancing food abundance during the coldest months. Backyard bird feeders have dramatically increased food supplies available to wild birds, reshaping entire avian communities. In Britain, for example, this supplemental food supply supports an estimated 196 million birds. This figure far exceeds the combined populations of many common garden species. The resulting long-term ecological impacts are unknown.
This study used data from annual bird counts over a 40-year period along with data from the Garden Bird Feeding Survey to determine how backyard feeders have contributed to national-scale population changes in wild bird communities in Britain. Early on, feeders were homemade affairs stocked with little more than kitchen scraps. In the mid-20th century, commercial interests began to advertise an ever-expanding array of feeders. Feed types on offer exploded from just a handful to over 120. In Britain, gardens account for about one-quarter of all urban land cover. At least half of British homeowners feed the birds in their gardens. Thus, more and more bird species benefited as homeowners put out greater varieties of food to attract them.
The efforts of British bird enthusiasts to attract more and different birds has been perhaps too successful. A total of 133 species of birds use garden feeders during winter, or 52.6% of all local species. Analysis of a 40-year time series showed a significant increase in the diversity of birds using feeders. Approximately half of all birds using feeders in the 1970’s belonged to just two species. By the 2010’s, the number of species making up half of the feeder users had more than tripled.
According to bird count data, populations of feeder-using species have expanded in numbers while the populations of species that do not use feeders have remained unchanged. To confirm that it was in fact feeders driving these changes, the researchers analyzed population trends from 1973 to 2012 for 39 species of birds that use garden feeders. Two-thirds of species showed significant positive population changes, and these effects were linked with feeder use. Such findings demonstrate the impact humans can have on not only the size but the structure of bird communities.
Further evidence linking feeders to changes in bird communities was found when researchers looked at population trends for urban-dwelling birds. Urbanization has had a negative effect on avian populations, but analysis of population data for 72 urban species found strikingly different population trajectories for those who did visit feeders in comparison with those who did not.
Wild bird feeding has become part of human culture all over the world over the last half-century. But this seemingly innocuous activity, through its pervasiveness, is primarily responsible for re-shaping the national populations of birds in Britain over the last half-century. Since feeding birds has clear conservation impacts, animal advocates can use this study to support coordinated, large-scale efforts to manage species of concern. This research can also inform public education campaigns around the effects, both positive and negative, of backyard bird feeders.