Chances are good that readers of this column maintain a bird feeder. The U. S. Fish and Wildlife Service estimates nearly a quarter of Americans feed the birds, a practice which has increased greatly in the past 20 years. We feed the birds to attract these creatures to us for our enjoyment and study. With all the bird feeders in the United States, it is logical to ask what effect all of these feeders are having on the birds.
One effect of bird feeding is to allow birds to expand their ranges. Over the past 30 years, several species have expanded their range north into Maine. These species include Tufted Titmouse, Northern Mockingbird and House Finch. Mourning Doves have become a lot more common in Maine. The provision of supplemental food at feeders has been used to explain these range extensions. Particularly in the winter, few individuals of these species are found far from a feeder.
Another effect of bird feeding is to increase the survivorship of birds. Members of the chickadee family have been the most-studied species in this regard because they tend to spend their entire lives in a restricted area. Failure to find a banded bird usually means that bird has died rather than moved to a different area.
The first experimental demonstration of bird feeders improving survivorship was done in Wisconsin by two ornithologists, Margaret Brittingham and Stanley Temple. Brittingham and Stanley had some sites with sunflower seed feeders and others without. In each site, Black-capped Chickadees were captured and banded with unique combinations of color bands. The two workers would regularly census their areas to see which chickadees they could find. Missing chickadees were presumed to be dead. Chickadees had higher survivorship in areas where bird feeders were present. Greater mortality occurred during extended periods of very cold temperature.
Brittingham and Erica Egan repeated this experiment in northwestern Pennsylvania. In addition to forested sites with and without feeders, these workers also monitored chickadees in suburban areas where there were many feeders. As in the earlier study, feeders had a substantial effect on winter survival. In forested areas without feeders, 16% of the chickadees died each month during the winter. In the forested area with feeders, only 7% of the chickadees died each month. In suburban areas with many feeders available, only 6% of the chickadees died each month.
We can all attest to the effectiveness of bird feeders in attracting birds to our yards. People often fret when they are away from their feeders on trips and can’t keep their feeders filled. Are their local birds in trouble when the feeders become empty? In other words, do birds become dependent on bird feeders for their food, particularly in the winter?
The answer to these questions appears to be no. Birds seem to know that they cannot count on even very rich food sources for long periods of time. Birds will not feed solely from a continuously stocked feeder. One study has shown that Black-capped Chickadees only take about 25% of their daily food requirements from well-stocked feeders. The birds are hedging their bets, looking for other sources of food in case a well-stocked feeder should become empty.
A definitive study to test for feeder dependency in Black-capped Chickadees was done in Wisconsin by Brittingham and Temple. These scientists studied two large populations of chickadees, banding most of the chickadees so that their survival could be monitored. For two years, one population was provided with sunflower seeds continuously. The other population was never given any food. In the third fall of the study, the feeders were removed from the first study area. Therefore, during the third winter of the study, neither population had access to supplemental food. If the chickadees in the first population had become feeder-dependent, we would expect them to have lower survival than the population that never had the benefit of supplemental food. Brittingham and Temple found that there were no significant differences in winter survivorship. The monthly winter survival rate was 84% (with an uncertainty of plus or minus 1%) for the population with feeders in the past and 85% (plus or minus 1%) for the population that never had access to feeders.
Remember that bird feeding is known to improve winter survival of a number of birds so keeping your feeder stocked will maximize this benefit. However, if your feeder empties while you are away, your local birds will not be at a disadvantage to other birds that never have the benefits of bird feeders.
[First published on November 28, 2010]