Tuesday, March 11, 2014
By Herb Wilson
I suspect that like me, you made sure your bird feeders were well stocked before last weekend's blizzard. Our feeders were pretty busy all through the blizzard. Clearly, birds readily accept our handouts.
This homeowner has assured herself a diverse, year-round population of songbirds by giving them plenty of incentives to stay around. And if those feeders go empty, the birds probably will find a way to survive.
Have you ever thought about the ethics of bird feeding? Is it OK if your bird feeder becomes empty for a while this winter? I get asked this last question quite a lot by people who plan to be away for part of the winter, depriving their local birds of a formerly constant source of food. Are we doing our feeder birds harm by giving them food and then taking it away? In other words, do our feeder birds become dependent on bird feeders?
To cut to the chase, local birds do not become dependent on feeders. One should not worry about a feeder becoming empty when high winds or a blizzard blow your feeder to the ground or when birds deplete the food while you are away.
We do know that feeding the birds increases winter survivorship. The best information for this effect concerns black-capped chickadees from studies conducted in Wisconsin, Pennsylvania and Ontario.
To investigate if birds do become dependent on handouts from humans, Stanley Temple and Margaret Brittingham performed an intriguing experiment. These two scientists extended their earlier work, showing that winter bird feeding improved chickadee survival. Their next step was a study to compare winter survival of chickadees between two areas. In one area, chickadees had been given sunflower seeds continuously for several years. In the second area, no bird feeders were ever present. The authors took away the bird feeders from the first area where birds had been feeding on sunflower seeds for years and monitored winter survivorship.
If the population given sunflower seeds in previous years had a lower survivorship than the population with no bird feeders, one could claim that the birds in the first area had become dependent on the sunflower seeds. But there was no difference in survivorship for the two populations. The previously fed chickadees did as well in the following year feeding on natural food as the unfed chickadees did.
Similar studies have not been done for other North American species that frequent feeders, but I expect that results would be similar. Depending on a single source of food is risky for any winter bird. Winter songbirds commonly range over areas of 10 to 25 acres. Much of this area is regularly patrolled and food is taken from a number of different parts of their winter area.
Bird feeding has a number of effects. It brings birds close to our homes where we can enjoy them from the comfort of our kitchen or living room. Bird feeding has strong effects on the birds as well. In addition to increasing survivorship, bird feeding has certainly facilitated the expansion of a number of birds. In Maine, house finches are rarely found far from feeders. Tufted titmice have been steadily expanding their range northward over the past 20 years, undoubtedly aided by the provision of food during the winter at bird feeders. These birds regularly occur in central Maine now and I have even seen one in the winter at the north end of Flagstaff Lake. Mourning doves, northern cardinals and Carolina wrens fall in the same category of birds expanding their range northward.
We know that bird feeders can change the habitat preference of birds. For instance, American goldfinches in the absence of bird feeders are equally likely to be found in areas dominated by coniferous forest, areas dominated by deciduous forest and areas we can classify as "edge" habitats (suburban areas, fields reverting to forest). But the goldfinches will quickly forsake forest habitat for edge habitat if one puts up bird feeders in the edge habitat.
Herb Wilson teaches ornithology and other courses at Colby College. He welcomes reader comments and questions at: