A free meal might seem like just the thing for your bird friends in winter, especially if that meal takes place in front of your picture window. But your feeder could be harming some bird species more than it's helping them. Even if it's a squirrel-proof seed tube or a pinecone rolled in peanut butter, there's still no such thing as a free lunch.
"We are really only in the early stages of understanding exactly what effects bird feeding is having on our wild bird populations," says Kate Plummer, an ecologist at the British Trust for Ornithology. Britons love to feed the birds. The BTO says that about half of everyone in the United Kingdom does it.
To find out how extra food during the winter months might affect wild birds, Plummer and her colleagues used populations of blue tits (Cyanistes caeruleus) that had never set eyes on a bird feeder. The birds lived at nine different sites in the woods. Researchers set up feeders to provide six bird groups with food (either straight fat, or fat plus vitamin E) while the other three got nothing. Over the course of three years, they rotated which bird populations were fed so they could better compare the results.
The scientists weren't interested in how extra food affected the birds eating it—they wanted to know what happened in the next generation. In nest boxes the following spring, they found that birds that had eaten at feeders hatched smaller chicks. Ultimately, fewer of these chicks grew up and left the nest.
Plummer says there are a few possible explanations. The fatty diet, for one, may have made birds nutritionally unbalanced by the time it came to egg-laying season. An earlier study that fed blue tits peanuts instead of straight fat found that it was beneficial to the next generation (though there were other differences between the studies too).
Feeders might also allow weaker birds to survive the winter, eventually hatching scrawnier chicks and bringing the whole population's average down. Or extra food in winter might encourage birds to invest resources in egg-laying, only to find come spring that their nests aren't in a great spot for food after all. The real answer may be a combination of factors, Plummer says.
Two other recent studies of woodland tits found that supplementing their food over the winter led to fewer chicks raised in the spring. But a similar study in woodpeckers found just the opposite. Bird feeders might be helpful for some species and harmful for others, possibly due to their different nutritional needs.
There may also be "winners and losers of bird feeding," Plummer says, that depend on the combination of species sharing a feeder. Dominant species might outcompete other birds, for example. Birds at feeders might also share diseases while they're pecking at the same seeds.
"People shouldn't stop bird feeding," Plummer says, at least not yet. It's too early to assume that feeding birds is always harmful. So go ahead and fatten up your feathered friends this winter. That is, as long as you can live with the possibility that the scene through your picture window isn't doing them any favors.
Image: David Lewis (via Flickr)
K. E. Plummer, S. Bearhop, D. I. Leech, D. E. Chamberlain, & J. D. Blount (2013). Winter food provisioning reduces future breeding performance in a wild bird Scientific Reports DOI: 10.1038/srep02002