The Outside Story: The trees of Christmas past and future
The birds, ever optimistic, still dropped by. I started writing dialogue for them:
Titmouse: "Nothing here yet. Still don't know why he stopped."
Chickadee: "He'll refill it, he's pretty reliable. He cares about us."
Blue Jay: "I don't know. He's cheap."
Me: "You blue jays are pigs."
Feeding wild birds is immensely popular. According to a 2013 study by the Wild Bird Feeding Industry Research Foundation, some 48.9 million households in the U.S. and Canada buy wild bird seed each year, creating a healthy $4 billion annual market.
Dr. Emma Greig, project director of the Cornell Laboratory of Ornithology's FeederWatch, calls it a "massive natural experiment" that may, or may not, be affecting bird behavior or benefitting particular species. No one really knows, she said.
"It's really hard to make generalizations about how feeders change behavior, or even how feeders change reproductive success and survival," Greig said. That's because it's difficult to do controlled studies comparing one population of a species with access to supplemental food to another without it. And it's hard to tease out the effects of winter feeding from "all the other changes people are imposing on the natural world, including changes to habitat and climate. Those are affecting bird populations as well." Another complication: there are hundreds of species of birds.
Greig said her intuition tells her that "supplemental feeding benefits some species, but developing a causal relationship is really, really tough."
In Britain, studies have shown that winter feeding helps with blue tit survival and reproductive success — in some cases. In other cases it had a negative effect. Studies found that Eurasian blackcaps, a type of warbler, are overwintering in colder areas and in greater numbers where they have supplemental food, said Greig. Anna's hummingbird has been increasing its winter range northward up the Pacific Coast and seems to be more prevalent in areas where there are people, she said. But are they increasing their range because of feeders? Or are people putting out feeders because they're seeing more hummingbirds?
The Cooper's hawk, an accipiter that preys frequently on small and medium-sized birds, has been increasing in numbers and more are wintering farther north. But is that because it can reliably find prey at bird feeders? Who knows.
Then there's the issue of whether winter feeding facilitates the spread of disease. The answer here is squishy, too. One study found that house finch eye disease, a type of conjunctivitis, spreads more easily in aviaries where finches shared a feeder. But does that apply to wild birds? "If you have sick birds wiping their faces on perches used by other bids it could facilitate the spread of disease," Greig said. But birds flock together anyway. And there's certainly no shortage of house finches.
"There does seem to be more evidence that feeding birds and having bird-friendly backyards does more good than it does harm," Greig said.
In spite of some built-in limitations, Greig said feeder-monitoring projects like FeederWatch (feederwatch.org) still have scientific value. They can help generate data on how bird populations are changing, for instance, or generate observations on dominance and predation rates at feeders. Scientists can then use that to craft experimental studies to explore the questions in more depth and try to figure out the causal relationships. The trick is not to generalize, "because different species may respond in different ways to bird feeding."
Backing up a bit, does start-stop feeding hurt the birds?
Probably not, said Greig. "As far as we know, birds will have an assortment of places they're checking for food within their expanded winter territory. If one source of food disappears, they have others they can rely on."
So, what do we know about the effect of winter feeding on the behavior of humans who do it?
Well, it seems to help bridge the human-natural world gap. The Cornell Lab's Project FeederWatch has some 20,000 participants. They regularly count birds and note species at their feeders. They love it because it takes their hobby to a new level, said Greig. They learn more about birds and get more vested in their welfare and nature. And studies show that citizen scientists "are more likely to be environmental advocates and actually take action when it comes to environmental issues," she said.
Joe Rankin lives in Maine. The illustration for this column was drawn by Adelaide Tyrol. The Outside Story is assigned and edited by Northern Woodlands magazine, and sponsored by the Wellborn Ecology Fund of New Hampshire Charitable Foundation: firstname.lastname@example.org. A book compilation of Outside Story articles is available at http://www.northernwoodlands.org.
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