It is a little ironic that all nature enthusiasts know that it is “bad” to feed the animals … they become dependent on the food, and in some cases will become a nuisance or dangerous, prying open cars or breaking into homes to get more food. Then the animal has to be put down or moved to a new habitat. But that sort of bad outcome is more common with, say, bears than it is with, say, chickadees. The irony here is that bird lovers, who are always nature enthusiasts, do not seem to balk at setting up bird feeders. In fact, approximately on half a million metric tons of seed is put out for the birds in the United States and the United Kingdom.
This must have an effect on the birds, for better or worse. Two studies just published by the same research team address this issue.
As one might expect, the impact is mixed. There are probably birds doing very well on account of feeding, but in some cases they do too well.
Few scientific studies have addressed the ecological effects of backyard bird feeders. So Gillian Robb, an ecologist at Queen’s University Belfast in the U.K., and her colleagues gathered as much information as they could about supplemental feeding in any context, for species from sparrows to owls.
Robb and her colleagues confirmed this effect with an experiment of their own in Northern Ireland, where they gave peanuts to dozens of blue tits at five woodland sites and let the birds fend for themselves at another five sites. The birds that nibbled peanuts all winter fledged more chicks in the spring, the team reports
when Florida scrub jays ate too much in winter, they laid their eggs so early that the hatchlings’ natural food sources weren’t available when they needed them…
If a region’s year-round residents plump up on extra seed all winter and get a head start on breeding territories and natural food supplies in the spring, they could present stiff competition to migrants returning from tropical wintering grounds.
Paige Warren, an urban ecologist at the University of Massachusetts, Amherst, says that based on the paper, she won’t discourage birders who ask her advice on bird feeders. But she brings up another potential downside, the socioeconomic impact: Birdseed is often grown in regions where farmers are treated poorly. And then there are the environmental effects of growing and transporting thousands of tons of birdseed.
The research was published in two journal articles. The first is in Frontiers in Ecology and the Environment. From the abstract:
… While alteration of the natural dynamics of food supply represents a major intervention in avian ecology, we have a remarkably limited understanding of the impacts of this widespread pastime. Here, we examine the many and varied responses of birds to supplementary feeding at backyard feeders – in large-scale management projects and in focused academic studies – and evaluate population responses to the bird-feeding phenomenon. Our review encompasses a wide range of species, from songbirds to raptors, and compares provisioning with a variety of foods, at different times of year and in different locations. We consider positive impacts, such as aiding species conservation programs, and negative ones, such as increased risk of disease transmission. It seems highly likely that natural selection is being artificially perturbed, as feeding influences almost every aspect of bird ecology, including reproduction, behavior, demography, and distribution. As the effects of bird feeding cascade through ecosystems and interact with processes of environmental change, we suggest areas for future research and highlight the need for large-scale experiments, with a particular focus on the backyards of an increasingly urban and generous, but sometimes fickle, human population.
The second study is in Biology Letters. Again, from the abstract:
Supplementary food given to birds can have contemporary effects by reducing the risk of starvation, increasing survival and altering movements and reproductive performance. There is, however, a widely held perception that birds benefit from extra food over winter, but that it is better that they ‘look after themselves’ during breeding. Here we describe a landscape-scale experiment showing for the first time that the effects of increasing food availability only during the winter can be carried over to the subsequent breeding season. Even though food supplementation stopped six weeks prior to breeding, birds living on sites provisioned over winter had advanced laying dates and increased fledging success compared with birds living on unprovisioned sites. Thus, supplemental feeding of wild birds during winter, in a manner mimicking householders provisioning in gardens and backyards, has the potential to alter bird population dynamics by altering future reproductive performance. With levels of bird feeding by the public continuing to increase, the impacts of this additional food supply on wild bird populations may be considerable.
It is remarkable that the intersection of science and recreational natural history (i.e., bird watching) has not produced extensive studies of this kind already. There is a great deal of folk knowledge about bird feeding …. that birds may get their moist eyes stuck on metal bird feeders in the winter, that if you feed birds in the fall you must make sure you continue to feed them all winter or they will die, and so on. The idea that feeding certain species will result in their out competing other, perhaps at-risk species does not seem to be part of that folk knowedge, but perhaps it should be.
Robb, G.N., McDonald, R.A., Chamberlain, D.E., Bearhop, S. (2007). Food for thought: supplementary feeding as a driver of ecological change in avian populations. Frontiers in Ecology and the Environment, preprint(2008), 1. DOI: 10.1890/060152
Robb, G.N., McDonald, R.A., Chamberlain, D.E., Reynolds, S.J., Harrison, T.J., Bearhop, S. (2008). Winter feeding of birds increases productivity in the subsequent breeding season. Biology Letters, 4(2), 220-223. DOI: 10.1098/rsbl.2007.0622