Winter bird feeding helps spring breeding
Bird-feeders can help feathered friends for a surprisingly long time.
Feeding the birds that visit your garden in winter has unexpected benefits, according to British research. Not only does it help species such as blue tits survive the cruel cold months, it also enhances their breeding.
The study is the first to show such a long-term effect of providing bird food. Previously, domestic wisdom held that nibbles such as nuts and seeds helped birds get through a lean spell — as well as giving you something to look at from your kitchen window — but that the benefit of such snacks was probably only temporary, and would not last past the winter.
Unlike migrating species that must build up large fat reserves before heading abroad for the winter, species such as blue tits and great tits that stay in cold climes are thought to live on a more hand-to-mouth (or claw-to-beak) basis, explains Stuart Bearhop of the University of Exeter, UK, who led the study.
But Bearhop and his colleagues found that, surprisingly, the benefits stretch into the spring. Birds with ready access to peanut-stocked bird-feeders from November until early March began laying eggs roughly 2.5 days earlier than birds without access to supplemental feeding, even though their breeding season does not begin until April.
What's more, the researchers also found that birds that had been fed had, on average, almost one extra chick that reached maturity. Bearhop and his colleagues describe their results in the journal Biology Letters1.
Many conservation charities recommend winter bird-feeding (see ), and bird-lovers in the United States and Britain typically hand out around half a billion tonnes of food each winter. The new research shows not only that they have been barking up the right tree, but that the benefits last long after the food is finished.
Although it might stand to reason that birds with extra food access might do better than others, no one had ever studied the effects on a large scale before. "We wanted to mimic the large scale of feeding that results from the feeding that people do in their back gardens," says Bearhop.
The research looked at the residents of some 500 nest-boxes installed in 10 wooded areas of the countryside in Northern Ireland. Postdoc student Gillian Robb of Queen's University Belfast, who led the field work, spent months stocking around 100 bird-feeders for some lucky birds, while others had access only to natural food sources. Overall, the catered-for birds chomped their way through more than six tonnes of peanuts.
Bearhop suspects that the improved breeding of the fed birds might be due to micronutrients in the peanuts that are important for laying healthy eggs. Peanuts are very rich in vitamin E, he points out. Alternatively, the constant availability of food might help the birds get into breeding condition more quickly after winter is over.
Feed the birds
The effects of bird-feeding on the overall ecosystem remain unclear. Most birds have a repertoire of feeding locations and are unlikely to rely entirely on feeders, but some populations can come to depend fairly heavily on the hand-outs. "If people stopped feeding them, it could be quite damaging," says Bearhop.
On the other hand, Bearhop notes that migratory species arriving to breed in spring might lose potential nesting sites as a result of the increased populations of birds that stayed at home for winter and found extra food at hand. "It is still unclear whether this has a knock-on effect on other species. This is something we are keen to investigate," he says.
Overall, bird-feeding is to be encouraged, says Martin Fowlie, a UK spokesman for the charity BirdLife International. But he warns that proper maintenance is essential to avoid feeding stations becoming transmission points for diseases such as trichomoniasis, which can kill young birds.
"You should never overfeed, because food goes off," Fowlie says. "If you're drawing birds in you should keep the area clean, whether it's a bird-table or window-ledge. If birds are perching on something and leaving their droppings and saliva there, and if they're ill, then it can be transmitted."
- Robb, G. N. et al. Biol. Lett. doi:10.1098/rsbl.2007.0622 (2008).
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