Some birds can come to rely quite heavily on human provided food, relying almost entirely on feeder handouts. The researchers are unclear about how this affects the migrant species that arrive in the spring to search out possible nest sites. By the time they arrive, prime nesting locations may be taken by the birdfeed reliant birds and result in lower numbers of the migrant species hatching each year.
As seen on the front page of Nature.com, feeding birds during the winter is not only fun for the people feeding them. According to a British study, feeding birds in winter benefits them during the breeding season by allowing them to lay their eggs 2.5 days earlier on average than birds that did not have access to winter food. Birds that had been fed all winter also managed to raise an average of almost one more chick each year.
The researchers looked at 500 nest boxes in N. Ireland, feeding some lucky birds more than 6 tons of peanuts and allowing the others to find their own natural food sources. The researchers think that the nutrients in the peanuts allow for better egg laying so the benefits may be different for various kinds of bird food. Another possibility is that the extra food allowed the birds to get into breeding condition more quickly.
So, it’s hard to say how feeding bird affects all species but for the birds that stick around for the winter, the extra food source can be a real boon. The extra chick/year could really increase certain populations which, depending on the species, could be good or bad. The article also points out how important it is to keep feeding areas clean to avoid the transmission of diseases such as trichomoniasis which can be fatal to young birds.