Garden bird feeders spread diseases

EACH year, millions of people stack their garden bird feeders with seeds and nuts to help birds survive the winter. But as valuable as they are to many species, for a minority of songbirds in Europe and North America bird feeders appear to be a death trap.

Little is known about the impact of bird feeders on wild populations, and some ornithologists liken them to a global experiment in manipulating nature. "We should have a huge amount of data, but we don't," says Jim Reynolds of the University of Birmingham in the UK. Reynolds will be speaking at a conference in London next month, which for the first time will examine the pros and cons of feeding wild birds.

The conference will hear that bird feeders have played a key role in two waves of diseases among songbirds, mainly finches (see map). Since 1994, an epidemic of an infectious eye disease called mycoplasmal conjunctivitis, which began in poultry, has wiped out 60 per cent of house finches in the eastern US. Undernourished and unable to see properly, they fall easy prey to predators. Experiments by André Dhondt of Cornell University in Ithaca, New York, show that the birds pick up the Mycoplasma gallisepticum bacterium from making contact with feeders as they peck at seeds.

The disease has just reached California, and has spread to other species such as the American goldfinch. A new and more virulent strain has emerged in North Carolina. "It spreads much faster, and the eye infections are more severe," says Dhondt.

In the UK, a feeder-related disease has been affecting greenfinches since 2005. Trichomoniasis, or "trike", is related to a disease thought to have killed some Tyrannosaurus rex. It triggers throat swelling, causing birds to starve, and has killed about a fifth of the UK's greenfinches. In 2007 alone, around 500,000 died, according to Rob Robinson of the British Trust for Ornithology (BTO).

Scott McBurney of the University of Prince Edward Island, Charlottetown, in Canada will report at the London meeting that trike reached Canada in 2007. There are also preliminary reports of cases in the US. This suggests the two outbreaks could overlap, with devastating consequences for finch populations.

None of this means that we should throw out our bird feeders. "This is the first big mortality effect," says Robinson, "but it's only in one or two species. We're safely feeding another 30 or 40." Other research shows that feeders help birds to survive the winter, and then to produce more young that have higher survival rates.

Mike Toms at the BTO says that simple measures like regularly washing feeders with clean water can reduce infection rates. He has also found that mesh or metal-frame feeders are less likely to spread disease than feeders with a single point of access.

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