26 July 2010, by Tamera Jones

Like the tortoise in Aesop's famous fable, slow and steady wood thrushes can still win the race. Scientists have found that birds which set off late and in poor condition for their autumn migration still arrive at their winter homes in good time.

Wood thrush

Wood thrush.

It turns out that this is because the amount of time they spend stopping over before they arrive can vary hugely.

'What I believed for the past 20 years is that delaying feather moult would mean that the birds would get to the tropics later and would have trouble getting the best winter spots,' says Dr Bridget Stutchbury from York University, Canada, who led the research. 'We've found that isn't necessarily the case.'

Wood thrushes may be late leaving their Pennsylvania breeding grounds during the autumn because they've bred several times and have ended up nesting later. The knock-on effect is to delay their feather moult.

But because moulting takes a lot of energy and birds end up losing weight, they need to spend time trying to bulk up again before they set off on their migrations.

'At the moment, we're not sure why they're spending so long stopping over. It's longer than they need just to re-fuel.'

Dr Bridget Stutchbury, York University, Canada

While scientists know a lot about how the quality of their winter habitat affects wood thrushes' success the following season, trying to figure out if late breeding, late migration and being in poor condition poses a problem for their winter season has, until now, proved difficult. That is, until miniaturised geolocators were invented by British Antarctic Survey scientists.

These tiny light-detecting devices show researchers where the birds have been – to the nearest couple of hundred metres.

Stutchbury and her co-authors tracked wood thrushes (Hylocichla mustelina) during their autumn migration from Pennsylvania in the US to the tropics - Nicaragua and Honduras. 'All the birds were adults, so they were experienced; they'd been to the tropics before,' explains Stutchbury.

Before they started using geolocators to track wood thrushes, Stutchbury and her colleagues assumed setting off late would mean they'd have trouble getting good territories in the rainforest. This can be a problem for them, because birds that are pushed out of the rainforest are more likely to die.

They found that although some of the birds were in poor condition when they set off late, they stopped off in places like Mexico's Yucatán Peninsula for a couple of weeks.

'At the moment, we're not sure why they're spending so long stopping over. It's longer than they need just to re-fuel,' says Stutchbury.

'What's puzzling is that these birds are territorial in their wintering grounds in the tropics. It begs the question, why aren't the earlier birds rushing to get the best spots, rather than relaxing during their stopover?' she adds.

'It might be that the males can take their time, but the females have to race down to the tropics. Most of the birds we tracked were males. What is clear is that some birds are fast, and some are slow.'

She muses: 'As is often the way with science, our findings raise more questions that we're keen to set to work on.'

The researchers are trying to understand the birds' migration route and which winter sites are critical for them. US wood thrush populations have been declining steadily, with a 30 to 40 per cent drop in numbers since the 1960s. One reason for this could be loss of their winter homes in the rainforests of Central America.

The research was published in the Proceedings of the Royal Society B.