WITH rising temperatures, courtesy of greenhouse gas emissions, you might expect birds to linger on at migration time, taking an extra ‘sus beag’, before the arduous journey to far-off lands.
However, according to a paper just published in the Journal of the Royal Society, they don’t. In fact, the opposite is the case; global warming, the authors argue, is leading to earlier migration, at least in one prominent wader species.
So what is going on? The elegant black-tailed godwit visits Irish and British estuaries in winter. The ones which come here nest in Iceland. After breeding, godwits roost in tight flocks at high tide along the shores of Icelandic wetlands where ringers catch them using nets attached to projectiles fired from cannons.
Over the last two decades, engraved plastic rings have been fitted to godwits’ legs. Bird-watchers can read the ring-codes through telescopes, identifying individuals in the field. It’s possible to tell the age of a young godwit when it’s caught. Older ones can’t be ‘aged’. After years of ringing effort, the natal years of most marked birds are known. Scientists at the University of East Anglia use sighting data to study godwit migration. Cobh-based author and film-maker, Jim Wilson, is the Irish member of the team.
Oddly, the birds are moving earlier in recent years than they did in the past and the pattern seems to correlate with rising temperatures. Global warming, it seems, is leading to earlier migration. Even more strangely, however, this does not apply to individual birds. Godwits, creatures of habit, are ‘set in their ways’ and extremely punctual when it comes to migration. An individual leaves Iceland on virtually the same date year after year and arrives back in spring at a set time, to within a few days. Younger birds, however, move earlier than their parents. They, too, have fixed departure and return dates year on year, but they are on the wing before their elders.
Those fledged prior to the year 2000 tend to arrive in Ireland and Britain in April. Ones hatched in the new century don’t appear until May. As the senior birds die off and are replaced by their descendents, godwit migration is taking place sooner. But why should a rise in temperature lead to a migration time-shift? Dr Jenny Gill, who leads the study team, argues that warmer weather induces birds to nest earlier.
By leaving for Iceland promptly in the spring, a bird is likely to secure a better breeding territory and this, in turn, boosts its breeding prospects. ‘Birds that hatch earlier are likely to have more time to gain the body condition needed for migration..’ she says. So, once they have completed the breeding cycle, the imperative is to moult, fatten up and migrate south as soon as possible to avail of the milder winter climate and the bounty of our food-rich estuaries. Like modern commuters trying to get ahead of the traffic, birds are leaving home progressively earlier.
This is not in itself an ominous finding. Black-tailed godwits migrate over relatively short distances; a flight between Iceland and Ireland may seem long to us, but to a godwit, it’s no big deal. There are implications, however, for species which migrate over longer distances. Studies show that they are not moving earlier in response to climate change than they did in the past. According to Gill ‘many long distance migrants arrive so late on the breeding grounds, that they only have a short time in which to mate and nest’.
‘Population declines are most widely reported in species that are not advancing migration’, the authors say, ‘short distance migrants, which generally return to breeding sites earlier and have greater scope for advance laying, are advancing more rapidly than long-distance migrants’.
* Why is Timing of Bird Migration advancing when Individuals are not, Jenny Gill et al. Proceedings of the Royal Society B. November 2013.
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