A 20-year study carried out with the help of hundreds of birdwatchers across East Anglia has revealed why birds are migrating earlier and earlier each year.

The University of East Anglia research involves placing unique combinations of colour rings on Icelandic black-tailed godwits, so the migratory routes of each individual bird can be tracked over many years.

But the work has only been made possible thanks to a network of more than 2,000 birdwatchers who report sightings of the colour-ringed wading birds along the whole flyway, from Iceland to the British Isles, Spain and Portugal.

Experts have long suspected climate change was somehow driving an advancing migration pattern, but the new research published today reveals that individual birds migrate like clockwork – arriving at the same time each year.

However, climate warming is resulting in earlier nesting and hatching earlier each year, and this appears to be linked to the advancing migration.

Lead researcher Jennifer Gill, from UEA’s school of biological sciences, said: “We have known that birds are migrating earlier and earlier each year – particularly those that migrate over shorter distances. But the reason why has puzzled bird experts for years. It’s a particularly important question because the species which are not migrating earlier are declining in numbers.”

The research team looked at the Icelandic black-tailed godwits over 20 years and during this period the flock advanced their spring arrival date by two weeks.

Dr Gill said: “The obvious answer would be that individual birds are simply migrating earlier each year. But our tracking of individual birds shows that this is not the case. In fact, individual birds do almost exactly the same thing every year – arriving punctually at the same time year-on-year.”

The team went on to investigate what could be causing the overall arrival time to creep forward.

“Because we have been following the same birds for so many years, we know the exact ages of many of them,” said Dr Gill. “We found that birds hatched in the late 1990s arrived in May, but those hatched in more recent years are tending to arrive in April. So the arrival dates are advancing because the new youngsters are migrating earlier.

“Climate change is likely to be driving this change because godwits nest earlier in warmer years, and birds that hatch earlier will have more time to gain the body condition needed for migration and to find good places to spend the winter, which can help them to return early to Iceland when they come back to breed.”

This can also explain why advances in migration timing are not common among species migrating over long distances. Dr Gill said: “Many long-distance migrants arrive so late on the breeding grounds that they have little opportunity to respond to warming conditions by nesting earlier. This research is very important because many long-distance migrant bird populations are currently declining very rapidly.”

Dr Gill plans to continue the research to identify why the birds that hatch early tend to be the ones that come back early and to determine how this benefits these birds.

The research was funded by Natural Environmental Research Council and significant contributions have been made by Tomas Gunnarsson from the University of Iceland; Peter Potts from Farlington Ringing Group; William Sutherland from the University of Cambridge; and Graham Appleton from the British Trust for Ornithology.