Climate change is likely to blame for birds migrating earlier and earlier every year, researchers from the University of East Anglia found in a new study.

"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," lead researcher Jenny Gill from the university's School of Biological Sciences said in a statement.

The researchers looked at the Icelandic black-tailed godwits, which have advanced their migration by two weeks over the past two decades. The changes, the scientists discovered, had little to do with individual birds, which migrated like clockwork, arriving at the same place at the same time year after year.

"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," Gill said.

Knowing the exact ages of the birds, the researchers were able to track the shifts in arrival times back to when they were born, finding that those that hatched in the late 1990s arrived in May while younger birds appeared in April.

"Climate change is likely to be driving this change because godwits nest earlier in warmer years," Gill said, "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."

The discovery also helps to explain why shifts in migration for those traveling long distances are not the same species-wide.

"Many long-distance migrants arrive so late on the breeding grounds that they have little opportunity to respond to warming conditions by nesting earlier," she explained.

Studies like this one are vital, Gill concluded, "because many long-distance migrant bird populations are currently declining very rapidly, and identifying how climate change is affecting these populations is a key part of understanding the causes of these declines."