Researchers at the University of East Anglia in the UK found that climate change is responsible for early bird migration.

In their study, they found that global warming lead to earlier nesting and hatching each year, and this appears to be linked to birds migrating earlier and earlier each year.

"We have known that birds are migrating earlier and earlier each year - particularly those that migrate over shorter distances," report author Jenny Gill said in a statement. "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."

Experts have long suspected climate changes to be somehow driving this advancing migration pattern.

The research team looked at a population of Icelandic black-tailed godwits for more than 20 years. During this time period, the flock advanced their spring arrival date by two weeks.

"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," Gill said.

The research team then went on to investigate what could be causing the early migration. They found that bird hatches in the late 1990s arrived in May, but those hatched in more recent years are tending to arrive in April.

They concluded that the arrival dates are advancing because the younger godwits are migrating earlier.

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

Gill said the findings would explain why advances in migration timing are not seen in birds that migrate over long distances.

"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 said.

She said 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 Gill said.