NORWICH, England, Nov. 12 (UPI) -- Researchers at the University of East Anglia in Britain say they've found out why some birds are migrating earlier and earlier each year.
While individual birds still migrate like clockwork -- arriving at the same time each year -- climate warming is resulting in earlier nesting and hatching each year, moving the overall migratory trend earlier, they said.
The researchers, observing a population of Icelandic black-tailed godwits over 20 years, found the flock advanced their spring arrival date by two weeks over that period, the university reported Tuesday.
"Because we have been following the same birds for so many years, we know the exact ages of many of them," biologist Jenny Gill said.
"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," she said. "So the arrival dates are advancing because the new youngsters are migrating earlier."
Birds that hatch earlier 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, she explained.
"This research is very important 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," she said.