A team of scientists at the University of East Anglia in the UK say they have discovered why birds are starting to migrate earlier each year: global warming may be to blame.
Experts have believed for awhile now that global warming is responsible for changes in the migration patterns, but they were at a loss to explain why. Dr. Jenny Gill of UEA’s school of Biological Sciences, who is the lead researcher on the study, says that solving the puzzle of why the birds are migrating earlier is important because those birds who are not migrating earlier are declining in number.
The difference seems to be that global warming is causing birds to nest and hatch at an earlier time, which appears to be linked to later migration.
To learn about the problem, the team of researchers examined a population of Icelandic black-tailed godwits over a period of 20 years. During this time, the flock of birds moved their spring arrival date up by two weeks.
While it would seem that perhaps the birds were migrating earlier each year, they team found that this was actually not what was happening. In fact, individual birds were quite punctual, arriving at the same time year after year.
Yet, something was creating an overall trend of the flock arriving later each year.
Gill says the key to the conundrum lay in looking at the ages of the individual birds. Because they had been following them for so long, the team knew the exact ages for many of the birds.
When they examined the problem from this perspective, they saw that the birds who were hatched in the late 1990s tended to arrive in May. However, those who were hatched more recently were arriving in April. So, as new members were being added to the flock, the overall trend was for the flock to begin to migrate earlier.
Gill and her team believe that global warming is the likely factor that is driving this trend. When the climate is warmer, the birds nest earlier and hatch their eggs earlier. And, in turn, they are ready to migrate earlier.
She says this may also explain why changes in migration pattern are not occurring among those bird species who migrate over long distances. These birds arrive so late to the breeding grounds that they do not have the opportunity to nest earlier in response to warmer temperatures.
Gill says that this work is extremely important because the bird populations who must fly long distances in order to migrate are rapidly declining in number. Understanding just how global warming affects these birds is essential to understanding why this is occurring.
The study regarding the effect of global warming on bird migrations was published on November 13, 2013 in the journal Proceedings of the Royal Society B.
By Nancy Schimelpfening