January 16, 2009

Birds Make Routine “˜Stopovers’ During Long Migrations

British scientists have discovered that the Manx shearwaters make routine "stopovers" during their 12,400-mile migration, most likely to feed and rest.

The researchers recovered data using electronic tags that were fitted to six breeding pairs of Puffinus puffinus, small birds weighing about 14 ounces.  The birds were from Skomer Island off the coast of Wales.

"Every one of the 12 birds made at least one stop during its migration in one place for up to two weeks," Tim Guilford of the Animal Behavior
Research Group at the University of Oxford, the study's co-author, told BBC News.

"We have interpreted this as being stopover behavior because this is common in terrestrial migrant birds; essentially, they stop to refuel," he said.

However, sea birds that migrated over open seas did not typically behave this way because, unlike terrestrial birds, they were unable to return to the same feeding location each year, Guilford added.

Nevertheless, the tagged Manx shearwaters seem to have engaged in the same behavior since it provided an "optimal migration strategy".
Professor Guilford hypothesized that the birds were more likely to survive if they consistently stopped along their migration, instead of flying directly to South America without interruption.

"If they flew directly, they would have to have a larger fat reserve in order to make the journey," he said.

"They could do that, but on the other hand that would mean the bird would be flying the first part of the migration weighing more than it needed to."
"It is a complex trade-off between the aerodynamics of long distance flight and the risks and time constraints of having to stop and refuel."
Professor Guilford said he was not overly surprised about the birds' migratory route.

"The route that they took was very broadly consistent with what people using more traditional methods, such as ringing, thought they had taken," he explained.

"It is gratifying that these techniques, which have been the mainstay of our understanding of avian migration for so long, turn out to be broadly correct."

However, the data obtained from the tags did present a few revelations.

"They go a little bit further south than we expected, but that was probably the result that human settlements were much more sparse, where there were fewer people to recover the rings."

The researchers used two types of logging tags designed and made by the British Antarctic Survey. "¨

"They are very similar except that one is a slightly more recent version, which records contact with salt water," Professor Guilford said."    

"This allowed us to know whether the bird was sat on the water or diving, or whether it was flying."

By correlating the two data sets of location and whether the birds were in flight or stationary, the researchers were able to ascertain the birds' migration pattern and behavior en route.

The researchers chose to utilize a "logging device", which stored data until it was physically retrieved from the bird, rather than a satellite tag, which returns data back almost immediately.

"This is the slightly nerve-wracking side of things," Professor Guilford said.

"The primary benefit is that they are much smaller than devices that transmit."

All of the tagged birds returned to the breeding site in Britain, indicating that the tags did not restrain them during their seven-month migration.

"We have gone to great lengths to try and minimize the impact of our devices," said Professor Guilford.

Although as a species the Manx shearwater is not endangered, Guilford said the findings help enhance our understanding of what is occurring in the Earth's seas and oceans.

"Although they are doing very well, they are still limited to big but compact populations on islands, which are very vulnerable to predators etc," he said.

"So it is nice to say that we can now begin to understand what these birds depend upon in terms of resources on land, and now at sea."

"We won't be stopping with Manx shearwaters, we are beginning to put them on puffins as well."


On The Net:

Department of Zoology, University of Oxford

British Antarctic Survey

Proceedings of the Royal Society B