News

Friday, 5 March 2010

--------- The Arctic Tern - Migrant Extraordinaire

This article arose from a suggestion by the Editor that it would be helpful if I could justify an assertion made in my Report of our January 2010 Indoor Meeting that some arctic terns, in migrating from the Arctic to Antarctica and back, cover approximately 50,000 miles every year. The flight distance between the north and south poles is just over 12,400 miles, he said, so even if they nested at the north pole and visited the south pole, the total distance covered on a return journey would only (only!) be about 25,000 miles. Not so. Recent research has demonstrated that some individual arctic terns do actually migrate 50,000 miles (80,000 kilometres) annually. How we know I will explain later but first, some background information.

The arctic tern (Sterna paradisaea) that graces our shores each Spring is one of our most remarkable summer visitors. Weighing a mere four ounces or so, with a wingspan of approximately two feet, this elegant little seabird has long been hailed as our most famous migrant, making an annual return journey from its breeding grounds at higher latitudes in the Arctic than any other tern to the Southern Ocean. According to The Migration Atlas published for The British Trust for Ornithology, most movements occur in small groups of a dozen or so individuals flying at high altitude across open oceans. Coupled with the fact that the breeding grounds of the arctic tern are sparsely populated and the wintering grounds largely uninhabited, it is clear that neither ringing recoveries nor at-sea observations will be sufficient to determine the full extent of its migratory journeys. A recent report by Dr Egevang from the Greenland Institute of Natural Resources, Nuuk and his co-workers, however, has thrown some light on the matter. The work was published in the January 2010 edition of the Proceedings of the National Academy of Sciences, and what the researchers did was to fit 70 arctic terns from Greenland and Iceland with miniature geolocators, each weighing about 1.4 grams, and track their subsequent movements. Geolocators, I should explain, record position by using a light sensor to record daylength (thus giving latitude) and the times of sunrise and sunset relative to a standard clock (thus giving longitude). These devices have been used for many years to record the movements of seabirds, but the development of miniaturised locators and improvements in attachment techniques to ensure that they do not interfere with behaviour or survival, for instance, have led to their being used in smaller species. Unfortunately, tagged birds have to be re-captured to retrieve information from the geolocator, but the development of remote downloading is under investigation.

The results of Dr Egevang's research are breathtaking. Loggers were retrieved from 11 terns re-captured at their breeding sites the following season. Ten of the loggers were successfully downloaded, providing a full year of migration data (July 2007 - July 2008) and six months of migration data were retrieved from the remaining logger. From this evidence it was found that, at the end of their breeding season in the northern hemisphere, tagged birds travelled southwest to a stopover region of deep water in the Newfoundland Basin. Here they remained for about 3-4 weeks before continuing their migration south-east towards the west coast of Africa. South of the Cape Verde Islands routes diverged, seven birds continuing parallel to the African shoreline and four crossing the Atlantic to follow the east coast of Brazil. All birds spent the austral summer (December to March) in the Southern Ocean and began their migration north in mid-April, the return flight of about 24000 km being completed in approximately 40 days. This northbound migration took less than half the time of the southern journey, and the average annual distance travelled (excluding movements within the breeding season) from departing the breeding sites in August to returning in late May the following year was 70,900 km (range 59,500 to 81,600km). The authors state that this is the longest round-trip animal migration ever recorded electronically. Even allowing for errors inherent in the technique, the tracked terns were considered to have travelled nearly twice as far as was previously supposed. Since arctic terns are known to live for longer than 30 years, the total lifetime journey could exceed 2.4 million kilometres. That is equivalent to approximately three return journeys to the Moon. A remarkable migration indeed.

Brian Shreeve