Carsten Egevang:
Migration and breeding biology of Arctic terns in Greenland

Date: 15-03-2010    Supervisor: Carsten Rahbek

This PhD thesis presents the results of a study performed on the Arctic tern (Sterna paradisaea) in the period 2002-2008. Data in the study were obtained from fieldwork conducted at two study sites in Greenland: Kitsissunnguit (Grønne Ejland), Disko Bay in Arctic West Greenland and Sand Island (Sandøen) in high-Arctic Northeast Greenland.

The level of knowledge of the Arctic tern in Greenland before 2002 was to a large extent poor, with aspects of its biology being completely unknown in the Greenland population. This thesis presents novel findings for the Arctic tern, both on an international scale and on a national scale.

The study on Arctic tern migration (Manus I) – the longest annual migration ever recorded in any animal – is a study with an international appeal. The study documented how Greenland and Iceland breeding terns conduct the roundtrip migration to the Weddell Sea in Antarctica and back. Although the sheer distance (71,000 km on average) travelled by the birds is interesting, the study furthermore showed how the terns depend on high-productive at-sea areas during their massive migration. On the southbound migration, the birds would stop for almost a month (25 days on average) in the central part of the North Atlantic Ocean before continuing south. Close to Equator (~10º N) a divide in the migration path way occurred: seven birds migrated along the coast of Africa, while four birds crossed the Atlantic Ocean to follow the coast of South America. The northbound migration from the winter quarters to the breeding grounds was performed particular fast (520 km per day on average) following a route of favourable wind systems.

Included in this thesis is also the first quantified estimate of the capacity to produce a replacement clutch in Arctic terns (Manus II and III). Although it is often mentioned in the literature that the species willingly relays, this study is the first where the reproductive response of experimentally manipulated breeding pairs is monitored. We found that approximately half (53.3 %) of the affected birds would produce a replacement clutch when the eggs were removed late in the incubation period. Surprisingly, growth and survival rates in chicks from these clutches did not differ from chicks reared four weeks later in the breeding season, although a shift in foraging pattern and prey size was apparent (Manus III).

At a level of more national interest, the study produced the first estimates of the key prey species of the Arctic tern in Greenland. Although zooplankton and various fish species were present in the chick diet of terns breeding in Disko Bay, Capelin (Mallotus villosus) was the single most important prey species found in all age groups of chicks (Manus III).

The thesis also includes a study on the fluctuating breeding found in Arctic terns (Manus IV). Although the Arctic tern may show an overall fidelity to the breeding region, our study showed a considerable variation in colony size between years in the small and mid sized colonies of Disko Bay. These variations were likely to be linked to local phenomena, such as disturbance from predators, rather than to large-scale occurring phenomena.

From the long periods spent in Arctic tern colonies it was furthermore possible to document the breeding association between Arctic terns and other breeding waterbirds. In a study conducted on breeding phalaropes (Phalaropus fulucarius and P. lobatus) at Kitsissunnguit (Manus V) a close behavioural response to tern alarms could be identified. These findings imply that the altered distributions of waterbirds observed at Kitsissunnguit were governed by the distribution of breeding Arctic terns as suggested by Egevang et al. (2004).

Included in the thesis are furthermore results with an appeal to the Greenland management agencies. Along with estimates of the Arctic tern population size at the two most important Arctic tern colonies in West Greenland and East Greenland, the study produced recommendations on how a potential future sustainable egg harvest could be carried out (Manus II), and on how to monitor Arctic tern colonies in Greenland (Manus IV).