Abstract
Colonial seabirds have adopted certain strategies to minimize breeding losses during fledging. This review focuses on the most numerous high-Arctic auks: Little Auk Alle alle and Brünnich’s Guillemot Uria lomvia . Colonies of Little Auks are usually situated farther from the sea than cliff-nesting guillemots. On departing the nest, Little Auk chicks are more advanced and can fly, while guillemot chicks, lacking flight feathers, jump from the cliff ledges and glide to the sea. Little Auk chicks lose weight before departure, thus improving their flight efficiency. In both species, fledglings have strong legs and a thick layer of feathers on the underside to prevent injury from hard landings. Some chicks reach the sea on foot. The fathers care for their fledglings, with whom they are in vocal contact. On land, chicks are hunted by Arctic foxes Vulpes lagopus and gulls Larus sp. When attacked by gulls at sea, young auks dive. In the air, however, only young Little Auks perform a characteristic nose-diving manoeuvre to confuse the predator. Guillemot parents can effectively counteract gull attacks on their young, while Little Auk parents do not directly defend their chicks. Unlike Little Auks, several adult guillemots usually accompany chicks during gliding and landing at sea. Adult guillemots gather near the colony during the fledging period and call loudly, thus indicating a safe landing place. In both species, fledging is highly synchronized and condensed into a few days and night-time hours, thus causing a swamping effect that reduces total losses among fledglings.