Camphuysen C.J. (2002) Post-fledging dispersal of Common Guillemots Uria aalge guarding chicks in the North Sea: The effect of predator presence and prey availability at sea. ARDEA 90 (1): 103-119

Chick growth and mortality characteristics (including predation risk) have been considered key selective factors to account for the evolution of the intermediate fledging strategy in Common Guillemots Uria aalge. The semi-precocial young leave the colony as small, partly feathered, flightless chicks at an age of c. 20 days at about one quarter of adult body mass and are accompanied by one of the adults (usually the male) when they swim out to sea. It is generally assumed that mortality rates in the colony are relatively low, while growth rates have been measured to be lower at the colony than at sea. Information on post-fledging dispersal of young and adults at sea in late summer (late June-September) is evaluated in this paper. Adult guillemots from North Sea colonies guide their offspring away from the colony. While doing so, they move rapidly through a coastal area with rich prey resources but with relatively high predation risk, and travel several hundreds of kilometres into the open sea where there are very few avian predators but where prey is low and patchy. Some were shown to end in distant areas with fewer predators and predictable prey resources. Meanwhile, the adults undergo a complete post-nuptial moult and become flightless for about 40-45 days. Laying, hatching and fledging in Common Guillemots are highly synchronised and with little variation in timing between years and very high success rates. In contrast, post-fledging chick survival is highly variable, which is understandable given the risky journey undertaken in often adverse environmental conditions. It is suggested that the selection pressure to defend or conquer breeding sites on proven successful ledges in autumn (starting only three months after 'normal' fledging) may strongly select for a timely departure from the colony. Chick care at sea allows the parents to become flightless while they undergo post-nuptial moult in time so that they can keep up with non-breeding adults and return to visit the breeding ledge in time