The cost of reproduction in Kittiwakes Rissa tridactyla were studied from 1990 to 1993 on Hornoy in northern Norway by enlarging and decreasing brood sizes of two chicks to three and one during the first week after hatching. No parents were able to raise the enlarged broods of three to fledging. Most chick mortality occurred in the 19—22 d age group. Chick body mass and fledging success were lower among enlarged broods than among reduced and control broods. Body mass of females, but not males, was lower at the end of the chick—rearing period in enlarged broods. Only those females rearing enlarged broods experienced a higher mortality during the next breeding season. Our results support Lack's hypothesis of clutch size regulation: Kittiwakes laid the maximal number of eggs they were able to raise successfully to fledging. Furthermore, as female Kittiwake mortality increased when clutches were enlarged, females have a trade—off between investment in current reproduction and investment in future reproduction. Because males and females responded differently to brood enlargement, fitness estimates showed that Kittiwakes may have a sex—specific optimal clutch size.

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Migratory species display a range of migration patterns between irruptive (facultative) to regular (obligate), as a response to different predictability of resources. In the Arctic, snow directly influences resource availability. The causes and consequences of different migration patterns of migratory species as a response to the snow conditions remains however unexplored. Birds migrating to the Arctic are expected to follow the spring snowmelt to optimise their arrival time and select for snow-free areas to maximise prey encounter en-route. Based on large-scale movement data, we compared the migration patterns of three top predator species of the tundra in relation to the spatio-temporal dynamics of snow cover. The snowy owl, an irruptive migrant, the rough-legged buzzard, with an intermediary migration pattern, and the peregrine falcon as a regular migrant, all followed, as expected, the spring snowmelt during their migrations. However, the owl stayed ahead, the buzzard stayed on, and the falcon stayed behind the spatio-temporal peak in snowmelt. Although none of the species avoided snow-covered areas, they presumably used snow presence as a cue to time their arrival at their breeding grounds. We show the importance of environmental cues for species with different migration patterns.

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