Within-season and between-year variation in the structure of Common Eider broods

Abstract

I studied within-season and between-year variation in Common Eider (Somateria mollissima) brood structure, by censusing broods for four years in the northern Baltic. I also examined within-season patterns of female aggressiveness, and explored whether between-year patterns could be related to female body condition at hatching. Multi-female tending was initially the dominant brood rearing strategy. Moreover, 25-38.1% of the lone-tended broods had more than six ducklings, indicating that they may contain adopted young. The number of ducklings per female increased with brood size, exposing some newly hatched ducklings to chilling due to unfavorably high ratios of ducklings to brooding females. Lone tenders became proportionally more common, the proportion of two-female tended broods was stable, and the proportion of broods with more than two females rapidly decreased during the season. Female aggression decreased in frequency over time, so the decline of broods with more than two females may relate to female aggressiveness. Family units began to break up when ducklings were 7 weeks old. Multi-female broods were initially more common in years when females were in good condition at hatching and less common in years when females were in poor condition. However, multi-female broods decreased in frequency over time in all years except the poorest year; multi-female tending strategy was most common in poor years and least common in good years. These between-year patterns may result from differences in body condition between lone tenders and multi-female tenders, indicating that female body condition may affect brood-caring decisions.