Second North American Sea Duck Conference - Patuxent Wildlife ...

SECOND NORTH AMERICAN SEA DUCK CONFERENCE
AGGRESSIVE COMMON EIDER FEMALES SEIZE CENTRAL POSITIONS
AND SHOW ENHANCED PARENTAL EFFORT
Markus Öst, Kim Jaatinen, and Benjamin Steele
University of Helsinki, Finland; markus.ost@helsinki.fi
Decreased risk of predation is a key benefit of group living, and selfish herd theory predicts
competition for central positions due to the relative safety of these spots compared to peripheral
ones. Spatial position is also likely to affect activity patterns, as individuals adjust their behavior
to mitigate the risk of predation. Female common eiders (Somateria mollissima) often pool their
broods and share brood-rearing, and antipredatory vigilance constitutes a core parental care activity.
Females are assumed to trade-off vigilance and feeding by diving; efficient condition recovery is
crucial for energetically stressed post-incubating females, which do not feed during most of egg
laying and incubation. Based on previous data, a female’s own ducklings are closer to her than
unrelated young in mixed broods, so a female’s spatial position is likely to correlate with the survival
prospects of her brood. We identified correlates of female spatial position within multi-female broodrearing
coalitions in a common eider population from the Baltic Sea, SW Finland, and explored how
spatial position affects female activity patterns. Neither clutch size, body condition at hatching nor
structural size (length of the radius-ulna) were significant predictors of spatial position; however,
females showing a higher frequency of aggression occupied more central spots. The investment
in vigilance increased, whereas the investment in feeding was uncorrelated with spatial centrality;
instead, central females devoted less time to other activities (resting, preening, and movement). Our
results demonstrate means by which socially dominant eider females may increase their reproductive
share in joint broods without jeopardizing their own survival by feeding less. These findings also
reveal that good body condition does not guarantee a favorable position in brood-rearing coalitions,
which may help explain our previous finding that female common eiders in best body condition at
hatching tend their brood alone.
68 ANNAPOLIS, MARYLAND, USA NOV. 7-11, 2005