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

SECOND NORTH AMERICAN SEA DUCK CONFERENCE
THE COSTS OF MATE GUARDING
TO MALE COMMON EIDERS
Ben Steele, Aleksi Lehikoinen, Markus Öst, and Mikael Kilpi.
Colby-Sawyer College; bsteele@colby-sawyer.edu
Male Common Eiders (Somateria mollissima) are known to guard their mates against interference or
copulations by other males during the laying period. This activity is accentuated in the archipelago
of southwest Finland where the male biased sex ratio (about 56% males) means many single males
approach paired ducks. We hypothesized that guarding involves a cost to mated males in less
time available for foraging during this energetically stressful time of year. Unmated males may
be adopting an alternative mating strategy, in which their survival is increased by feeding more
and their reproductive success may benefit from occasional extra pair copulations. Scan sampling
of 96 males in 2003 and 71 in 2004 showed that unmated males foraged more in 2003, but not in
2004. The number of dives per hour showed a similar pattern: in 2003, unmated males dove more
than mated males. Unmated males participated in more aggressive encounters per hour than mated
males, during preincubation, but spent a lower proportion of their time in aggression, suggesting that
their aggressive encounters were shorter than that of mated males. In both years, mated males were
clearly dominant over unmated ones, winning most aggressive encounters. These results suggest
that there is no consistent feeding advantage of not being mated, and that unmated males may just
be subordinate, perhaps younger males, excluded from breeding. Scan sampling omits “harassment
groups” consisting of 5 to 60 unmated males that chase a mated pair for up to two hours. During this
time, no foraging takes place and it appears as if large amounts of energy are expended by the mated
male defending the female. The number of males participating in these harassment groups increases
during the breeding season as some females leave to incubate, thereby increasing the male bias of the
sex ratio. These groups may constitute an occasional, but intense cost to mated males.
NOV. 7-11, 2005 ANNAPOLIS, MARYLAND, USA
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