Effects of mate removal on the fecundity of common eider Somateria mollissima females

Behavioural observations, measurements of male and female reproductive success, and DNA microsatellite loci were used to investigate parentage of common eider Somateria mollissima clutches and productivity of widowed females in Finland. In an experimental study simulating spring harvest of male eiders, a total of 20 males were shot while attending 16 females (four males after rematings, 16 initial males) during the pre‐laying and laying period in 1994. Of the 16 widowed females, 11 nested and five did not. Mean clutch size of breeding widows (4.55) did not differ from that of control females in the same year (4.47; N = 32). However, the hatching success of widows was significantiy lower than that of control females (53 vs 81%) because of a greater proportion of addled eggs and dead embryos (38% vs 11%). Male removal also appeared to change male and female behaviours resulting in higher incidence of intra‐specific brood parasitism and mate replacement. Occurrence of foreign eggs averaged 9.5% during the treatment year but was not observed during a year without disturbance during mating and egg laying (1997). Behaviours of widowed females related to remating attempts varied from active seeking of new mates to total rejection of courting males. Rematings did not lead to full clutch fertilisation among widowed females. Neither did we observe evidence of immigration of new males into the hunting area to court the widows. Male removal clearly lowered the fecundity of eider females, reducing nesting success by 35% of long‐term averages. The potential for remating appears to be reduced by the female‐biased sex ratio caused by simulated male‐only harvest.