Sexual Size Dimorphism and Disassortative Mating in the Dunlin Calidris alpina schinzii in Southern Sweden

Abstract

In a South Swedish population of Dunlins, males were on average smaller than females. Differences were most pronounced in bill length and body mass. Males arrived at the breeding sites in spring on average 12 d earlier than females, with a tendency for large and long-billed females to arrive earlier than small and short-billed females. A significant positive correlation between date of arrival and start of egg-laying was found in females, but not in males. Previously mated pairs started egg-laying on average 4 d earlier than newly formed pairs. Disassortative mating in relation to size was found in new pairs, so that small and short-billed males, and large and long-billed females nested earlier than averagesized birds. A significant positive correlation was also found between female size and mean egg volume. To explain the observed patterns of mating and sexual size dimorphism, I suggest a combination of sexual and natural (ecological) selection. Small size in males may be energetically advantageous during aerial display and male parental care. Short bills may be selected for when males are feeding in terrestrial habitats during broodrearing. In females, selection for large size and long bill is probably associated with the production of larger eggs and more efficient feeding in marine littoral habitats, respectively. Selection may also favour large females in intra-sexual competition for mates and food resources.