long-term study (10 years) aimed to check if the sex-ratio of dunlins Calidris alpinaat a stopover site in
the southern Baltic region was biased. Two age classes among non-juvenile
dunlins were recognised: immatures (2ndcalendar year) and adults
(> 2ndcalendar year). There was a significant male bias in the
sample of 4,406 non-juvenile dunlins captured during their southward migration.
Overall, 60.3% of immatures and 59.4% of adults were males. Particularly among
adults, the proportion of males increased significantly after the start of
autumn migration. The annual sex-ratios were consistently male biased, but
varied somewhat and fluctuated in parallel for adults and immatures. One
plausible explanation for the male bias is that males and females differ in
migration strategy. Females may make longer flights and avoid stopover sites
with unpredictable feeding conditions, such as the southern Baltic coasts,
which provide low quality habitat. Assuming a balanced non-juvenile population
sex ratio, the ‘missing’ females could stopover elsewhere in the Baltic or fly
directly to the tidal areas of the Wadden Sea. The sex ratio in the study area
may depend on wind conditions during the early phase of autumn migration. In
some years, adverse weather may force more females than usual to stopover in
the study area.
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