Sex bias in Icelandic Black-tailed Godwits Limosa limosa islandica caught in the Swale tidal channel, UK, during autumn and winter

The Icelandic Black-tailed Godwit Limosa limosa islandica is a sexually dimorphic subspecies. Birds of this subspecies that migrate through, moult and winter in the Swale tidal channel, a Ramsar site located in south-east England, between 1991 and 2020, have been studied. The biometric data of a trapped sample showed an uneven bimodal distribution of bill lengths, indicating an uneven ratio of the sexes. Using a published discriminant function analysis, and a population-level statistical analysis to separate the normal mixture distribution of bill lengths, we confirmed an excess of females (73%) in the sample of captured birds. To assess if this uneven sex ratio is in the Swale population as a whole, we compared adult Black-tailed Godwits captured on the Swale with those colour-ringed in Iceland, Portugal, Ireland and sites elsewhere in the UK and resighted on the Swale. We found a more even sex ratio of 55% female, 45% male. It has not been possible to establish why there is a statistically significant difference in the sex ratio of the captured sample compared to the whole population. Hypotheses related to both capture methods and Black-tailed Godwit behaviour, and methods to test them, are discussed.