Coastal grassland wader abundance in relation to breeding habitat characteristics in Matsalu Bay, Estonia

Wader populations have been declining worldwide, providing a fundamental question as to which environmental factors limit population growth. Many studies have focused on the effects of habitat change on wader populations as a result of climate change, agricultural intensification or abandonment of arable land. However, there are few studies investigating the relationship between wader distribution/abundance and prey abundance. This study focused on the relationship between breeding wader abundance, habitat characteristics and prey abundance on different types of coastal and floodplain grasslands. The study was carried out in the Matsalu Bay area, Western Estonia between 2001 and 2005. Results showed that most wader species were strongly related to habitat flooding type but not to plant species richness or evenness or mean vegetation coverage. Abundance of epigeic earthworms at a site was positively correlated with wader species diversity and abundance, as well as at the individual species level for abundance of Northern Lapwing Vanellus vanellus, Black-tailed Godwit Limosa limosa and Redshank Tringa totanus. Endogeic earthworm abundance was not significantly related to total wader abundance, although for some species there was a weak negative relationship. We conclude that epigeic earthworms are an important food source for most wader species on Matsalu grasslands and endogeic earthworms are not likely to be utilised as a prey source. Wader abundance was most strongly related to flooding as a determinate factor for breeding habitat characteristics and with epigeic earthworms as a food source. These factors are important environmental variables influencing local abundance and distribution of coastal waders.