Phenology of migratory bat activity across the Baltic Sea and the south-eastern North Sea

We compiled the available information on the occurrence and timing of migratory bat activity across the Baltic Sea and south-eastern North Sea coasts and islands, based on ultrasonic monitoring projects at 19 localities in 2007-2009. The data refer to three species; Nathusius' pipistrelle Pipistrellus nathusii, soprano pipistrelle P. pygmaeus and common noctule Nyctalus noctula. Pipistrellus nathusii occurred at all sites (north to 61 degrees N in Finland), while the other species were scarcer, particularly at the northernmost sites. The status of the recorded individuals is unknown. However, the activity most likely was of migrating individuals or individuals on migration stopover, because very few observations were made during the maternity period. Spring activity occurred predominantly in May, with the median observation date of P. nathusii 20 days earlier in the south (Germany) than in the north (Finland). Autumn migration was observed throughout August and September and activity that may or may not indicate migration was also observed in October and November. The median date of such activity in autumn usually occurred in September and without any significant difference in timing in relation to latitude. Migratory bats in the Baltic area apparently move on a broad front in most cases. The estimated speed of migration for P. nathusii in spring was 55 km/day. The entire coastline and islands around the Baltic Sea are of potential importance for migrating bats in spring (April-May) and autumn (August-September) and should achieve relevant protection according to EU legislation and its implementations.