Post‐fledging migration of common murres Uria aalge in the Baltic Sea: management implications

In this study of common murre Uria aalge chicks, at Slora Karlsö in the Baltic Sea (Sweden), we used two data sets to investigate the post‐fledging migration to learn more on potential threats from human activities. Firstly, we radio tracked 12 chicks on average 5.1 h during their first night at sea, after jumping from the breeding cliffs in early July. Their average speed was 1,9 km h‐’and all chicks swam in a similar direction (SW) with a mean bearing of 235°. By searching an area of ca 400 km2 further off the island between two and six days after the radio tracking, we concluded that they did not stop, even temporarily, within 40 km off the island, but had continued the swimming migration. Our second data set was made up by 400 recoveries of ringed chicks. Only those recovered within eight months after fledging (until February) were included. Two birds recovered at the Polish coast in the beginning of August had moved on average 10.7 and 12.3 km day‐’(i.e. 0.45‐0.51 km h‐). In August, all chicks, except one, were found south of Gotland > 70 km away. The average distance to the recovery sites in. August was 253.5 km (n = 11). The pattern of distribution of birds reported in September. October and during the winter period from November to February was about the same as in August. The average direction from the colony lo the recovery sites was S to SW. Important findings from nature management perspective are: 1) our data suggest that the route of the chicks at least partly coincide with ship routes with at least 42000 passages each year. Hence, there is a potential for a great part of the common murre population to get in contact with oil spillage at this stage, notably in the first half of July. 2) From the 400 reported recoveries of ringed chicks we concluded that the most important over wintering areas are around Bornholm and the Bay of Gdansk. Hence, these are the most likely areas for conflicts with the fishery and oil spillage during the winter.