The nocturnal flights of migrating waterfowl and songbirds (passerines) were tracked by radar at the Utgrunden Lighthouse in southern Kalmar Sound on a total of 23 autumn and 26 spring nights from 2006 to 2008. Both the routes and the altitudes of the birds’ flights were studied. The radar echoes were classified as follows: birds that flew at no more than 20 km/h were considered songbirds, whilst those that flew at least 45 km/h were considered waterfowl waders (the report calls them waterfowl. For eight autumn nights and eight spring nights, there was heavy bird migration. A great amount of data was gathered on a total of 14,172 songbird echoes in the autumn and 1,014 in the spring, as well as on 1,105 flocks of marine birds in the autumn and 295 flocks in the spring. Southern Kalmar Sound is known as a location frequented by many marine birds, with heavy migrations both in the autumn and spring (daytime about 6 – 8,000 bird echoes/h/km). The peak reading for this study was 1,840 echoes/h/km for autumn nights and 355 echoes/h/km for spring nights. These figures can be compared with readings taken at Falsterbo, where the peak readings in the autumn were about 6,600 bird echoes/h/km, and at Kriegers Flak on the southern Baltic, about 3,000 echoes/h/km. Migration over southern Kalmar Sound is thus relatively heavy in the autumn, but in the spring, nocturnal songbird migration is fairly light, and involves relatively few birds in the area studied.

The nocturnal bird migration above the sea occurs at higher altitudes for both marine birds and songbirds. On autumn nights, marine birds fly at an average altitude of 156 metres above the sea, as compared to 17 metres during the day. In spring, the corresponding figures are 106 metres at night and 24 metres during the day, respectively. The average altitude for songbirds in the autumn is 330 metres by night and 35 metres by day. On spring nights, the corresponding figures for songbirds are 529 metres at night and 50 metres by day.

Waterfowl fly so high at night that they risk colliding with wind turbines that are 150 metres tall (most commonly off-shore). About 50 – 90 % of the migrating waterfowl are affected. They need to either veer off or fly above the wind turbines in order to avoid a collision.

This study shows that waterfowl veer off from the wind turbines. This veering off occurs closer to the turbines at night than during the day. The study does not demonstrate that the risk of collisions is either greater or less than that shown in previous studies.

Regarding nocturnal flying in conditions of poor visibility, the marine birds either veer off somewhat closer to the wind turbines at night, but not closer than an average of 500 metres (compared to an average of 570 metres on nights without fog) or flew above the turbines, with their average flight altitude being higher on nights with poor visibility.

These distances at which birds veered off at night differed from the distances found during the day (i.e. 1– 3 km before the wind turbines). Only 0.1-0.5 % of the marine birds flew between the wind turbines during the day (the distance between each of the area’s seven turbines is about 400 metres). On nights without fog, 5 % of the flocks flew between the turbines, and this figure rose to 9 % on foggy nights, which may indicate a higher risk of collisions at night than by day.

The large number of songbirds that migrate across this stretch of sea at night flew at an average altitude that was high above the turbines (330 metres in autumn and 529 metres in spring). They seem to fly a little higher on foggy nights (343 metres as compared to 330 metres when there is no fog). This flight altitude in fog only applies to autumn nights, and the difference is not statistically significant. However, on certain nights, there are statistically significant differences. On nights without fog, songbirds fly about 100 metres higher than on foggy nights.

The great majority of songbirds fly above the wind turbines at night, but there is a great range as to where these songbirds fly. In spring, 8 % of the migrating birds are affected by wind turbines, which are 150 metres tall, and in autumn, this figure is 17 %. However, this study cannot give any answer as to how low-flying birds pass the turbines, as the area studied for songbirds was more than 1,500 metres away from the lighthouse where the radar was located.

However, it was shown that songbirds flew higher above the sea on two of three foggy nights, and thus clearly flew above the approximately 100 metre high fog. The observations of night-flying marine birds also show a higher flight altitude on nights with fog (averaging 240 metres) as compared to nights without fog (156 metres).

The study shows that there are some (albeit a few) songbirds that rest after a night of migration. This most often happens when a night of migration is followed by a foggy morning. Even under those conditions, there are few birds out around Utgrunden. The great danger involving songbirds and off-shore wind turbines arises when mass landings occur. This happens when birds are flying over the water and encounter a stormy area of rain and mist, which makes them fly lower and search out places to land. No such phenomenon has been observed on Kalmar Sound in this study.

Based on new data, a rough calculation of the risk of collision encountered by songbirds at the seven existing wind turbines located at Utgrunden indicates that 16 songbirds will be killed out of the approximately half million songbirds that pass that point at night. The collision risk for waterfowl is not considered to have changed as a result of these data, and remains at a total of about 10-15 waterfowl being killed annually by the seven wind turbines at Utgrunden and the five at Yttre Stengrund (Pettersson 2006).