VULNERABILITY OF SCOTTISH SEABIRDS TO OFFSHORE WIND

3.4 Nocturnal flight activity (Factor 4)

Nocturnal flight activity is difficult to score, as detailed data are not available for many species, although geolocation data logger data are starting to change this situation, though so far mainly for large Southern Ocean seabirds (Phalan et al. 2007, Mackley et al. 2010, 2011). However, similar data will soon be published for some North Atlantic seabirds including gannets (S. Garthe and colleagues) and great skuas (E. Magnusdottir and colleagues) based on geolocation data logger deployments where geographical distributions have already been reported but activity data are still being analysed (Kubetzki et al. 2009, Magnusdottir et al. 2011). According to ICES (2011) expert group on seabird ecology ' birds are somewhat less inclined to avoid turbines at night'. In contrast, according to Anon (2006), ' extended periods of infra-red monitoring at night using TADS at Nysted provided unexpected evidence that no movements of birds were detected below 120m during the hours of darkness, even during periods of heavy [seabird] migration'.

We used scores published in Garthe and H◘ppop (2004) for the species where these were available ( Table 8), so followed the score values established in that study. Score 1 (hardly any flight activity at night) to score 5 (much flight activity at night). We used published data where possible, and information (often qualitative rather than quantitative) from individual species studies or from handbooks (Glutz von Blotzheim and Bauer (1982), Cramp and Simmons (1983), del Hoyo et al. (1992, 1996). Classifications were also moderated by experts. It is possible that, in the near future, collection of quantitative data on time spent in flight from geolocation data loggers (for example based on salt-water switch recording time spent with the logger immersed in seawater) will allow this scoring to be converted into a quantitative scale rather than the present qualitative one.

Garthe and H◘ppop (1996) reported that in the southern North Sea, lesser black-backed gulls frequently forage at fishing vessels during the night, and that great black-backed gulls, herring gulls and black-legged kittiwakes will also forage at fishing vessels at night. However, Kotzerka et al. (2010) reported from GPS tracking data that black-legged kittiwake foraging trips mainly occurred during daylight, and that while some birds appeared to undertake foraging trips that lasted overnight, their travel speeds indicated that they were mostly inactive during the night. So it is possible that nocturnal foraging in gulls is mainly limited to situations where the birds are unable to obtain adequate resources during daylight. Similarly, geolocation data loggers indicate that breeding gannets rarely fly during hours of darkness, but do so slightly more during their migration period (Garthe and colleagues, MS in review). Greater scaups are 'mainly night-active making regular feeding flights to the sea in the evening and returning at dawn' (Nilsson 1970). During the breeding season, red-throated diver foraging flights occurred during daylight but extended into twilight in the two weeks following hatching of chicks (Reimchen and Douglas 1984). Divers apparently rarely fly during darkness. As with Factor 3, there may be seasonal variation in these scores, possibly with seabirds flying more in dark conditions while rearing chicks, or during the shortest days of winter, but there are no suitable data on this.

Table 8. Nocturnal flight activity scores