Sample Size Required to Characterize Area Use of Tracked Seabirds

Conflicts in resource use between humans and wildlife populations are increasingly determined through quantitative approaches. To better understand interactions between birds and human activities in the marine environment, telemetry is routinely used to characterize the area use of species, but evaluations are often based on a small number of individuals taken as representative of a local population studied. Furthermore, the relative importance of the number of animals required and for what duration they should be tracked has received little attention. We examined the central-place foraging movements of 24 lesser black-backed gulls (Larus fuscus) from a protected population from 1 March to 31 August during 2010–2013. Using bootstrapping and non-linear modeling, we investigated whether sample sizes were sufficient to characterize offshore area use by considering the cumulative area use for an increasing number of birds and duration of tracking. Box-and-whisker analysis suggested a minimum of 13 birds and a precautionary upper maximum of 41 birds were needed to describe 95% of the estimated area use of the population (defined by 100% occupancy). Tracking fewer birds for longer was more important than tracking more birds for less time. A period of 145 days was required to characterize area use for 13–41 birds; however, offshore areas were used primarily after May, meaning that a 97-day tracking period from May onwards was also representative. Predicted and observed areas were strongly correlated, and the predicted area of 15 birds for 151 days was 91% of the total estimated for the population. These findings suggest that the data were suitable for determining interaction with offshore developments, and were characteristic of the population. This study has revealed the power of a long-term tracking dataset, and has uncovered further complexities surrounding study design and analysis that may shape conclusions drawn. The method and considerations raised have wider applicability for other datasets where human-wildlife resource use conflicts need to be assessed

Relative effects of static and dynamic abiotic conditions on foraging behaviour in breeding Sandwich terns

Where and when animals forage depends on the spatio-temporal distribution and catchability of their prey. In dynamic environments, animals can repeatedly target areas that provide predictable availability of prey or may search for ephemeral conditions of high prey availability. However, how foraging behaviour is initiated in response to static versus dynamic environmental conditions is difficult to study, since both environmental data sources are often lacking. In this study, central-place foraging Sandwich terns were tracked using GPS loggers during foraging. Hidden Markov models showed that the probability of switching between transit and foraging was most strongly affected by the static variable sediment type. Wave period (a dynamic variable related to weather), salinity (a dynamic variable) and water depth (another static variable) affected the transition probability to a lesser extent. Cloud cover, wind speed and current speed were only included in lower ranked models. Air and water temperature were not included in any model. Consistent with the greater importance of static versus dynamic abiotic conditions, consistency between foraging trips of the same individual varied irrespective of tidal, diurnal or seasonal cycles, although trips made close in time within a season were slightly more similar than trips with a larger time gap. We suggest that Sandwich terns target broad areas with coarser sediments, where sandeels (Ammodytidae) are more common, and that weather variables may be related to prey visibility. Our study suggests that even in highly dynamic environments, static environmental variables may more strongly affect foraging behaviour of coastal seabirds than dynamic variables

Great skua (Stercorarius skua) movements at sea in relation to marine renewable energy developments

Marine renewable energy developments (MREDs) are an increasing feature of the marine environment. Owing to the relatively small number of existing developments and the early stage of their associated environmental monitoring programmes, the effects of MREDs on seabirds are not fully known. Our ability to fully predict potential effects is limited by a lack of knowledge regarding movements of seabirds at sea. We used GPS tracking to improve our understanding of the movements at sea of a protected seabird species breeding in Scotland, the great skua (Stercorarius skua), to better predict how this species may be affected by MREDs. We found that the overlap of great skuas with leased and proposed MREDs was low; particularly with offshore wind sites, which are predicted to present a greater risk to great skuas than wave or tidal-stream developments. Failed breeders overlapped with larger areas of MREDs than breeding birds but the overall overlap with core areas used remained low. Overlap with wave energy development sites was greater than for offshore wind and tidal-stream sites. Comparison of 2011 data with historical data indicates that distances travelled by great skuas have likely increased over recent decades. This suggests that basing marine spatial planning decisions on short-term tracking data could be less informative than longer-term data