Red-throated divers (Gavia stellata) are known to be displaced by marine industries, including offshore wind development and vessel traffic. Displacement is analogous to habitat loss, with areas used by divers prior to displacement no longer being available to them. However, the energetic, physiological and demographic consequences of displacement are currently unknown. If divers are already energetically constrained in the non-breeding season, they may struggle to meet the additional energetic demands following displacement. The aim of the Red-throated Diver Energetics Project (https://jncc.gov.uk/our-work/rtdeproject/) is to collect and compare novel data on foraging and movement behaviour of redthroated divers in the breeding and non-breeding season. Whilst this will not give direct empirical measures of the consequences of displacement for individuals, it will give insight and inference into whether divers are currently energetically challenged in the non-breeding season.
During the 2018 breeding season, 74 red-throated divers breeding in Finland, Iceland and the Northern Isles of Scotland were tagged with time depth recorder (TDR) and geolocator (GLS) bio-logging tags. In 2019, 27 divers were recaptured, and tags removed but, due to tag loss and failure, data from only 21 TDR and 15 GLS were acquired. Tags provided data from June 2018 to March 2019 (TDR) or October 2018 to February 2019 (GLS, avoiding the equinox periods). GLS data were processed to reveal non-breeding season core use areas during October to February. TDR data were processed to extract dive information, with multiple sequential dives classified into dive bouts. TDR data from both the breeding and non-breeding seasons were obtained to compare foraging behaviour in different seasons.
Red-throated divers showed consistency in non-breeding season core use areas amongst individuals from the same breeding site (i.e. country). The divers from Finland for which location information was available (n=4) moved westwards during the non-breeding season, from the Baltic Sea into the North Sea. By contrast, Scottish (n=4) and Icelandic (n=7) divers for which location information was available, remained close to their breeding grounds, with Scottish divers wintering around NW Scotland and Northern Ireland and Icelandic divers remaining around the north coast of Iceland.
From June 2018 to March 2019, a total of 275,091 dives in 14,917 dive bouts were recorded, with a mean of 710 bouts per individual or 16 bouts per day (n=912 days of data, across all individuals). Almost all dives had a maximum dive depth (MDD) of <20 metres, with 2-6 metres being the most frequent MDD. Individual divers showed substantial variation in MDD, dive bout duration (DBD) and daily total dive bout time (TDBT). In Iceland, mean MDD remained relatively consistent between the breeding and non-breeding seasons, whereas in Scotland and Finland, mean MDD increased with birds tending to dive deeper as the nonbreeding season progressed. Mean DBD was relatively consistent in Scotland across the whole study period, at around 10-15 minutes per dive bout, whereas in Iceland and Finland mean DBD increased as the non-breeding season progressed to a peak of approximately 25-30 minutes per dive bout. Mean TDBT was lower for Scottish birds, at approximately 3 hours per day, compared with Finnish and Icelandic divers, at approximately 4-5 hours per day. TDBT was relatively consistent across the study period at all sites, showing no substantial changes with season, although there was substantial individual variation in time spent diving each day.
We did not have data on light levels experienced by all birds at all times, so instead inferred diurnal and nocturnal foraging behaviour by assessing diving activity by hour of day, termed here ‘Diel Dive Activity’ (DDA). DDA was found to be consistent among individuals, with no divers diving for long periods either side of midnight. For Icelandic and Finnish birds in midwinter, almost all divers were diving for a period either side of midday, suggesting that all birds were foraging only during periods of daylight. This suggests that divers choose to forage only during daylight hours.
During the 2019 breeding season, more TDR and GLS tags were deployed on divers at the same three breeding locations. Attempts will be made to retrieve these tags in the 2020 breeding season. Once these data are available, further analyses will be conducted on the foraging and movement behaviour of red-throated divers during the non-breeding season and conclusions drawn about the behaviour and energetics of this species at this time. The preliminary results presented in this report do not attempt to evaluate whether divers undergo an energetic bottleneck in the non-breeding season as we currently have insufficient data to make this assessment and comprehensive detailed analyses have not yet been undertaken. Consequently, results from this report should not be used to make inference about diver foraging behaviour and energetics.