Responses of Great Skuas to climate
Forecast climate scenarios indicate global surface temperature to rise by up to 5.8 Celsius degrees (C°) by 2100, although considerably more at high-latitudes. Possible responses of long-lived species, such as seabirds, to climate change are less easy to predict. The Great Skua, Catharacta skua, adapted for breeding at high-latitudes, may be restricted by heat stress at southern range margins and is an ideal species for which to determine responses to climatic change. A biophysical model constructed for the Great Skua, indicated that the upper limit of the thermoneutral zone was ~10˚C and the critical limit, above which evaporative heat loss was no longer sufficient for thermoregulation, was ~20˚C. Within the next 80 years, critical levels will only be exceeded regularly at colonies in arctic Russia or south of current range margins. Field data from Foula, Shetland, in 2002 and 2003, indicated that breeding Great Skuas were currently responding to heat stress by increasing the time spent bathing at the expense of other activities. When foraging conditions were poor, however, bathing was traded-off for extra foraging time and heat was lost by panting. Within Foula, mean operative temperature (a measure of heat stress) at low altitude breeding sites was consistently greater than at higher altitudes, and adult bathing activity was correspondingly more frequent at lower altitudes. Even so, breeding performance (laying date, hatching success and productivity) was not influenced by differences in heat stress exposure, even when adult energy expenditure was high. The flexibility of adult behaviour therefore accommodated current levels of heat stress. Dispersal models indicated that Great Skua breeding populations were still expanding from artificially low levels and, if not restricted by changes in food availability, would spread throughout the coasts of western Scotland and Northern Ireland by 2100. The European Great Skua distribution is probably in equilibrium with climate and future distributions are expected to track changes in food availability. In arctic areas, the timing of spring snow melt constrains the length of the breeding season and was the most probable climatic mechanism restricting seabird breeding distributions. The Great Skua, being primarily limited by pelagic food availability, illustrated the likely impacts of climate change on predominantly sub-arctic seabirds. Uncertainty concerning climatic impacts on marine productivity, combined with interspecific variation in foraging ecology, however, will cause species to respond in individualistic ways to climate change.