Abstract
Diving seabirds that overwinter at high latitudes experience persistent cold exposure, shorter days and associated declines in ocean productivity that can challenge their ability to balance daily energy budgets. We used dive-immersion geo-locators to test the hypothesis that pursuit-diving Common murres (Uria aalge) will respond to the challenges of winter in the North Atlantic through increased daily energy expenditures (DEE) that will be met by increased foraging effort and adjustments in dive tactics. Largely flightless in winter ( 85% resting and swimming). Accordingly, when sea surface temperatures (SST) were consistently near freezing in late winter (1.9 ± 0.8 °C), mean DEE (2463.2 ± 10.9 kJ day-1) exceeded the theoretical limit to sustainable energy expenditure in vertebrates (i.e. 7 X Basal Metabolic Rate or 2450 kJ day-1 for murres). Consistently deep (70% > 50m) and long dives in late winter, 38% of which exceeded their calculated aerobic dive limit indicate that targeted prey was distributed in deep (dark) waters. Consequently, foraging was highly constrained to daylight hours; likely because capture efficiency of deep-water prey is very poor in low light. Murres responded to late winter time and energy constraints with a nearly 2-fold increase in daily time spent diving (95.2 ± 5.6 mins and 178.3 ± 6.3 mins day-1 during early and late winter, respectively), an increase in dive bout frequency and duration, and correspondingly less time resting between bouts. Uniquely adapted for deep-diving, pursuit-diving murres push their dive limits in order to maximize daily energy intake when energy demands are high and prey are distributed in deep water. This study highlights late winter as an extremely challenging phase in the annual cycle of North Atlantic murres and provides critical insights into the behavioral mechanisms underlying their winter survival.
Highlights
Diving seabirds that over-winter at high latitudes contend with high-energy requirements for thermoregulation and challenging foraging conditions during seasonal lows in ocean productivity and short days
Using fine-scale behavioral information gathered from pursuit-diving murres equipped with dive-immersion geo-locators, we demonstrate overlapping trends in mean daily energy expenditures (DEE) (Figure 2) and mean daily time spent diving (Figure 3) over winter, both showing a steep increase through early winter (EW) followed by a late winter (LW) asymptote
Our results contribute to our understanding of the tolerances of murres in response to a prolonged and demanding LW period in a Low Arctic ecosystem, and demonstrate that murres are capable of coping with predictable, seasonal environmental extremes over an extended period
Summary
Diving seabirds that over-winter at high latitudes contend with high-energy requirements for thermoregulation and challenging foraging conditions during seasonal lows in ocean productivity and short days. That are primarily visual predators (c.f. Regular et al, 2011; Berge et al, 2015) face additional constraints on foraging time during the shorter days of winter and correspondingly longer periods of nocturnal fasting (Grémillet et al, 2005a,b; Daunt et al, 2007). 2011; Berge et al, 2015) face additional constraints on foraging time during the shorter days of winter and correspondingly longer periods of nocturnal fasting (Grémillet et al, 2005a,b; Daunt et al, 2007) This conflict between resource demand and availability in winter raises the critical, but poorly resolved question of how diving seabirds meet the energy demands of survival
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