Arctic great cormorants are poorly equipped to cope with harsh polar winters; they have very meagre fat reserves and their partly wettable plumage offers little insulation. Yet a small population of these diving birds stubbornly overwinters north of the polar circle, along the coast of Greenland. Suspecting that the birds have physiological or behavioural adaptations to survive in their hostile home, David Grémillet and his colleagues painstakingly collected physiological data from great cormorants for an entire year. Incredibly, they found that cormorants don't appear to have any such adaptations, yet still make it through the Arctic winter(p. 4231).
How does a warm-bodied diving bird with poor insulation survive in the Arctic? Grémillet reasoned that the birds could cope with the polar night (the period during the Arctic winter when the sun remains below the horizon) by depressing their metabolism or by decreasing the time they spend diving for fish in the freezing water. Grémillet travelled to Greenland to find out if cormorants adjust their physiology or behaviour to save energy during the winter. He caught ten cormorants nesting during the summer, and Grégoire Kuntz and Caroline Gilbert implanted tiny data loggers into the birds' abdomens to record their heart rate, body temperature and dive depth over a year.
The team returned the following summer, anxious to see if the birds had survived the dual perils of the harsh Arctic winter and the Inuit hunting season. Luckily, all ten had made it through the winter unscathed and were happily breeding again. Collecting the data loggers, the team first investigated whether the birds depress their metabolism during the winter. They expected to find that cormorants drop their body temperature in winter to reduce the temperature difference between their body and the environment and cut heat loss from their body. But the birds had a normal, steady temperature(38-41°C) year-round. A second tell-tale sign of depressed metabolism is a lowered heart rate, but the team found that the birds don't lower their heart rate during the winter.
Since cormorants don't adjust their physiology, the team supposed that the birds adjust their behaviour instead and spend less time diving for fish in the winter. So they were shocked to discover that the birds did not spend less time diving during the coldest months of the year; instead, the birds spent more time swimming as the days got darker. `This is probably because cormorants are visual predators, so during the dark polar night they can't see the fish as well and have to spend more time hunting', Grémillet says.
The team suspects that cormorants simply cannot afford to stop fishing if they want to survive the winter. When the team dissected unfortunate cormorants that had been trapped in fishing nets, they discovered that the birds are incredibly lean; their fat reserves would only last for three days. While marine mammals like seals rely on a thick layer of blubber for insulation and energy reserves, cormorants don't have this luxury. `Cormorants are true athletes; they dive every day throughout the winter. The more we learn about these birds, the more amazing they seem', Grémillet concludes.