Have you ever experienced really cold water? I mean, REALLY cold water? I remember once my wife, Marg, and I were backpacking along the Spray River Trail in Banff National Park. It was early June and the sun was heating up the countryside. We took a breather beside a small lake, created by melt water directly off the icepack high above on the mountain – this was the source of the Spray River. To cool off I unthinkingly dunked my head in the lake and was hit hard with an instant headache that lasted over a minute. It made brain freeze created by drinking a slurpee too fast seem like child’s play.
A few years ago when I was doing field work on Svalbard we went through 5 days of fairly intensive training prior to heading into the field in a variety of useful skills. One was how to get into an immersion suit – a very important and necessary piece of equipment as we had to zodiac 20+ kilometers between the town and the field site, sometimes in pretty stormy conditions. Once we were in the suits we had to get into the water of the fjord (which was about 0 C.) and then swim 20 km [haha….it has been pointed out to me that this is an incredibly long distance to swim in any conditions…it should read metres rather than kilometres.] to the zodiac and haul ourselves out. The “trick” here was that we couldn’t wear the protective insulating mitts so that we could experience how cold the water was and how it impacts your ability to use your extremities – getting hold of the ropes and pulling yourself out in these conditions was very difficult – just getting your fingers to work.
These types of experiences have caused me to wonder how birds, that routinely are in frigid water for large parts of their lives, survive it. How does a murre or penguin or other diving bird keep its eyes open under water without experiencing extreme pain. Thick-billed Murres have a very dense set of feathers that shields them from direct contact with the water they swim in….except when they’re incubating an egg. Both sexes develop a brood patch – an area of bare skin that they place over the egg to impart the maximum amount of heat. But how does this affect their heat balance when they leave the nest to fish and sit on the surface for long periods of time or dive?
The question came up again two days ago when I was doing a bird count along the Grand River between Caledonia and York. Although I nearly missed it I noticed a Great Blue Heron hunkered down beside the river. The water temperature would have been around 5 or 6 C. How do these birds manage to stand in water this cold for long periods while they patiently wait for a fish? Why don’t their legs and feet freeze up?
Birds that stand in cold water or on ice/snow for long periods – ducks, gulls, this particular heron….- use a technique called “regional heterothermy” whereby they maintain their core body temperature while letting their legs/feet drop to close to outside temperatures. The system for doing this is referred to as the “countercurrent heat exchange”. In the bird’s leg arteries leading through it to the foot are very close to the veins bringing blood back from the foot. Arterial blood starts off warm but is cooled by venous blood as it descends and vice versa: cool blood returning from the foot is warmed by the arterial blood and reaches near normal temperatures by the time it reaches the core. It obviously works; how many birds have you seen standing in cold water or on ice and not the worse for wear.
Of course the obvious question for this heron is: why hasn’t it flown south where it’s nice and warm?