Photo: Tufted Titmouse, Catherine McEntee/Audubon Photography Awards
Libby Lyons, President
I expect that many of you have wondered during the past frigid, snowy, and icy weeks, “How can birds survive the winter here?” That question has had me thinking too, and the answer seems to boil down (pun intended, see below) to two types of responses, i.e., behavioral and physiological, or a combination of the two.
Birds have several challenges to overcome in winters like this one. One is staying sufficiently warm. Some of the common adaptive bird behaviors I’ve seen include birds basking on the sunny side of a lake or river or hunkering down out of the wind in a dense evergreen tree. A visit to my brother in Port Angeles, WA one February suggested another type of beneficial behavior. Early one morning, with ice covering the bushes, I was gobsmacked to see an Anna’s Hummingbird flitting about. My brother said that he knew of one that had found a nice warm spot under the eaves of a house near his.
That hummingbird also may have benefitted from going into torpor, a physiological mechanism that’s akin to a short-term hibernation! This type of daily night-time torpor results in a lower body temperature and a lower metabolic rate. In this physiological state such birds, which usually require high levels of energy for their metabolism and flight, can conserve energy and survive very cold temperatures.
What are other physiological mechanisms birds use to stay warm? When we might go out and buy a down coat, many birds grow extra down feathers before the winter! The downy feathers are short and fluffy and lie below the larger outer feathers. These feathers trap air near their bodies where the warm air pockets act like insulation, shielding them from very cold temperatures outside. Birds often combine such feather growth with a behavior where they use muscles to fluff up their feathers, boosting the thickness of that insulating layer, and in some cases, their comical appearance!
OK, but how can birds stand on ice and not have their body temperature plummet? Some birds have specialized scales on their feet and legs to help insulate them. Their anatomy can also help via a phenomenon known as counter-current exchange. This is an exchange of heat which occurs when the arteries carrying warm blood from the heart are in very close contact with the veins carrying cold blood from their feet. The cold, heart-bound blood is thus warmed up, helping the bird maintain a stable core temperature. Interestingly, the same counter-current exchange mechanism helps birds tolerate the other heat extreme (recall my mention of boiling up above.) When we lived in Kenya I often wondered “How can those flamingos stand in near-boiling hot springs?” The temperature of very warm veinous blood from their legs is reduced when it races upward in close proximity to the cooler arterial blood coming down from their hearts!
There are other simple ways that birds change their behavior to stay warm. Some tuck in their bills and legs in to keep them warm. Others huddle together. I recall one night seeing six Swallow-tailed Bee-eaters packed like sardines on a branch in Kgalagadi Transfrontier Park. They were trying to stay warm in that desert locale where winter temperatures can be in the seventies or eighties during the day but can dip well below freezing at night.
Another challenge for birds this winter has been finding food when so much of the water and ground is frozen. An obvious behavioral response is to fly to a more suitable habitat. For many birds this means flying (further) south. For others it means flying from frozen-over ponds and lakes to very deep, unfrozen lakes or to flowing rivers, like parts of the Potomac River. I was surprised last month to see several Wilson’s Snipe just upstream from the Potomac at Hunting Creek Bridge, but it was one of the few places around with unfrozen mud! And another day, from one spot along the Potomac River in Alexandria I saw Hooded, Common, and Red-breasted Mergansers, species not normally found together, but all brought to a rare ice-free spot where they could dive for fish.
Another way birds find food in such frigid times is by locating birdfeeders. My husband tends several feeders in our back yard which have brought in more than two dozen species in winter, including a Cooper’s Hawk. The hawk is not after the seeds because it eats birds and small mammals. Its normal behavior has been to hunt our birdfeeder-visiting birds from a nearby tree, but on one snowy and windy day in late January it was sitting just two feet above the feeders. Perhaps the cold had altered its behavior as well, and it had decided it was too cold for stealth!