Great Egret

Great Egret

Sunday, 8 February 2015

How Birds Do It......

.....keep warm in the winter, that is.....just in case you were expecting something a bit more 'racy' :-).

I'm not really as much of a winter person as some people are. I don't mind a bit of it, and I used to relish winter for tobogganing, playing hockey on the pond, snowshoeing, cross-country skiing and the like. But I am fascinated by the way the numerous types of plants and wildlife survive the wintry elements, and I don't mind sitting in a warm environment pondering some of these mysteries.

I have been reading a book entitled "Life in the Cold: an introduction to winter ecology", by Peter J. Marchand, a professor in Vermont. It deals a lot with the physics side of things, regarding heat transfer, the effects of short-wave vs long-wave sunlight radiation, etc. and how the many species of flora and fauna deal with the cold. Physics was never one of my strengths, and I found my eyes glazing over in some parts of the book, but other parts were much clearer to me. So some of the following is what I learned from this book regarding birds.

Of course as we know, many birds fly south, especially those which require insects or open water. But we have lots of resident species which may alter their diet from insects during the warmer periods to seeds during the colder months. By not flying hundreds or thousands of kilometres south to avoid winter, they save a lot of energy. Those that do migrate risk numerous hazards along the way. That isn't to suggest there aren't hazards by staying here in the winter. And to some of the really northern species, such as this Snow Bunting in the following photo, southwestern Ontario is already hundreds or thousands of kilometres south, so they've already experienced some migration hazards and may experience more while they are here.

The key to survival is to minimize heat loss. For mammals, eating enormous amounts of food in the fall to add fat layers is a common strategy. Birds cannot add much fat, however, since it would interfere with the mechanics of flight. In most cases, especially for smaller songbirds, they can only add enough fat to enable them to survive for a day or two before refuelling becomes more critical.

Dark-eyed Junco

Song Sparrow

Tufted Titmice

Red-breasted Nuthatch
Therefore once birds have discovered the sustenance at your feeders, it is imperative to keep the feeders well stocked for the winter. If they go empty and there aren't good alternatives, they just might perish.

Some birds will find places of shelter in which to roost. It may be an old woodpecker hole, or a dense evergreen from which to escape the effects of the wind and extreme cold. On occasion, more than one bird will use the same shelter, using the 'huddling' effect to share heat and conserve energy. However they had better ensure that a Raccoon or an Eastern Screech Owl isn't already using the hole!

used Pileated Woodpecker hold
 Some birds, like American Goldfinches and their relatives, actually increase the amount of feathers, which enables them to be better insulated.
Winter goldfinch
Fluffing the feathers increases the thickness of the layer of insulation between the body and the cold. This Pine Warbler isn't normally around in the winter, but it found a home in a friend's backyard and adjacent area, where it has survived some of the recent very cold weather. A combination of a well-stocked feeder, some nearby evergreens and being able to fluff its feathers, has helped. During the spring, it won't look nearly as bulky.

Pine Warbler
Other species such as American Crows, when they aren't producing a certain amount of heat via muscle activity when in flight, will shiver almost constantly. It isn't the type of shivering that we can see, but it is more of an internal shivering that requires muscle action to create heat.
As in most things, there are limits to the benefit of constantly shivering. I remember the infamous blizzard of January, 1978. It had been above freezing, windy and raining. And then over night, the temperature dropped precipitously and stayed there for several days. The rain changed to snow, the wind howled and the temperature dropped to at least minus 18C. We were living near Rondeau at the time, and from what we heard from those in Chatham, many hundreds of crows perished.

But please, residents of Chatham, do not start praying for those conditions again. While many crows perished, they were the obvious ones victims of this extreme weather. Many smaller birds died as well!

Another strategy used by some birds, including Black-capped Chickadees, is to go into a state of partial torpor. Hummingbirds are well known for this strategy.
Black-capped Chickadee
The birds shiver to a certain extent, while their body temperature drops a bit. They do this alternate shiver and body temperature drop so that their body temperature can be several degrees below what it would be when they are fully awake and functional. But it works to enable them to conserve body heat.

What about those birds that spend a lot of time on the snow, or in water that is just barely above freezing? Wet feathers are an immediate danger, as mentioned above in the discussion on crows, but some species of waterfowl dive into the freezing water frequently to obtain food. The natural oil in the feathers keeps the feathers from getting wet as long as it is replenished by the bird's oil glands.

Food supply can be a problem, when the ice gets too restrictive and prevents the birds from accessing what they need. Then alternate food sources much help them through the tough times. However in some cases, such as this Redhead below, feeding on zebra mussels isn't part of its normal diet. It may help them through for a short time, but some of them won't make it.

But what about their appendages? One would think that feet are particularly vulnerable. Sometimes you will see ducks sitting on the snow, with their feet pulled up into their feathers.

Mallards, American Black Duck
 Some birds such as these Canada Geese will rest on only one leg, with the other pulled up into its feathers.

Herring Gulls and Ring-billed Gull

Mallard feet
But often, the feet are exposed to the almost constant ravages of the cold for long periods of time, whether they are walking on ice or swimming in the water. Another strategy that comes into play to make the difference. It is what is called 'counter current heat exchange'. The following illustration, taken from the book mentioned at the beginning, may help understand this.

In essence, the veins on the exterior most part of the legs and feet constrict, forcing the blood flow to veins that are closer to the middle. As the cold, de-oxygenated blood from the veins pass in close proximity to the warm arterial blood coming from the body, the heat is exchanged before the venous blood gets back into the body cavity. Therefore the arterial blood is cooled off while it carries oxygen to the feet so heat is not lost, and is slightly re-heated as it returns. What an amazing strategy!

So next time you see a goose standing in the snow on one leg, or a songbird such as this Eastern Towhee doing the same on a branch near a feeder, you will know why it can survive.

Eastern Towhee

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