Surviving the winter

George Walter

Photo by Jill Severn

As January turned to February, we got our first blast of winter: a week or more of subfreezing, snowy weather.

For a bird to successfully reproduce, which is the principal aim of its life, it must successfully survive such conditions; and it’s interesting for us, living in our comfortable homes, to explore how our avian neighbors manage this feat.

You probably already know more about this topic than you might realize. For example, that birds are “warm-blooded.” Like us, they maintain a high internal temperature and use their heart and blood system to circulate warmth from their core to their extremities. That helps birds survive those cold winter days.

A second bit of common knowledge is that some bird species can survive our cold weather while other species have adapted to winter by opting to migrate south for the winter. Like their human counterparts, these are known as “snowbirds.”

Anna's Hummingbird in winter

Photo by Liam Hutcheson

Hummingbirds are among our very favorite local birds and our two common species, Anna’s Hummingbird and Rufous Hummingbird, follow these opposite approaches. The Rufous hummers migrate south in September, flying all the way to southern Mexico, nearly 4,000 miles, and in the early spring they repeat the flight. For such tiny creatures, that’s an amazing physical feat.

Rufous Hummingbird

Photo by Liam Hutcheson

In contrast, Anna’s hummers stay here for the winter. If you have kept your hummingbird feeder up, Annas are the ones that continue to visit all winter. How do they manage it? They have the ability to temporarily semi-hibernate, called torpor. Their body functions, especially temperature and metabolism, slow way down. When your feeder is frozen, chances are the hummers that were regularly visiting are hunched up in a sheltered location, waiting for better conditions.

This simple contrast between two species tells us something very basic about the evolution of species. Birds have to survive to reproduce. Migration versus staying around are two winter survival approaches, both posing substantial stress and likely killing off those individual birds that are not able to accomplish the needed life tasks.

When I wrote the first paragraph of this column, I almost added “bitter” in describing our cold, but compared with the winter conditions of the interior of North America or the Arctic, our coastal Pacific Northwest is a “banana belt.”

Almost all the waterfowl and shorebirds we see in winter here have migrated from a breeding territory where the ground and water, even the ocean, is frozen solid. Now that’s a winter that deserves the term bitter.

If challenged to name avian adaptations to winter weather, some quick-witted wag might answer that they wear down coats. Their outer feathers, the bristly ones, fit together smoothly and can readily shed water like, you guessed it, water off a duck’s back.

Golden-crowned Kinglet

Photo by Liam Hutcheson

The inner feathers, in contrast, are downy. Birds have the ability to fluff these inner feathers, trapping air within (as illustrated by Liam’s accompanying photo of a Golden-crowned Kinglet). This insulation quality of feathers has been recognized for centuries and resulted in using bird feathers, especially those of waterfowl, for winter clothing and bedding. Rather than for meat, this use was probably the principal reason for domesticating geese. 

Most bird bills also present a “keeping warm” challenge since they have no feathers. Many of the owls that winter in very cold areas have an interesting feather adaptation, growing long overlapping feathers around a large part of the exposed bill. A close-up photo of a Great Gray Owl shows this feather cover-up very well.

Great Gray Owl

Photo by Liam Hutcheson

If you take another look at the kinglet photo, you may notice a limitation to that down jacket approach to winter survival — those naked legs. Small birds like the kinglet hunch down when perched, somewhat covering their legs, and they will tuck their bills under a wing and into their side feathers.

But what about larger birds? Gulls, waterfowl and shorebirds stand out in the cold all day and night. How can they maintain body warmth with those naked legs and feet? And, for that matter, why don’t their feet freeze or get frostbite?

First, birds’ feet do get cold, but they don’t seem to mind. Experiments suggest that they don’t have the same type of cold sensors in their feet as mammals do. Also, bird feet can withstand low temperatures without damage in part because there are mostly tendons and bones with little muscle or nerve tissue.

Birds also have a specially adapted circulation system. The blood vessels in their feet and legs are very close together, allowing heat exchange between the vessels themselves. Thus, the warm blood entering the feet warms the returning chilled venous blood, and also reduces the energy needed to keep the feet warm. Thus, their feet are protected from freezing, and their bodies’ core is protected from the excess chill produced by cold blood returning from those naked feet.

Many birds also have behaviors that protect against very cold temperatures. Some smaller species snuggle up together in a communal roosting site. Waterfowl and shorebirds tuck their bills under their wing feathers and will also stand on one leg, alternatingly tucking the other into their feathers for warmth. The small shorebirds in the accompanying photo exhibit all three adaptations.

Sleeping shorebirds

Photo by George Walter

Finally, every day, each bird that is not going into torpor needs to find something to eat. That’s why, on especially cold days, your bird feeder will seem to be especially busy. In fact, some people find it necessary to refill feeders several times a day.

And here’s the final piece of knowledge you probably already have about what it takes for birds to get through winter:  Since they need food every day, your yard and feeders can be an essential stop on their daily survival mission.

George Walter is environmental program manager at the Nisqually Indian Tribe’s natural resources department; he also has a 45+ year interest in bird watching. He may be reached at george@theJOLTnews.com

Photos for this column are provided by Liam Hutcheson, a 17-year-old Olympia area birder and avid photographer.