In the Bleak Midwinter

Insulation - chickadee warming its feetIt was minus 40 Celsius with the wind chill the other morning. The bite of the air stung any carelessly exposed skin and the snow squeaked like Styrofoam underfoot. Wrapped up in my shearling coat, I couldn’t help but watch in fascination as a nearby mountain ash came alive with foraging Pine Grosbeaks and the cheerful chirps of chickadees and nuthatches filled the frosty air, reminding me just how incredible these tiny winter residents really are.

Chickadees, for example, weigh not much more than 10 g, about the same as two nickles. Yet, they can survive quite comfortably in temperatures that would leave us frostbitten and shivering.

Winter birds accomplish this seemingly unfathomable feat in a number of different ways. Firstly, they’re wearing a down coat. Those of you who own one know just how warm they can be and for birds, that insulation is part of the standard package. Feathers are a remarkable insulator. Comprising only about 5 – 7 % of a bird’s body weight (that’s half a gram on a chickadee), the air trapped within them makes up 95% of that weight’s volume, creating a thick layer of dead air that traps heat generated by the body, preventing much of its loss even on the coldest of days. Many winter residents grow a thicker winter coat, much like mammals, augmenting their feather count by up to 50 %. Fluffing feathers increases their insulation factor even further (about 30%), making them a very efficient way to keep warm in the winter, so efficient, in fact, some birds, like Great Gray Owl can actually overheat in the summer.

While some species, like Ruffed Grouse and many owls, grow feathers, along their legs and feet, like fluffy winter boots,  most songbirds’ legs are bare, thin sticks of sinew, blood and bone exposed to the elements. Although birds can tuck these delicate structures up into the warm cover of down when temperatures really plummet, most of the time they’re out in the open. So, why don’t they freeze and why isn’t all of a bird’s body heat lost through these naked limbs? Bird legs are marvels of biological efficiency, having been streamlined by millennia of evolution into sleek structures with very little muscle and few nerves, using instead pulley systems of tendons and bone to accomplish movement. These tissues, along with their scaly coverings have very little moisture and are less likely to freeze than flesh and skin.

Birds also have cold feet. Using a common natural system called a countercurrent heat exchange, our feathered friends keep their feet upwards of ten to twenty degrees colder than their core body temperature. Countercurrent Heat Exchange System in a bird's leg. by Heather HinamWarm arterial blood on its way to the feet pass right next to colder blood coming back towards the body through the veins. Heat wants to reach a point of equilibrium, so warmth from the arteries passes into the veins which carries it back into the body. Because the flows are running opposite to each other, it’s impossible for the heat balance to ever reach equilibrium, so by the time the blood gets to the feet, it’s much cooler than when it entered the leg and all that precious body heat has been kept where it needs to be, in the core.

However, as most of us who have experienced a true northern winter know, a coat alone isn’t always enough. There has to be heat to trap in order for insulation to work over the long term. To generate that heat, many winter birds shiver constantly when they’re not moving. Ravens, whose feather count isn’t as high as some of its more fluffy distant cousins, actually shiver constantly, even when flying, the repeated contractions of their massive pectoral muscles acting like a furnace. Powering that furnace takes energy and cold-weather specialists meet those needs by upping their metabolic rate, in some species, to several times their normal levels. As a result, food is always a going concern in winter.

Many winter residents can only forage for food during the day, so keeping the internal fires burning at night can be a challenge.  Finding a warm place to settle in for the night reduces those metabolic needs.  Densely-packed spruce boughs or old tree cavities are perfect nighttime microclimates and many birds use them. Chickadees will often take it a step further, piling as many fluffy little birds as possible into an old woodpecker hole to share body heat, which may just be too much cuteness in one place. Ruffed Grouse take advantage of the insulative capacity of snow in a somewhat comical way. One cold nights, the birds dive head first into a drift and tunnel deeper into the snow, creating a cave known as a kieppi. Temperatures inside the kieppi can hover just around the freezing mark, even when it’s minus thirty outside.

So as we close in on the shortest day of the year and sink deeper into the cold clutches of winter, take a moment, now and then, to marvel at those tiny survivalists outside your window. Much of the technology that keeps us from succumbing to winter’s icy grip was adapted from them. Nature truly is our greatest teacher.

It’s a Hard-Knock Life

Juvenile Northern Saw-whet OwlsThere isn’t much else in the world that’s cuter than a baby Northern Saw-whet Owl. I should know; I handled dozens of them over the course of my doctorate research. Between their huge, blue, soulful eyes and the round, fluffy, ewok-like body, they’re guaranteed to evoke an ‘aww’ out of even the hardest-boiled egg of a person.

Still, most people will never have the opportunity to see one, at least not in their juvenile plumage. They’re notoriously hard to find.  Northern Saw-whet Owls nest in old tree cavities, moving into empty woodpecker holes and other crevasses in rotted out trunks. To study them more closely, researchers put up nest boxes in the hopes of coaxing them into more accessible real estate. It’s a lot easier to climb a ten-foot ladder up to a nest box than to have to figure out a way to get 25 feet up into a poplar or worse, a hydro pole.

Even once they’re out of the nest, they’re difficult to spot. Being not much bigger than a coffee mug full-grown, these little owls rely on camouflage to stay safe in the forests and woodlots where they make their home.  Their first line of defence when threatened is to go stock still against a tree trunk or in a mess of branches. It’s a very effective manoeuvre.  Adult saw-whets have stripes of brown and white on their breast feathers and spots on their heads that break-up their profile, helping them melt into the shadows. I can’t tell you how many times I’ve tracked a radio-tagged bird to their daytime roost and still couldn’t spot the little guy among the leaves. The brilliant white V on the forehead of juvenile birds is to help parents find their mouths in the dark of a nest cavity. Still, in daylight, this natural beacon manages to blend into the dappled sunlight on the foliage.

Adult Male Northern Saw-whet Owl

Adult male Northern Saw-whet Owl blending into the background.

So, any time I get to spend with these little guys is a treat, one that I never take for granted. It’s always such a pleasure to get to observe their individual personalities up close.

Don’t let their size and adorable expressions fool you. These are tough little birds. They have to be. Life for a Northern Saw-whet Owl is hard from day one. Females lay their eggs two days apart, but start incubating before they’ve completed the clutch. As a result, you end up with a nest full of young where the oldest may have a ten day head start on life over the youngest. In years where the small mammal population is high, the provisioning males can make their nightly quota of about seven or more prey items a night, making it possible for all the young to make it out of the nest. However, in years where food is scarce, that age difference suddenly comes into sharp relief and it’s not uncommon to find only one or two of the oldest nestlings surviving out of a clutch of 4-6.

Even if they make it out of the nest, life doesn’t get much easier. After a month crammed into the nest hole with mom and all their siblings, you’d think these newly-fledged saw-whets would want to move on and take advantage of their new-found freedom as quickly as possible.  However, despite having fully-feathered wings by the time they leave the nest (unusual for owls), juveniles tend to hang around the homestead for another month or so. They spend their days tucked away in the shadows in nearby trees and their nights calling insistently for food deliveries from their already beleaguered father, their mother having taken off around the time the oldest hit 21 days for a much-needed break.  During this post-fleding period, young saw-whets practice flying and refine their hunting skills.

Eventually, it’s time for them to strike out on their own into the great unknown. It’s actually a great unknown for us researchers as well. Despite a number of long-term banding programs for the species all over North America, we still don’t have a very good handle on saw-whet owl movements outside of the breeding season.

So every year, my colleagues across the country and I will keep adding new nest boxes and checking the ones we have, spending as much time as we can peering into the lives of these adorable and enigmatic owls in the hopes that one day we might unravel a few more of their mysteries.

* If you would like to entice owls to your backyard, let me know, and I’ll send you the plans for building a nestbox.

The Edge of Darkness

Owl SilhouetteAs I’ve mentioned before, I have always had a love for obscure words, especially those that find everyday use in the lexicon of certain specialties.

Crespuscular is one of those words.

I use it all the time, but it’s definitely not common knowledge, something that’s become increasingly obvious over the many years that I’ve been a nature interpreter. I’ll throw it out there, along with other natural history terms, like ‘nocturnal’ or ‘carnivore’. While my charges usually nod sagely in understanding at these other adjectives, ‘crespuscular’ usually elicits furrowed brows and working tongues as they try to wrap their mouths around the syllables, eyes rolled up towards their brains, as though watching it try to divine the word’s meaning.

It’s too bad, because it’s a good word. It’s also a great way to be. A crepuscular animal is one that is most active at twilight, straddling the line between night and day in the muted light of either dawn or dusk. It certainly my favourite time to be out and about, probably because I’m in such good company.

Many animals are crepuscular in their habits; the most notable of which,  for me, are the owls. Species, like the Great Gray Owl, are at their best at this hazy time of day, making use of their enormous eyes and highly-tuned hearing to pick up the slightest rustle of prey along the forest floor. Owls, however, are not the only birds that enjoy this shoulder time. Common Nighthawks and Wilson Snipe also come alive in the dusk, the former swooping and diving through the gloom, scooping up millions of flying insects that have taken to the air after the heat of the day before the cool night temperatures slows their metabolisms and forces them back to earth. Most songbirds reserve their choruses for the crepuscular hours; Olive-sided Flycatchers announcing the dawn and Hermit Thrushes heralding the dusk, their refrains rounded out by the harmonies of breeding frogs.

Most boreal mammals are also crepuscular in their habits. The dull grey winter coat of the white-tailed deer is at its most invisible in the murky hours of twilight, especially to the mostly colour-blind vision of their carnivorous predators. Bats join the nighthawks in their aerial quest for a meal and rabbits emerge from the shadows, taking advantage of the low light to grab a quick nibble before complete darkness makes it difficult to spot approaching danger.

In reality, the busiest time of day, in whatever habitat you might live, is twilight. So, whether you are an early bird, who rises before the dawn, or a night owl, like me, who takes comfort in the release of the day as the sun slips below the horizon, get outside at these tenuous moments and discover the beauty and wonder of becoming crepuscular in your habits.

Diving into things

Northern Hawk-owl We’re heading into owl season around here, my favourite times of year. Of course, after nearly 15 years chasing these denizens of the dark around in the forest, I may be a little biased.

Actually, as soon as the snow blankets the ground, I go into owling mode, scanning tree tops and hydro poles for distinctive silhouettes. This heightened awareness comes from the knowledge that winter and early spring are the best times to spot one of the more unique members of the Order Strigiformes.

I’m referring to the Northern Hawk-owl (Surnia ulula). Their name alone suggests that they may be something a little different. They’re actually quite aptly-named. Unlike most of their fluffy, almost stocky cousins, these smallish owls are really quite streamlined, with relatively small heads, long tails and slightly more narrow wings. They’re also diurnal, hunting during the day, instead of exploiting the darkness like most other owls.

In the winter, these little stealth bombers come out into the open, perching high on the tops of spruce trees or old snags, sweeping the area with their eyes and ears, searching for scurrying rodents. Their sharp-eyed gaze can spot a vole bouncing over the snow up to 800 m away. When they find one, watching them pounce is a sight to behold. Most owls are relatively slow flyers, almost floating to a stop over their prey before they plunge to the ground for the final kill.

Hawk-owls are another matter. They usually don’t slow down. If the vole is on the surface of the snow, they don’t stop at all, snagging the morsel on the wing and swooping up to a perch to settle in with their kill. Their flight patterns are actually more like that of a Goshawk or a Cooper’s Hawk and that, coupled with their daytime habits, likely led to their common name.

These unusual birds are one of the least-studied of northern owls. Most of what we know about them comes from studies done in Scandinavian countries, where banders have worked diligently for decades to understand their owl populations. Here in North America, only about a dozen nests have been studied extensively.  The fact is that they’re hard to work with. Although you can spot them with a fair bit of regularity during the winter, hawk-owls disappear into the bogs and old burns to breed during the spring and summer months, making them hard to find, right at that period in their natural history that we want most to understand.

What we do know is that they like tree cavities for nests, laying their eggs in old woodpecker holes or burnt-out snags.  Like their northern neighbours, the Great Gray Owl and Boreal Owls, their reproductive success and thus their population numbers are likely tied to the cycling abundance of their rodent prey. Indeed, every now and then, large numbers of these birds will wing their way south out of their normal range in search of food, stirring southern birder populations into a frenzy. Still, we don’t really understand the mechanisms that drive them yet.

The scientist in me abhors this unknown and wants to work up a study and dive right into fray, teasing out patterns from the apparent chaos. However the writer in me likes that there are still mysteries left in our world.

Can you hear me now?

Close up of a Great Gray OwlSomehow I’ve managed to make a good part of my living for the last decade standing around in the dark. Owls have been a source of fascination and a subject of study for me for quite a while now.  I’m not alone in that fascination. Many people are drawn to their seemingly all-knowing eyes and wise faces.

I hate to destroy any long-held beliefs, but owls aren’t really all that wise. There are many bird species (the crow family, for example) that have much greater levels of intellect.

That doesn’t mean owls are any less remarkable. This group of birds has evolved some amazing adaptations to help them make the most of their nocturnal realm.

What I find most fascinating is their ability to hear in three-dimensions. While we can perceive depth with our eyes, they can also do it with their ears.

Owl Skull (without sclerotic ring) showing ear openings.

The secret is asymmetrical ear openings. In most animals (ourselves included), the ears tend to be at approximately the same level on either side of the head. However, in the especially nocturnal owl species or ones that predominantly use hearing to locate their prey, one ear is usually higher than the other.

As a result, sounds do not reach both ears at the same time. It’s this delay that allows the bird to figure out where the sound is coming from.  They can be quite precise, detecting differences in timing up to 30 millionths of a second. When an owl hears a sound, the medulla of the brain creates a three-dimensional mental map of where it’s located. They then hone in on that point by moving their heads until both ears are hearing the sound at the same time. When they reach that point, they are facing their prey. Adjustments can be made while in flight, moving their heads until they’re lined up to strike.

Because they are operating at such a high level of precision, it’s important to have the best hearing as possible. In many owls, like this Great Gray Owl above, most of their face is made up of a disk of feathers. This facial disk acts like a satellite dish, funneling sound to the ears. Muscles under the skin allow them to adjust the shape of the disk as needed to get the best reception. Cup your hands around your ears and you’ll get an idea of how it works.

The size of this disk of feathers is a pretty good way to quickly assess just how nocturnal an owl species is. Larger disks, relative to the overall size of the face (like in Boreal Owls or Barred Owls), means that the species tends to hunt mostly at night.

Great Grays are a bit of an exception.  They have huge facial disks, but hunt mainly at dawn and dusk. These owls, however, hunt prey that are hidden by layers of snow in the winter. Like hunting in the dark, they can use their ears to pinpoint a vole running under the snow up to 150 metres away! It gives a whole new meaning to good reception.