Surviving the cold – part II

In yesterday’s blog post, I summarized the challenges of living in (and surviving) the harsh weather of northern latitude winters and described a few of the solutions to those challenges.  But there are more solutions available to animals — and humans.

4. Turn up the heat:  For those animals (and humans) that can afford to do so, the best solution to surviving cold temperature extremes is to fire up the metabolic furnace.  No one enjoys standing out in the cold shivering intensely, but that and physical exertion are the first line of defense in staying warm.  The trick is to preserve the heat inside the body by improving insulation.


Flying around looking for food generates body heat for this Red-bellied Woodpecker, but the bird must fluff out those feathers when sitting still to retain the heat produced by activity.


Mammals, like this Gray Squirrel, fight the cold by increasing both shivering and non-shivering heat production, using an extra source of heat production from their “brown fat”.  It doesn’t hurt to have a furry tail to keep your back warm either.

Brown fat (more vascularized than regular white fat) is more prevalent in mammals acclimated to cold (even humans!) and especially in young mammals and in hibernators that undergo dormant sleep for most of the winter.  Localized in the trunk and back, brown fat heat production preferentially warms the spinal cord and brain.

5. Don’t spend what you don’t have:  In cases where food is limited or costly to obtain (i.e., resulting in a net loss of energy), the opposite strategy from #4 above is to be more conservative in expending energy by turning down the metabolic furnace when resting, decreasing activity, sleeping more, etc.  A variety of mammals, some birds, and even some humans employ this strategy in the winter.  Hibernation, or winter sleep, is key to survival in many rodent species (except tree squirrels), because there is little energy wasted on heating up their body that is essentially the same temperature as their burrow.


Hibernating chipmunks store food in their underground burrow, and rouse every couple of weeks from their torpid sleep to snack a little before becoming dormant again.  Photo from

human hiberanation

The British Medical Journal described a case of “human hibernation” in a group of Russian peasants, living in an impoverished area with inadequate food, who typically slept through the winter, rousing only once a day to eat a little bread, drink some water, and add fuel to their fireplace.

Just turning down the furnace and lowering body temperature a few degrees at night can make the difference between survival and succumbing to the cold.  Birds as small as Black-capped Chickadees and as large as Red-tailed Hawks save 30-40% of their overnight energy expenses by cooling off a few degrees.

Red-tailed Hawks sitting on phone poles

It’s not unusual to see Red-tailed Hawks perched on telephone or light poles along highways in the winter, where they can get a good view of potential prey moving around below them.  But if these hawks miss a few meals, they may not have enough energy reserves to make it through the night. Better to reduce night-time costs and save energy.  Photo by Allan Block

6.  Tolerating net energy loss:  This is kind of a last-ditch effort to survive winter, but may be a viable strategy in larger-bodied, well-fed animals.  For example, White-tailed Deer may not find enough forage to sustain themselves over an entire winter, so they put on weight by eating a lot in the Fall and coast through the winter, using up their reserves.


Winter cold must be especially tough on smaller-bodied fawns with less energy reserves than the adults.

Longer winters with more extreme temperatures may mean lower survival rates, and may even compromise an animal’s ability to recover in the spring.

white-tailed-deer-late winter

This White-tailed buck looks pretty emaciated after a long, cold winter.  He may not be able to rebuild his muscle mass over the summer in time for the Fall rut season.

A more atypical illustration of this strategy is that of hibernating bears.  They aren’t really hibernating in the true sense, since their body temperatures are only a few degrees lower than normal, but they purposely fatten up in the fall, and then metabolize that fat over the months of winter sleep, losing 25-40% of their body weight before they emerge from the den in the spring.  Females use an additional portion of energy reserve to nurse cubs born during the winter sleep.

black-bear-hibernating -

Hibernating Black Bear and cub; photo from

In summary, animals use a variety of strategies to offset the cost of surviving winter cold; it’s not really mysterious or magical, but is a product of selecting what works best in a particular situation.  Animals using the wrong strategy are quickly removed from the breeding pool, and thus solutions get better and better over time.

baby, it’s cold out there!

The thermometer read -13F (that’s -25C) in the backyard this morning — birds, squirrels, and deer were sleeping in until the sun rose high enough to hit the trunks of the trees and the bird feeders (around 10 a.m.).  And then the rush was on to grab peanuts, eat suet, and grab a few sunflower seeds before disappearing back into some warmer niche.

black-capped chickadee

Chickadee waiting patiently for a turn at the peanut feeder. 

These little birds look like balls of fluff, with their feather insulation expanded out for maximal warmth.  But a stiff wind today makes heat conservation pretty difficult with wind chill temps hovering around -35 F!

chickadee eating peanuts

Got one!  One small peanut weighs about 1 gram — providing about 0.5 grams of fat, 0.2 grams of protein, and 0.2 grams of carbohydrate.  Just the fuel needed for a cold chickadee.  The bird might come back to the peanut feeder several times, but will probably stash most of them for consumption later.  The chickadee itself only weighs 10-12 grams.

red-bellied woodpecker eating peanuts-

Red-bellied Woodpeckers are peanut specialists, and return many times for these high energy capsules.

white-breasted nuthatch eating peanuts-

White-breasted Nuthatches prefer peanuts over sunflower seeds on a day like this.

I really dislike going out in weather like this, but animals have no choice.  Foraging hours are shorter on extremely cold days, and the birds prefer the high caloric fuel to make energy ends meet overnight.  Of course, retreating to a nicely insulated nest hole helps too.

The other day when I was out walking in the far backyard (marshy swamp area) just before dusk, I stopped to look at a snag (dead stump) with a variety of different sized holes in it, when suddenly a Hairy Woodpecker flew out from one of the holes.  A moment later another woodpecker flew out from the same hole.  They might have been a breeding pair that decided to share their heat in an old nest hole — which is a great way to survive this frigid weather.

We love fat!

Yesterday’s climatic drama here in MN included thunder and lightning (in the winter!), rain, freezing rain, sleet, snow, and then bright sunshine for 5 minutes before more gray drizzle.  There was also a lot of wind, making outdoor activity quite uncomfortable.  The birds visited the feeders frantically early in the morning, perhaps in anticipation of the weather drama soon to hit them. It was entertaining to watch the woodpeckers scramble for access to the suet feeder — I usually never see all four resident species within the space of 10 minutes.

Northern Flicker-

Mr. Yellow-shafted Flicker was first and dominated for a few minutes until he was completely sated and went and sat in a tree to digest his heavy meal.

yellow-shafted flicker

So full of fat now — that’s about 100 calories per gram of suet eaten, and this guy ate a lot!

Hairy Woodpecker

Mrs. Hairy Woodpecker fed noisily, chipping continuously as she warned others to stay away until she had her fill.

Red-bellied Woodpecker-

However, Mrs. Red-bellied Woodpecker was not to be denied access and finally drove Mrs. Hairy off. She vigorously attacked the suet, drilling multiple holes in the block.

Downy Woodpecker-

Finally when everyone else was satisfied, Mr. Downy Woodpecker got his chance, and spent a few minutes cramming big chunks down his gullet.

You’ve probably noticed the order of my suet visitors seemed to be organized by size, and it certainly seems to be the case that smaller birds do yield to larger-bodied ones when both want the same resource.

But the intensity of the feeding activity observed made me wonder if precipitation that day was more of driving force, or was it the drop in temperature??  Do all birds respond the same way to these two climate variables?  Well, it turns out that someone else was interested in these questions, using the data generated by Project Feeder Watch collected from backyard feeders in the northeastern U.S. by the folks at the Cornell Lab of Ornithology.  And some of the results were a little surprising, summarized in the graph below.

Zuckerberg et al. 2011 fig 2

Gray bars next to the species name indicate the extent to which birds might increase their feeder activity in response to decreased temperature. Black bars indicate the feeder activity response to increased precipitation.  Obviously, not all species respond the same way!

According to the authors’ findings, Yellow-bellied Sapsuckers showed the greatest probability of increased feeder activity with decreased temperatures, but the diminutive Downy Woodpecker’s feeding activity did not change with temperature — probably because they are such regular visitors every day anyway.

Red-bellied Woodpeckers are more responsive to temperature changes than Hairy Woodpeckers, even though the two species are very similar in size, and presumably in their daily energy needs.  So, perhaps this reflects differences in the two species’ reliance on backyard feeders for their total food intake?

House Finches and Cardinals are more likely to be seen at feeders when there is marked precipitation, but the House Finches, like Downy Woodpeckers don’t seem to care if it’s colder — just wetter.

It’s amazing what one can learn from observing birds at a feeder.  Do these results jive with what you’ve seen at your bird feeders?

morning conversation

On a very rainy day recently, there was a lot of chatter in the backyard between a White-breasted Nuthatch and a Red-bellied Woodpecker.

red-bellied woodpecker with peanut-

It seems this male Red-bellied Woodpecker had peanuts he wanted to stash right where the Nuthatch was resting higher above.  You can see how matted down the woodpecker’s head feathers are from the constant drizzle on this morning.

red-bellied woodpecker-

Either because he was annoyed with the Nuthatch or wanted to create a new cavity for storage, Mr. Red-bellied WP decided to create a few more holes in the walnut bark while waiting for the Nuthatch to move.

red-bellied woodpecker with peanuts

Now he’s back with two peanuts this time — and the nuthatch stays his ground. There is a constant stream of chatter going on between these two the entire time.

red-bellied woodpecker with peanuts-

“Move it, Nuthatch — this is my tree.”

white-breasted nuthatch

Nope, this bird is happy to have a little protection from the raindrops and stays put.  You can see the rectangular gouges the woodpeckers make in the bark, not deep enough to store nuts, but enough for the sap to flow and perhaps attract insects the woodpeckers and nuthatches can then dine on.

Woodpecker delight

After 10 days of fog so thick I couldn’t see the neighbor’s house across the street in the morning, we have a partially sunny morning at last.  That, and temperatures hovering at 0 F brought the woodpeckers into the backyard suet feeders all at once. In the space of about five minutes, four woodpecker species snacked on the peanut-enriched suet in my new feeders.

downy woodpecker female

The diminutive Downy Woodpeckers are frequent visitors to the suet.

hairy woodpecker male

Mrs. Downy was quickly chased away by a much larger Hairy Woodpecker — this one a male, indicated by the red on the back of his head.  The Hairy Woodpeckers don’t seem to like suet feeders as much as the rest of the birds do in my backyard.

male red-bellied woodpecker

Mr. Red-bellied Woodpecker never can decide which hole provides the best tasting suet. He tries several each feeding bout.

northern flicker male

And over on the other side of the yard, a Northern Flicker male pounds away at the peanut suet in a natural log feeder.

I have learned that all suet is not the same — woodpeckers really delight in the peanut variety, over just fat and bird seed pressed together.  They also seem to prefer logs over cakes, whether because they like the idea of drilling in a hole, or because the log variety of suet just tastes better.  So this is what I buy for them now.


It costs a little more than regular suet, doesn’t melt in the summer, and can easily be molded and pressed into the holes in the feeder logs.

I found I could save quite a bit on buying suet feeders by just converting appropriate sized logs from tree branch clean-up into log feeders.  My husband drilled the 1 inch holes with his largest drill bit, fastened an eye-bolt into the top of the log, and voila — log feeder ready for filling.  Plus, with the rough surface of the log easier to cling to, no additional grooves need to be cut into its surface. Chickadees, nuthatches, woodpeckers, and even Blue Jays visit these log feeders often.

And so it begins…

Earlier this year than last, but probably average for this northern climate, winter began with a flurry of flakes this morning.


The backyard takes on its typical winter hues:  white, brown, and gray.  How long before we see bare unfrozen ground again, I wonder?


Some Minnesotans just love snowfall — me, I keep thinking of warm, sunny days.

red-bellied woodpecker

Even the birds complained this morning — Mrs. Red-bellied Woodpecker was quite vocal about the snow cover on her favorite feeder.

chickadee in snow

There was little activity at the bird feeders this morning, but I can always count on a chickadee or two. I guess most of the birds were smart enough to stay huddled in a protected place.

Missing something

Red-bellied Woodpecker juvenile

Is it the angle of this shot or does this juvenile Red-bellied Woodpecker seem to be missing something? Its tail.

Red-bellied Woodpecker juvenile and House Finch

Yes, definitely missing its tail, as well as the signature red feathers on the crown and nape of its neck.  But the latter is typical of juveniles, the former (tail loss) is a liability.

Some predator may have tried to grab the youngster by its tail and pulled the feathers out. Normally, wing and tail feathers would develop simultaneously in young birds, so a fully feathered wing and absence of tail indicates some foul play here.

Woodpecker tails form the third leg of a tripod with their feet to support them on vertical surfaces, as the photo below illustrates.  The central tail feathers are pointed and especially stiff, propping the bird up while holding the body away from the tree surface.

Red-bellied Woodpecker illustrating the tripod of support provided by two legs and stiffened tail feathers

Woodpeckers typically spread their tail feathers as they land on a vertical surface, increasing the surface area of that third leg.  The central quill (rachis) must be strong enough to function as a support but must also flex and not bend, like a drinking straw would. This is accomplished by increasing the diameter of the central tail feather rachis, filling it with a high density of interlocking keratin proteins surrounded by a rigid keratin shell.

hairy woodpecker tail cross section-from

Cross sections of the central quill (rachis) of body and tail feathers of a Hairy Woodpecker illustrate well the strengthening provided by higher density of keratin in the shaft of the tail feather.  From a presentation by Tiffany Lee (MIT open courseware).

Without its tail support, young RBW will find it difficult to forage for grubs under the tree bark.  Perhaps that’s why it has been a constant visitor at the birdseed feeders recently.


There are woodpeckers that specialize on obtaining nutrition from tree sap, like the four species of North American sapsuckers.  Typically, they bore a hole into the cambium layer beneath the bark to get to the sapwood where sugary fluid is moving up from the roots to the developing leaf buds in the spring.  But sapsuckers aren’t the only birds that enjoy the spring sap flow.

A Red-bellied Woodpecker found this sap flow on a small maple tree, and enjoyed several minutes of sap-lapping.

A Red-bellied Woodpecker found this sap flow on a small maple tree, and enjoyed several minutes of sap-lapping.

It's possible that a sapsucker created the hole through which sap was actively dripping.  But perhaps other woodpecker species know how to do this as well.

It’s possible that a Yellow-bellied Sapsucker initially created the hole through which sap was actively dripping.  But perhaps other woodpecker species know how to do this as well.

I could see the bird retracting its tongue after each probe into the hole in the bark.

I could see the bird retracting its tongue after each probe into the hole in the bark.

Seeing this bird make use of a fairly rare event in the forest (sap flow in the spring) raises the question of how it learned to utilize this resource in the first place:  trial and error, watching other birds, or was it attracted to the flies that were on the sugary sap?

Pairing up

At various times throughout the winter, I have seen either the male or the female Red-bellied Woodpecker at the bird feeder or on the trees near the feeder, but never together at the same time in the same place.

Seeing them together makes it easy to see how much more red the male has on his head.

Seeing them together makes it easy to see how much more red the male has on his head.  Still can’t see the “red belly” though.

I have to admit that this shot is fake — I inserted the female into the photo of the male, just to test out another, different feature of Photoshop, and I haven’t quite got the knack of cutting and pasting an object into another photo yet.

But my purpose in showing them like this is to emphasize how similar in size they are, often not the case in many species where either the male or the female is substantially larger.  Similar size and anatomy means that males would potentially compete with females for food resources in their breeding territory, unless they harvested it in different ways somehow.

If you watch closely, it does seem that the two sexes prefer to forage on different parts of the tree — the males more on the trunk and thicker branches, the females more on the higher, thinner branches.  And, even though they are similar in body weight, males have a stouter bill and a longer tongue with a broader tip and barbed end, for harvesting insects deeper in the wood.

"Did you know that my tongue is twice as long as my bill!"

“Did you know that my tongue is twice as long as my bill!”

Basking tree

The trunk and top branches of the Buckeye tree outside my (somewhat dirty) porch windows are brightly illuminated with the morning sun, making it the perfect place to bask in whatever heat the sunlight can provide on this chilly morning.

A chilly start...

A chilly start…

Early in the morning, I’ve noticed a variety of birds and squirrels using the buckeye as a basking spot.

You don't normally see White-breasted Nuthatches at rest in this posture (head up).  Notice how the bird is plastered right up next to the trunk of the tree with its feathers maximally fluffed.  Is it possible that the tree surface is actually "warm"?

You don’t normally see White-breasted Nuthatches at rest in this posture (head up). Notice how the bird is plastered right up next to the trunk of the tree with its feathers maximally fluffed. Is it possible that the tree surface is actually “warm”? (well, probably warmer than the air…)

Red-bellied Woodpeckers apparently like the buckeye as a basking spot as well.

Red-bellied Woodpeckers apparently like the buckeye as a basking spot as well.  This bird was sitting at rest, not foraging in this spot.

Even the much, much larger-bodied Gray Squirrels enjoy a little basking time on the trunk of the tree when the temperatures dip into the -20 F range.

Even the much larger-bodied Gray Squirrels enjoy a little basking time on the trunk of the tree when the temperatures dip into the -20 F range. Belly and tail are plastered tightly to the trunk of the tree to soak up whatever warmth it can provide.

Usually we associate basking behavior with reptilian thermoregulation — sun-loving turtles and lizards, for example.  Even crocodilians lie around in the sun letting its heat warm them while they digest their latest meal.  But basking becomes important to birds and mammals as a means of economizing on the high metabolic expense of staying warm in extreme cold.

Here's a bird that specializes in basking to warm up on cool mornings.

Here’s a bird that specializes in basking to warm up on cool mornings.