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.

red-bellied-woodpecker

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.

gray-squirrel-

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 chipmunk-sni.schlastic.com

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 sni.scholastic.com

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.

white-tailed-fawns-feeding

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 - bearlakereserve.com

Hibernating Black Bear and cub; photo from blackbearreserve.com

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.

Let me look into your eyes…

Eye color in birds is rich and varied, spanning a range from white to yellow to orange, deep red, brown and black iris color.  The color of the iris might change seasonally, as it does in Brown Pelicans and Double-crested Cormorants (see below);

brown-pelican

White head and neck feathers are part of the breeding plumage of Brown Pelicans, whose eye color also changes from brown to blue during breeding.  Photo taken in New Orleans, February 2013.

or might vary with age, as it does in various raptor species from pale yellow in juveniles to dark brown in adults;

Red-tailed hawk eyes-PentaxForums.com

The amber color of this Red-tailed Hawk’s iris will darken to deep brown as the bird ages.  Photo from Pentaxforums.com

or it might be associated with other flamboyant colors of males advertising their sexual prowess, like the bright red eyes of the Wood Duck,

wood-duck-male-Backyard Biology

Once they molt their full adult plumage, male Wood Ducks retain their bright red eye color permanently.  Photo taken Fall, 2013 on Lake Vadnais.

or the deep golden-orange eyes of the Glossy Starling seen in South Africa a month ago.

The bright orange-yellow eye of this  Starling from South Africa is one of its key identifcation characters.

The bright orange-yellow eye of this brilliant blue-purple feathered Starling taken in South Africa is one of its key identifcation characters. 

With such a diversity of eye color, within a species or over the bird’s lifetime, it seems strange that birds seem to lack the green and blue-hued eyes that are common in other animals, and especially in humans.  A couple of the very few examples of green/blue eyes in birds:

double-crested cormorant

The bright blue iris color indicates breeding condition in Double Crested Cormorants.  Photo taken in New Orleans, February, 2013.

brown_pelican4_bill_stripling-national audubon-

Are blue eyes considered sexy in birds?  Brown Pelicans also sport blue eyes during the breeding season.  Photo by Bill Stripling – National Audubon

Green-eyed birds are either really rare, or rarely photographed; I found only one image of a bird with this rare iris color — a Great Cormorant. (Note:  there are other images of green-eyed birds posted on Google, but some definitely look photo-shopped — see below.)

great cormorant

This might just be a trick of the lighting, but this Great Cormorant’s iris does appear to be green, or at least blue-green.  Photo from sight-touch.com

I didn’t realize until I was looking for more information about “green eyes” that they are rather rare in the human population as well.  Only 1-2% of humans have truly green eyes (not hazel with flecks of green).  That makes my paternal grandmother, me, my daughter, and my grandson rare!  Hmm…that doesn’t sound too rare.

Green color, being a combination of yellow and blue, requires a yellow pigment and a refractive property that reflects blue light (because there are rarely blue pigments that produce color).  In humans, amber and green eye color is produced by a yellow pigment called lipochrome.  Varying amounts of lipochrome and melanin present in the iris generate a host of eye colors varying from yellow-brown to deep, intense grade shades.  Click here to read a fascinating discussion of the derivation of eye color on a blog devoted entirely to that subject.

green-eyed owl

Are you for real?
Or is this a hypnotist’s trick to make you “look into my eyes”?

Flying lessons

I saw this hawk land on a tree when I was driving back roads at Carlos Avery Wildlife Management Area last week.  But even with binoculars I couldn’t really tell what species it was from a distance.

red-tailed hawk

Doing its "head on backwards" pose prior to taking off.

Doing its “head on backwards” pose prior to taking off.

But once the bird took off from the perch and began to soar overhead, I could get a look at its tail, and its ID was obvious.

There is a little hint of red on the tail there.

There is a little hint of red on the tail there.

Backlighting illuminates the red tail, and shows how every secondary feather in the wing overlaps its neighbor to provide a smooth airfoil.

Backlighting illuminates the red tail, and shows how every secondary feather in the wing overlaps its neighbor to provide a smooth airfoil.

Notice how the primary feathers at the tip of the wing are rotated and widely separated from each other.  This rotation reduces the resistance to moving the feathers through the air mass as the wing is raised after the powerful downward thrust.  The smooth plane of the secondaries provides lift and allows the bird to rise in the air column.

Lift and drag

Bird flight fascinates me.  The wing is both an instrument of power and a specialized surface that generates the lift that carries them aloft.

Long wings with a huge surface area -- Brown Pelicans are superb at dynamic soaring.

Long wings with a huge surface area — Brown Pelicans are superb at dynamic soaring.

The feathered surface of the elongated forearms is central to flight, and humans have tried to copy the principles of avian flight in designing aircraft.   Two forces govern the operation of the wing in generating flight:  lift provided by the differential air masses moving over the upper and lower surface of the wing and frictional drag induced by the bird’s body (and wing) moving through the air.  To optimize flight, birds need to maximize lift, while minimizing drag.

Some examples of how this is accomplished.  Birds can reduce drag by presenting as little body surface to the airstream as possible, as illustrated by the flattened profile of an Anhinga, with its outstretched neck in perfect alignment with its body and tail.  Airflow is directed rapidly over the relatively flat upper surface of the bird, reducing air pressure there, and causing the body to rise.

cormorant in flight

Ducks, Geese, Swans, and Cranes adopt the same strategy (flying with outstretched neck) to minimize frontal drag, but long-necked wading birds (Herons and Egrets) double-up their necks while flying.  This doesn’t seem as aerodynamically efficient, but these species don’t tend to fly long distances, anyway.

Great Egret or American Egret

Great Egret or American Egret.  Photo taken at Bayou Segnette Park in New Orleans.

Louisiana Heron at take-off.  Photo taken at Bayou Segnette Park in New Orleans.

Louisiana Heron at take-off. Photo taken at Bayou Segnette Park in New Orleans.

Lift is directly related to wing surface area — the more area, the greater the lifting power.  Long, wide wings make the best lifters, as seen in hawks, eagles, vultures, pelicans, albatross, etc.

Long, wide wings of this juvenile Red-tailed Hawk are ideal for riding thermal air currents.

Long, wide wings of this juvenile Red-tailed Hawk are ideal for riding thermal air currents.

Adaptable wings serve a variety of purposes in osprey:  they can soar in circles over water while hunting, make rapid course changes, even hover briefly as they put down their feet to grab a fish.

Adaptable wings serve a variety of purposes in osprey: they can soar in circles over water while hunting, make rapid course changes, even hover briefly as they put down their feet to grab a fish.  Photo by co-blogger Alison.

But big, long wings dictate slow, coursing flight, and some birds need to get there faster or make quick changes in direction as they chase prey or evade predators.  Most song birds, pigeons and doves, parrots, and a few raptorial birds like Accipter hawks and Falcons have short, round (elliptical) wings that enable high maneuverability and agility while flying.  Lift is generated by the wing as it forces the air mass under it down. Pectoral muscle contractions generate the power both to move forward and to move up in the air column.

Elliptical wing shape of the Monk Parakeet (seen in downtown New Orleans)

Elliptical wing shape of the Monk Parakeet (seen in downtown New Orleans).

The king of speed (Peregrine Falcon) has the short, swept-back wings of a fighter plane.  Its flat flight profile reduces drag, but the key to its success is its short but powerful wings that allow the bird to change direction rapidly as it chases its prey.

Powerful down strokes of the wings propel the bird through air horizontally, like a oars on a rowboat.  But when the bird folds its wings next to its body in head-downward flight, it can reach speeds up to 200 mph.

Powerful down-strokes of the wings propel the bird through air horizontally, like oars on a rowboat. But when the bird folds its wings next to its body in head-downward flight, it can reach speeds up to 200 mph.  Photo by Mike Baird from Wikipedia

And how about the ability remain stationary in mid-air or to even fly backward?

Male Anna's Hummingbird from Berkeley Botanic Garden, November 2012

Male Anna’s Hummingbird taken at the Berkeley Botanic Garden, November 2012.

Hummingbird wings move in a figure-eight pattern back and forth above the plane of its body, and the mobile shoulder allows the wing to rotate so that it presses down on the air mass both on the down-stroke and on the up-stroke of the wing beat.  So, unlike other birds that develop almost 100% of their lift during a downward wing flap, hummingbirds can generate up to 25% of their lift during the up-stroke was well.

For some really spectacular photos for birds in flight, please check out Phil Lanoue’s Photography (e.g., October flights).  At the end of each month, Phil showcases some of the dramatic photos of airborne birds he has shot that month.

A morning visitor

Sitting on the porch reading my book this morning, I just barely caught some activity out of the corner of my eye.  A big, juvenile Red-tailed Hawk landed near the wildflower garden and looked like it had caught something.  Usually when hawks catch prey, they “mantle” (spread) their wings out around the prey, so that other predators can’t see what they are doing/eating.

I know it’s a juvenile (young of the year) Red-tailed Hawk because it doesn’t have a red tail, and it has pale eyes.  The red tail will grow in next spring and the eyes gradually become darker brown in the adult.

This is a widespread and very common hawk that occupies a wide variety of habitats from western Alaska through Canada and the U.S. and on down to Mexico and Central America.  There are local races of the species that differ slightly in their plumage characteristics, some darker, some lighter, some banded, etc.  The striped breast feathers of this juvenile can be characteristic of the species, although some individuals have a completely white breast.

One subspecies of Red-tailed Hawk (Krider’s Red-tail) is a spectacular looking pale, whitish hawk of the Great Plaines states.  You can read more about this handsome bird here.

(photo from http://www.wildlifesciencecenter.org/red-tail-hawk-kriiders/)

Red-tails can nab a wide variety of prey, ranging from small mice to large rat snakes and even gray squirrels (rarely).  You often see them perched on power or light poles along highways, scanning the grassy roadside for small mammal activity.  Their binocular vision enables them to see detailed images with much greater magnification than our eyes do.

Having finished its meal or its sunbath, whatever it was doing on the ground, the hawk flew up to a perch and gave itself a thorough cleaning, posing nicely for more photos.  There is a characteristic white patch of feathers on the back of the head, called an occipital spot.