Bird Feeder Week Part 2: Avian Thermoregulation, And A Big Tangent About Sex (determination)

As regular readers of this blog know, I am a complete Cold Wuss. Although I make the best of our snowy Utah winters, I always have thoughts of hot, sunny summer days in the back of my mind as I shiver away the dark months. That’s why I’m totally in awe of birds that don’t fly South for the Winter. There they are, all winter long at the feeder, outside in sub 20F or colder temps, tiny little things, apparently doing just fine. How do they do it?

But before we answer that- why do they do it? Why don’t they fly South, like any sane bird does?

But that leads to an even bigger question, and that is why birds migrate in the first place? Seriously, long-distance migration is a monumental undertaking. Every Spring, millions and millions of birds fly thousands of miles North. They hang out, mate, hatch a few chicks, then come Fall turn around and fly thousands of miles South to stay ahead of the cold. The migrations are long, dangerous, and require tremendous preparation, risk and effort. Huge numbers of birds fail to complete these migrations (and failure here generally means death.) Any way you look at it, long-distance migration is a huge dick-dance. Why do it? Why not just stay in the tropics year-round, and instead of screwing around flying North, spend one’s time doing important bird-things, like foraging, nest-building, courting, mating and raising chicks?

Because the pay-off is a reproductive home run. The Northern climes may only be habitable for 4 or 6 months or the year, but during those months life explodes to make up for the short living year. And that explosion is a bonanza of seeds, fruit, bugs and worms for the birds who can reach it. The plentitude of food resources, combined with lots of open nesting/foraging space (something sorely lacking in the tropics, due largely to intense competition) gives migrating birds a set of caloric resources that enable successful chick-rearing on a level they could never dream of realizing if they stayed home in Columbia.

Migration, difficult as it is, has been a winning formula for countless thousands of bird species for millions of years. Migrating birds tend to funnel into common “flyways” that utilize favorable geography, winds and water resources.

The birds that stay behind- the Finches, Goldfinches, Chickadees, Juncos and many others- are the cold-climate specialists, those that followed an evolutionary path that allowed them to thrive in a climate at the margins, a climate that thins out competitors for resources. In this respect, they’re analogous to Polar Bears among the Ursids (bears), or even the Inuit among pre-technology humans. They figured out how to make a decent living where others couldn’t, and have thrived as a result.

OK, so now back to the original question- how do they do it?

In the last Bird Feed Week post we looked at the parallel rise of both birds and mammals. One of the interesting things about this rise is that birds and mammals have often developed different mechanisms for solving the same problems. Both have developed an insulating outer covering- hair for us, feathers for birds.

Tangent: And both make great coats. Though I’ve never owned a fur coat, here I am modeling my favorite Christmas gift: the Puffy Jacket, filled with goose down. This thing is so warm I break a sweat just thinking about it. I can’t wait for my next winter camping trip. My days of shivering are at an end. (And don’t I look sharp in it?)

Both have developed genes to enable and manage color vision, with birds building upon and enhancing their reptilian chromatic legacy, while we primates have re-evolved the ability via a completely different set of genes, following our long, evolutionary “night” of nocturnalcy (“Nocturnal-ness”? “Nocturnaltude”?), which we talked about in this post. Both birds and mammals have evolved distinct genetic mechanisms for sex determination, a trait lacking in many reptiles.

Big Tangent About Sex (determination)

Tangent: The whole subject of sex determination in mammals, birds, reptiles and insects is mind-blowingly fascinating. Here’s the short version:

In most mammals, including us, sex is determined chromosomally. If we have 2 “X” chromosomes , we grow up female. If we have 1 “X” and 1 “Y”, we grow up male. And of course since females have no “Y” chromosomes, it’s our father’s genetic contribution that determines whether we turn out male or female.

Nested Tangent: The specific gene on the “Y” chromosome that makes us male is called the SRY gene. If this gene is missing or damaged or otherwise non-functional, we turn out female anyway, even if we have an XY combination. In this sense, female is the “default” or “normal” state of a human being. Males, with their almost-freakish strength, size and aggression, are the result of that one gene’s effects upon the otherwise “normal”/female embryo.

In birds, 2 very different chromosomes, “W” and “Z”, determine sex. But here, the mixed version, WZ, is female, while the “same” or homogametic variant, ZZ, is male. And since males have no “W” chromosomes, it’s the mother’s contribution that determines sex.

Nested Tangent Revisited: Whether some gene on the “W” chromosome imposes “female-ness” on a default male state, or whether the double “Z” combo somehow imposes “males-ness” on a default female state is as yet unknown.

In reptiles, sex determination is all over the place. In many reptiles, alligators being a classic example, sex is determined by the temperature at which the eggs are incubated. In other cases, such as the Green Iguana, sex is determined genetically via the mammalian-style XY system. But in yet others, such as snakes, sex is determined by the WZ system.

With insects it’s different still- Hymenopteran insects (bees, ants, wasps) use the haploid-diploid chromosomal mechanism I explained when we looked at Honeybees, and yet other insects use an “XX/X0” system, where 2 “X”’s make a female, 1 “X” makes a male, and there’s no such thing as a “Y”. Got it?

This whole topic of course leads to the fascinating question of why all these animals have 2 sexes in the first place, but this tangent’s gone on long enough…

Anywho, back to birds in Winter, and the point I was trying to get to, which is that birds and mammals have also tackled the challenge of thermogenesis (heat-generation) differently.

2 things mammals do to get through the Winter is grow more hair and more fat. Birds do both of these things as well; the American Goldfinch, Carduelis tristis, (pic left) increases it’s feather mass by 50% in winter. Birds also add fat. The Dark-eyed Juncos, Junco hyemalis, (pic below, right) in my yard carry an average of 14% more fat right now than in summertime, but there’s a very practical limit to how much fat you can pack on when you’re in the business of flying.

But there’s a 3^rd trick we mammals have that birds lack. Way back in September, when we got that first early snow and I went on about how unbelievably cold I was, I explained the 2 mechanisms we mammals have for making heat. First is shivering, a strictly temporary/emergency measure designed to crank up our core heat quickly for a short period. But the more effective, longer-term method of thermogenesis we employ is brown fat, a specialized type of fat that is specially optimized for heat generation, the chemistry of which I explained in this post.

Birds never evolved brown fat. And what this means to a Winter bird such as a House Finch or a Crow or a Magpie is this: when they are not flying, they are shivering pretty much all the time. This probably isn’t as miserable for the birds as it sounds. Shivering for birds isn’t a quickie-emergency fix; it’s something they’ve evolved to do for extended periods, and they’re able to shiver far more efficiently than we can for much longer periods.

Tangent: Another thing birds can’t do as well as mammals is change posture. People, dogs, cats and squirrels can all curl up to conserve heat. Birds can tuck their bills down a bit, or maybe hide their head under a wing, but that’s pretty much it.

Birds have other tricks, the most obvious of which is the fluffing of feathers, which creates air pockets that help improve the insulating capacity of their feathers. And one bird at my feeder has developed a really neat trick: controlled hypothermia.

Unlike the other birds at my feeder right now, the Black-Capped Chickadee, Peocile atricapilla, doesn’t shiver nearly continuously. Instead it repeatedly allows its internal body temperature to decline a few degrees, and then at regular intervals, shivers vigorously to bring it back up a bit. Chickadees exercise this ability most often at night, lowering their body temps by as much as 20F, which provides estimated energy savings of up to 50%. (And in fact Chickadees lower their body temp nocturnally year-round, including on cool summer nights.)

By way of comparison, if a human’s body temp is lowered by just 9F, they’re in stage 3 hypothermia. That’s the stage just before death, from which it’s pretty much impossible to emerge without external heat/assistance. But the Chickadee pulls off this amazing trick routinely throughout the Winter, apparently without even thinking about it.

Next Up: The star of my winter feeder…