A Varied Thrush has been hanging around the yard for the last week or two, and possibly has been here for at least 3 weeks. It appears to be a male. I check for him every morning and spend a few minutes watching him if I have time before I go to work. Watching birds in winter always gets me thinking about the physics of winter survival.



The Varied Thrush is almost always hanging around a small cluster of trees and tall shrubs. The cluster includes a Ponderosa Pine, several junipers, a tall, dense Mock Orange bush, a dying crabapple, several Chinese (or Siberian?) elms, and a healthy Macintosh-type apple tree. This last year was a good one for apples. The Macintosh produced a heavy crop and most of its apples are now on the ground. I've tossed branches and other debris in a few places under the trees to create brush piles.



The thrush appears to have a regular routine. When I see him, he is usually under the shadows of the trees and shrubs. He pecks away at a selected apple for five or ten minutes, spends a few minutes kicking leaves around, presumably looking for bugs, then perhaps pecks at the apple a little longer than disappears for a few minutes, presumably to digest food. If I wait, he shows up again to eat more apple, kick around a few more leaves, etc. Today, after one of his eating episodes, I saw him sitting motionless on the ground among the fallen leaves under a low-hanging juniper branch. When he's not moving, he is difficult to see in the shadows among the mix of gray, brown, and orangey leaves and red and yellow apples.



Apples seem to be the "bread" in his diet and the bugs the "butter." He bulks up on apples, searches for a few bugs for dessert, then naps. Then repeats. He conduct all these activities within an area of a few square feet. He forages mostly on the ground, but scurries under the brush piles or flies up into the junipers when something startles him.



His cozy little thrush haven has one big drawback: It's on the Palouse. So far this winter, temperatures have climbed above freezing on most days and lately, they've even been above freezing or barely below freezing at night. So, the apples next the ground, especially those with an insulating blanket of leaves around them, are unfrozen for at least part of the day.



Water has an unusually high latent heat of melting. (Remember that phrase from that high school physics class you napped through?) It takes 80 calories to convert one gram of ice at 0 C (32 F) to liquid water at 0 C. It takes 1 cal/gram to raise the temperature of liquid water one degree Celsius. To raise the temperature of liquid water at 0 C to thrush body temperature of approximately 40 C thus takes about 40 calories. To raise the temperature of ice at 0 C to thrush body temperature takes 80 cal (to melt the ice) plus 40 calories (to heat the water to body temperature), or a total of about 120 calories/gram. So, it takes about 3 times as much energy to warm ice to body temperature as cold liquid water.



I got to wondering whether the caloric value of apple tissue would be enough to pay the cost of heating the water in the apple if the apple were frozen. I didn't do a real experiment with a real apple and a bomb calorimeter. I did a little lazy web research and a few back of the envelope calculations. According to my brief web search, a typical apple has about 116 g of water and about 8100 calories. (Note to those gasping in disbelief. The "calories" on a typical human diet chart are really kilocalories, or calories time 100. Apples are usually listed as having 80 or so "calories" which means they have about 8000 "real" calories.)



(Is anyone still reading this?)



Anyway, that means that, for every piece of apple that contains a gram of water there is about 67 calories. If the apple is very, but not yet frozen, the bird will spend about 40 of the calories in the apple warming the apple bite to body temperature. He will still have a net caloric gain. In contrast, if the bird eats a bite of frozen apple, it would take 120 calories to melt the water in the apple and heat it to body temperature. That's a losing proposition; it would take nearly twice as many calories to process a frozen apple bite as the bird gets from the apple.



Now, of course, things aren't so simple. The sugar in the apple lowers the freezing temperature, so the apple will not freeze until the temperature drops a few degrees below 0 C. Also, as the cells rupture in the apple from freezing and thawing, water leaks out. Dry conditions may also cause sublimation of water from the apple. The apples laying on the ground for a while likely have a higher sugar content and lower water content than my imaginary fresh apple. Still, there has to come a temperature when the caloric economics of eating apples, or any fruit with a substantial amount of frozen water, don't "pencil out" for the thrush.



On average, the coldest temperatures on the Palouse come during the last week of January and the first week of February. So, my thrush still has six weeks to go before temperatures turn around and begin rising again. There will be times on the Palouse when the apples on the ground will likely be frozen for days on end or the ground will be covered by too much snow for the thrush to easily clear away. The thrush will have to try to find enough seeds (which usually have little water and a high fat content) or bugs, or it might try to huddle in the junipers and tough it out on stored fat. Birds can't store much fat because of the rigid weight restrictions of flight, so my thrush probably can't live very many days without food. His last option is to move down into the warmer canyons or into the warmer part of the Basin, where he will face more competition and more predators.



What prompts him, I wonder, to pack up and move? Does he leave at the first deep snow? Does he wait until he has suffered several days without much food, at which point he will be in a poor condition to travel?



There's a lot about birds we don't know.



Kelly Cassidy

Pullman