Wednesday, March 09, 2005

Red-tail Sleep and Temperature Control

Red-tail Sleep and Temperature Control

I asked John Blakeman for some information about how and where Red-tailed Hawks sleep, and about their night roosts. Here is a part of his answer. [I'll post another part soon.]

First, how does my falconry red-tail Savanna spend her nights? She stays out doors year round, especially in the winter. Red-tails are a bit put off with room-temperature warmth in the cold seasons. After all, they wear a remarkably effective down coat year-round. Beneath the outer covert feathers, the ones we see, is a thick layer of the finest, insulating down. This is what keeps the birds warm. The outer covert feathers are oiled daily by diligent preening, and this repels water rather well. The down feathers retains heat. To be kept indoors all winter is somewhat like wearing a thick winter coat indoors.

Actually, the bird could do this. It can be parked indoors at room temperature with little problem. Red-tails, along with other similar hawks, have a remarkable way of coping with temperature changes, which also often relate to food availability.

Red-tailed hawks are "homeotherms," otherwise known as "warm-blooded," like all mammals and birds. But unlike humans, who tend to keep body temperatures narrowly near 37 degrees Celsius (the classic 98.6 degrees Fahrenheit), red-tail body temperature can vary greatly, which allows them to sleep as they do, changing body temperatures with the weather and body food reserves.

In my undergraduate years at Bowling Green State University (the Falcons, where I kept the university mascot), the Ohio Division of Wildlife donated a wing-injured red-tail that was permanently disabled. I was able to "man" it, to "tame" it, for some body temperature research BGSU's ornithology professor, Dr. Eldon Martin, wanted to investigate. He, too, was interested in the maintenance of body temperature as weather changed. He had a graduate student talented in electronics. This fellow devised a peanut-sized radio transmitter that could be surgically embedded in the abdominal cavity of a large bird. This was long before modern digital electronics. This little device could transmit data for only a few days, and the receiving antenna had to be just a foot or so away from the bird.

The transmitter broadcast frequencies of beeps that varied by temperature. Dr. Martin surgically implanted the transmitter in the body of the red-tail, which was housed inside a climate-controlled environmental chamber, a large room. Daily seasonal temperature regimes were created, and the results were unexpected.

In summary, deep body temperature varied by as much as 10 degrees Fahrenheit. When highly active, the bird's temperature would rise to 105 degrees or more. But at night, particularly in cold weather temperatures, body temperature would drop to the low 90s. I believe that this was one of the first studies to show such wide temperature variation in hawks. It is known in other birds, but this settled it for red-tails.

This means that when sleeping, red-tails can turn down the furnace, conserving fuel during the inactive night period. In winter, at New York and Ohio latitudes, night periods can approach 16 hours.

But back to the question, where do the birds roost? Here's one place. On the way to school each morning, I used to watch resident pairs I knew of. There were several tall utility poles way out in the middle of several hundred acres of row-crop fields, upon which I frequently saw a red-tail perched. This was just at dawn. The bird surely had taken the perch the previous evening. I saw this hundreds of times, and was able to discern that red-tails will never perch out on these poles when windy weather might be approaching. The birds know what the next 16 hrs of winter weather is going to be when they elect to spend the night on those poles.

So where were they at night during inhospitable weather? I don't know. I have never seen a wild red-tail roosting as you people so frequently have done in Central Park. I simply presumed that the birds fly into a woodlot and park themselves on the downwind side of a tree trunk. For me, your Central Park observations are new and helpful.

One other consideration, one that presently isn't a concern in Central Park. In rural areas, the great-horned owl occupies exactly the same habitats as the red-tail. This owl is larger and even more muscular. It can kill anything it chooses to attack, including skunks, house cats, and other very large prey. When banding young at a great-horned owl's nest, one must wear a leather jacket, helmet, and probably a face guard. The bird will almost always attack with fury.

The bird could therefore easily dispatch a sleeping red-tail. Red-tails seem to be able to see at night just about as well as humans. Owls, of course, see well, and a sleeping red-tail would appear to be a tempting meal vulnerably sitting on a pole or branch.

When sleeping, red-tails turn their heads back over the shoulder and bury it in the feathers of the back. They look utterly decapitated. They certainly can't see any approaching hungry great-horned owl.

But great-horns rarely, if ever, kill sleeping red-tails. They seem to abide their nocturnal presence. This is probably an evolutionary outcome. Great-horns are incapable of building a nest. Most frequently, they expropriate a recent nest of a local red-tail, forcing the red-tail to build a new nest. And this relates to what we've seen with the 927 nest. This is why I was not overly concerned about the "disruption" at the site. Many rural red-tails have their nests taken over by great-horned owls every year, and they have to build new ones. Red-tails are genetically programmed to do this. Those that weren't able to re-nest in new, adjacent locations, simply didn't reproduce and their genes were lost. Established red-tail pairs such as Pale Male and Lola can withstand a great deal of nest disruption, probably a result of the ecological interactions of great-horned owls over many millennia. The ecological interplay great-horned owls and red-tailed hawks is a close and delicate one. Conversely, a red-tail could easily snatch a young owl from a nest during any unprotected moment in the daytime, just as our birds pluck pigeons out of Central Park. But red-tails seldom pester the owls. There is a continuing, delicate detente between the species.