Thursday, April 07, 2005

More on Sexual Dimorphism -- Donna's Q's and John's A's

On 3/24/2005 Donna Browne sent off a few questions to John Blakeman, Ohio red-tail expert and falconer. As on a few previous occasions [saee archives] she was still trying to fathom the mysterious disparity of sizes between male[tiercel] and female [hen] raptors: the males are considerably smaller than the females.
Hi John,

In the possible advantage of Small Dads category, do
tiercels need less food than the larger hens year

Does anyone know if the tiercel eats only after prey
for the hen has been bagged, or divides a kill or eats
his fill then hunts for her?

As the tiercel's activity level is higher during
nesting does he need more calories than the mostly
sedentary hen...and if he were bigger, even more?

Donna Browne

A few weeks later Blakeman answered:

As always, good, incisive questions.

No. 1, do tiercels require less food? This is so, year round. But it's not a significant factor. In large falcons such as the peregrine, where the tiercel weighs a third less than the falcon, there is a significant difference in daily food requirements. But no so much in red-tails. Tiercels are indeed smaller than hens, but not by a significant amount. A red-tail tiercel can typically weigh 1100 grams, a female tips the balance at, say, 1500 grams. Although this seems to approach the one-third size reduction in peregrine males, this 400 gram difference is essentially much less. That's because the red-tail is a much larger bird, so the surface to volume ratio of both sexes of this larger species is smaller. As falconers who keep and feed both male and female red-tails know, the big females do eat more than the slightly smaller males, but not much less.

And in fact, the smaller size of the male actually requires more frequent feeding experiences, although less actual food over time. (Hmm? I am getting to something I've never thought of. Is this a real selective advantage of big sedentary incubating females over the smaller males? We may be on to something here. But I get ahead of myself.)

During the training and hunting season falconers must carefully weigh their birds each day, keeping minute track of their birds' weight changes. A hawk only hunts when it's motivated by hunger (or in the case of a spring male, it's motivated by desire to feed its mate and young eyasses). A falconer who tries to fly an over-weight hawk is likely to find the bird flying off to a tree where it sits the night through, utterly unresponsive to the falconer's food enticements. (Now don't anyone misunderstand this. Falconers never, ever starve their birds to motivate them to hunt. A weak, starving bird can't hunt, and will completely disregard a falconer's caring efforts. The bird will fly off and never be seen again. Why should it voluntarily return to the falconer's care? Falconers carefully feed their birds to maintain them at the height of athletic tone -- never too fat, and never too thin. Falconers know the proper weights of their birds, and consequently also experience the results of over- or under-feeding.)

Back to the story. Falconers know that it can take an inordinate period of time to bring down the weight of a fat red-tail. Well-fed fed-tails can go for many days, probably 10 to 20 days without food before experiencing lasting health problems. This is a trait of all of the large raptors. Ancient falconers in Japan are known to have fasted their Japanese hawk-eagles for 45 days before the hunting season. These large raptors (perhaps twice the size of red-tailed hawks), like all eagles, can live without food for extended periods of time on accumulated body fat. The red-tail can do this, too, but not for a month and a half.

Could this be a factor? Because of their larger body fat reserves, big females could sit on eggs without being fed for several days, if not a week or more. Larger females can have longer periods of fasting than the smaller males. This might be a tipping factor during really bad spring weather events. The male, too, has to feed himself during episodes of bad weather, but he's free to move around and search for food. The incubating parent is confined to the nest. If the smaller male had to sit during a week of bad weather (a heavy March snow storm that covers all of the vole runways), he'd be off the eggs looking for food before a female would be. Over time, is this the deciding factor? (Of course, that works for New York or Ohio, where early spring is often not so spring-like. This mechanism doesn't seem to apply, however, to the American South or Southwest -- where females also do most of the incubating.)

Oh, well.

No. 2. Is the male altruistic in putting the welfare of his mate ahead of his own? Does he eat first, then give the rest to his mate? Actually, it appears that he usually separates and offers food to the sitting female before he eats any himself. This, however, may more reflect the ample food resources of his territory than it does any putative altruism. The key is not the male's good natured put-the wife-and-kids-first attitude, but rather the fact that he's previously chosen a territory with so much food that who eats first is not a concern. There's enough for everyone.

If a male consistently eats before his mate or eyasses, there is trouble in his territory. If there aren't enough prey animals each day to feed everyone, the entire social structure of the breeding family breaks down. The breeding pair becomes reproductively dysfunctional, alluding again to my contention that the availability of ample prey is the foundation of everything red-tail. If there are enough prey animals, who eats first is not a question that arises, as it makes no difference. The tiercel may, indeed, eat first. But he can then just go take another vole (or in CP a pigeon) and offer it up.

No. 3. Because the male is out on the wing hunting during incubation, does he require more food than the sedentary female? Proportionately, yes, but effectively, no. Unlike peregrines and accipiters, both of which approach a form of avian hyperactivity, red-tails actually don't expend increased amounts of energy when flying. First, red-tails spend a great deal of time sitting, watching everything below. When they take flight, as we all know and love, they expertly use the wind to their soaring advantage. Because they are such large raptors, red-tails must conserve energy when flying and hunting. For brief episodes, red-tails can expend great energies flying after and killing large prey. They are very muscular and capable of great effort. But they must reserve these events to isolated, infrequent occurrences. Most of the time, red-tails either sit for long periods, or fly by adroitly playing the winds to reduce energy expenditures. Just watch Pale Male and Lola as they take flight advantages of the winds swirling around the buildings of Fifth Ave.

Altogether, I know of no evidence that suggests that the elevated hunting activities of the male during incubation require any significant additional food. It does mean that he must spend more of the day hunting, instead of just sitting around for most of the morning to preen and loaf, as both birds do in the last half of the summer when the kids are kicked out of the house (or territory).

Again, the size differences between the sexes doesn't seem to be a deciding factor related to hunting and food resource restrictions. Except for a week's long heavy snow storm (as mentioned above), it still appears that a male could just as easily incubate as the big female. Back to square one.


John A. Blakeman