Saturday, April 09, 2005

Blakeman on the Proliferation of CP Redtails

Donna Browne sent John Blakeman a report of some of the other redtail pairs making nesting attempts around Central Park and its periphery -- the Trump-Parc pair that seems to be incubating eggs high on that building on Central Park Sout, the 97th Street pair that suxccessfully nested in a tree in mid-park last year, the Cathedral of St. John the Divine pair [technically not CP, but near enough] and several other poissible nestings. Here is John's response : At the very end I have one more comment in response to his letter.

Thanks for the updates on the "other" CP red-tails. As you pointed out, the topic of the other CP pairs has not been very publicly addressed. As a raptor biologist I think the CP red-tail story must include all of the red-tails in and near Central Park. As remarkable as Pale Male is, he's no longer the only story. The developing, greater story is how the species itself is colonizing Central Park, and that involves a host of related phenomena, including (as always) prey availability, hunting competition, territory definition and defense, post-fledging hunting experiences for the new eyasses, migration away from NYC, and a bunch of others. It's a classic population biology problem, no longer just the study of a single pair.

I'm not suggesting that continued attention shouldn't be paid to the 927 pair. It should be. But the effects of adjacent pairs and other unmated and immature birds ("floaters") all competing for the same food and nesting territories is the real story. You can see, then, why I'm so interested in learning about the other pairs, too.

I find interesting the several accounts of other pairs failing in first and second nesting attempts, just as Pale Male did. This frequently happens in wild rural pairs, especially, I think, when the involved birds are young and inexperienced, just as in CP. What, then, will be the nesting success rates when there is a larger experienced resident adult population? Right now, except for Pale Male and Lola, all the birds trying to nest are just two or three years old. Things will get interesting when these birds get into their fourth and fifth years. I'm wondering if they won't then take on more typical rural territory sizes and be less accommodative (dismissive?) of nearby adjacent pairs and floaters. Will a population of older adults competing among themselves be less tolerant of either interloping young or nearby adults?

Conversely, is the CP red-tail environment so favorable that when some sort of population equilibrium is attained the birds will be -- as they appear to be now -- a bit social? How will this sort out as the Central Park red-tailed hawk population ages into maturity, when the age curve of the entire population has the shape of established wild rural ones? In the wild, aging adults predominate. Not yet so in Central Park. Right now, the population is bloated with young. Pale Male is the only bird that's been around and knows the entire Central Park score. (And he reads it like a great conductor, from memory, never missing a cue.) The others are still feeling their way through it all.

Wish I lived in New York, so I could watch this first hand. (Well, actually I wouldn't wish to live in NYC - it's an otherwise foreign environment, far too hectic, space-confined, and fast for my laid-back approaches to too many things. But Central Park and its hawks, those I can relate to.) So please be my biological eyes, as you have been. Thanks much.


John A. Blakeman

My additional comment:

John notes a parallel between Pale Male's two unsuccessful nesting attempts and the failures of some of the other CP redtails. But as I see it, their main problems in years past have been the absence of spikes to hold down the twigs at the various ledges they have chosen for their attempts. There were three
unsuccessful nesting attempts in a row on a high ledge on the Annenburg Building of Mt. Sinai hospital. [So these birds must have been older than 2 or 3 years. Don't forget that if, by any chance, these are Pale Male offspring, the first brood fledged in 1995!] The ledge had no spikes, needless to say. In the spring of the 4th year a successful nesting was observed in mid-park -- at 97th Street -- in a tree. Now the 97th St. nest was directly to the west of Mt. Sinai. It is logical to conclude that the Mt. Sinai pair finally gave up and moved to a tree a bit to the west. So their failure was not due to immaturity, as Pale Male's failures in 1993 and 1994 probably were, but were due to the spikelessness of their chosen ledge.

Similarly, the failure during the last two [or was it 3] years of the Trump- Parc pair might be attributed to the smooth ledge they have chosen, rather than to immaturity.

Friday, April 08, 2005

Blakeman replies to Watson's defence of Kestrels

Blakeman replies to Steve Watson's letter about Kestrels and their diligence at egg-sitting:

Steve Watson is quite correct about kestrel tiercels being equally good at incubating, compared to the females. I intended to refer only to red-tails. The males of kestrels and other falcons are very accommodating and accomplished incubators. Kestrels were the first falcons to be bred consistently in captivity, and their reliable incubation was noted early on. This prompted captive breeding efforts of peregrines, which likewise incubated well in captivity.
And I think that some tiercel red-tails are rather eager incubators, too. But in general, they tend not to be as enthusiastic about sitting as the females. Different species, different traits.
Steve's kestrel nestbox camera is providing views seldom seen by anyone of this otherwise common species. A wonderful site.

John A. Blakeman

Thursday, April 07, 2005

Let's hear it for the males -4/7/05

Steve Watson, my California correspondent who has sent in his Great Horned Owl photos, and who has an amazing kestrel webcam near his house
[] responds to one of Blakeman's posts:

Just as a comment...I take good-natured exception to John B.'s comments about male hawks not doing quite as good a job as females at incubating! :) Our kestrels (now named Dash and Lilly, btw) seem to very nearly share equally incubation duty, and the male seems just as at ease settling on the eggs, fluffing his feathers over them, turning them, etc. He often sits on them for several hours at a time, perfectly poised.

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

Tuesday, April 05, 2005

Blakeman on Redtails in the Rain - Q&A

Bob Brooks, a Central Park birdwatcher, sent in the following question:

Is there any truth to the following statement? A nature writer from Maine wrote it.

"I've noticed, for example, that squirrels feed en masse on rainy and drizzly days, making me wonder if they perceive that hawks, being so soft-feathered, aren't apt to be hunting in wet weather."

Here is what John Blakeman answered:

No, I don't think squirrels are instinctive about feeding safely en masse on rainy days, to lessen hawk attacks. Any squirrel that feeds with this presumption (on the thought that squirrels actually cerebrally ponder such questions -- they don't) is likely to be lethally surprised. Red-tailed hawks hunt very effectively in wet weather. They may have "soft" feathers, but they preen and oil them diligently and the birds are seldom "soaked." In light rain, red-tails have no predatory inhibitions whatsoever. In such conditions water rolls off their feathers as it does on a duck.

Only in episodic drenching, windy downpours will a red-tail's feathers get saturated. When this happens, the hawk is often seen sitting after the rainstorm with its wings slightly open and outstretched. This tends to occur, I think, most often in first year birds. Old adults have enough sense to keep oiled and preened, just as Lola did during New York's recent bout of heavy weather.

Actually, woodland squirrels are seldom attacked by red-tails. Hawks don't express much selective power on squirrel populations Squirrels are not a preferred food. Their skin is very tough, and the big rodents can bite very severely. Red-tails surely take squirrels, as is so often seen with the Central Park hawks. But in larger, more typical rural red-tail territories, bushy-tails are taken only infrequently. From my experiences of watching my trained falconry red-tails hunt in rainy weather, I think any squirrel that believes it's safer out on wet days is a bit deluded. Sounds like a bit of typical hawk (or squirrel) mythology -- of which there is still a preponderance.

Speaking of which, I could quickly "de-ingratiate" myself to many here were I to debunk the most common bit of popular hawk mythology, one believed by even many professionals who should know better. It's the question of whether or not wild hawks control vermin. The discussion on this would surely get pretty heated -- but who here would let emotion get in the way of ecological truth? Anyone interested?


John A. Blakeman