Saturday, April 16, 2005

Blakeman: I'll answer questions soon

More really fine queries. But I'm catching up on lots of paperwork. . . I'll try to have responses in a day or two. The observations on hawk wing aerodynamics are important and interesting.
I drove past a local red-tail nest today on returning from a prairie design workshop, but didn't have time to even stop. Nonetheless, a parent was standing on the rim of the nest leaning over it. I'm certain that it was feeding a newly hatched eyass or two. Good stuff.
And I know everyone will become ecstatic when the 927 eggs hatch. These good questions may have to wait until things settle down, should an eyass appear this weekend. I'll try to get something in as soon as I can.
And I want to counter your rejoinder. But later.

John A. Blakeman

Friday, April 15, 2005

Blakeman on Progeny -- a retreat of sorts

Below, Blakeman's new thoughts on the progeny question. At the end I add a small rejoinder of my own, just to keep the conversation going.

You know that I went on at length back in December about how all the other red-tails in the Central Park area weren't likely to be Pale Male progeny. I expounded on how the adults will drive off the young to protect the larger adult hunting territory. Secondly, I noted that an incursion of Pale Male kids would promote mating of close kindred hawks, resulting in genetic problems.

But from your many descriptions of pale colored birds, and other cogent observations you and others have made, I've retreated from my former answer of rather standard field biology, that the kids shouldn't be there. It looks like they are, for whatever reason(s) and in whatever densities.
Now the question. Did the new NYC ledge-nesting red-tails select those sites because they were raised in and fledged from the 927 ledge nest? Or, might there be another explanation?

I really don't think the birds are nesting on ledges because they were hatched on ledges. As I mentioned before, little eyasses are rather stupid and involve their minute, developing brains with vocalizing, feedings, flapping wings, and defecating. Frankly, until an eyass can fly, it doesn't know at all what’s supporting the nest – nor does it care.

Why, then, the recent preponderance of ledge nests? I'm guessing that it involves the multitude of people under the trees that would be the conventional sites of red-tail nests. Even in the West, where big trees are available, trees are used over ledges. Red-tails prefer trees when they are available.

I've never elaborated upon the inordinate tolerance of humans, dogs, and who knows what other animal and mechanical disturbances the Central Park red-tails have endured. Out here in rural areas, a sitting red-tail is likely to abandon the nest and eggs when a single human walks within 100 or 200 yards of the nest. In Central Park, people are walking right near perching and nest trees. For overnight perching, the birds have learned to accommodate the multitude of strange bipeds (people) strutting hither and thither below.

But when it was time to build a nest, I think the presence of dogs and people might have been a bit over the edge. Trying to get a few sticks to lodge in the small crotch of a tree 40 ft up is no easy task. It requires full attention to the multiple construction tasks inherent in getting a secure nest erected. I'm guessing that the birds simply chose nest sites remote from any human experience, and those were way up on the sides of tall buildings. Up there, they could poke and probe and thrust and re-arrange sticks without any diversions.

One of the first anomalies of the 927 nest I noticed was not that it was on the side of a “cliff.” I've seen wild Western red-tails do that. But it was the inordinate height of the selected nest site. The western nests I recall observing were seldom much taller than tall trees, usually in the 75 to 100 ft range. Many were lower. I never recall seeing a red-tail nest either so high, or so close to the canyon rim (here, the roof line).

<>But no one can deny that both the 927 nest and the Trump-Parc nest are about as far from humans as physically possible. I think that’s the factor, the desired remoteness from humans and dogs. This might also involve a perceived nest predation threat on the part of these two-legged ground animals. Who, among red-tails, knows if humans couldn't just shinny up a park tree with a nest in it and take a pair of eggs for breakfast? Perhaps the hawks are building high, remote nests on ledges because it’s the only place in Manhattan where a sitting hawk can be confident that a human won't disturb its calm incubation. As urbanized and acclimated to The City as these hawks are, they may still retain a requirement for a bit of private remoteness, some wild solitude and security. The high ledges are the only places that provide these.

But as always, merely thoughts off the top of my head (reflected, however, from watching both wild and captive red-tails for 35 years or so).


John A. Blakeman

John: You note that the little eyasses are too stupid to take note of their ledge origins, and therefore are not likely to be drawn to ledge nests when they're ready to breed themselves. But would this be a matter of intelligence? Might it not involve instinct.?Just like the principle of philopatry brings phoebes and orioles back to their former nest locations by instinct, not intelligence, might there not be some instinctive drive on the part of these eyasses raised on a ledge to replicate their early experience?

Wednesday, April 13, 2005

Blakeman answers Donna about stick size

Let me summarize your good questions submitted to me about nest sticks (or twigs).
1. Why are only little sticks being used in NYC?
Frankly, I'm not sure it's so, although I first suggested this. It was only an hypothesis I related to a perceived (but errant) notion that Central Park had no dead trees or limbs from which large sticks could have been broken off. Marie corrected me on this. NCY red-tails have dead limbs to break off, and they have twigs and limbs of all sizes to use from other trees, too. It’s no longer fair for me to suggest the NYC nests are fundamentally different from other red-tail nests.
Except, to refresh my memories of field studies long ago (1970s), I looked up the fine photos of red-tail nests in G. Ronald Austing’s seminal 1964 “The World of the Red-tailed Hawk.” Ron Austing, another Ohioan in the Cincinnati area, became entranced with red-tails like the rest of us. He was (and is) a remarkable wildlife photographer. And like myself, he engaged in falconry with red-tails. Ron’s book, although written for a general audience, is still a major work on the species. The photography is remarkable, including the numerous telephoto shots of nests taken from carefully erected towers Ron placed next to nest trees.
It’s from these photos that I believe the wild RT nests tend to have slightly larger sticks than Pale Male’s. When I compare the unexcelled nest telephotos of Lincoln Karim with Ron Austing's photo on p. 50, the Ohio twigs do look larger than the NYC material. So I still contend that rural wild nests are made of slightly larger sticks. But whether that accounts for anything on NYC buildings is still an open question.
2. Do red-tails build nests similar to the ones they were fledged from?
Seems as though they do. But from me, that’s a useless statement because I've only seen conventional tree nests (except for two summers of cliff-nest studies in northern Nevada, where I could no watch any progeny build nests.) I think the species builds its nests primarily on instinct and materials available, not on anything learned or seen as an eyass. Eyasses are plainly stupid. They just sit there, vocalize, eat, defecate, and try to learn to fly. They don't intelligently peruse the building materials and techniques used by Mom and Pop. I think it’s all instinct.
3.Just how big are the "larger sticks" the Western Red-tails use?
I don't know. I failed to record their size when I studied those nests. At the time, it seemed to be a useless bit of information. What difference did it make? (Now, I know that it might be important, but I have no info. An unrecognized, lost opportunity.)
4. We've only seen Pale Male and Lola break twigs off trees, in their case live ones. Do your RT's take their bigger dead sticks ONLY off trees or have you ever seen them forage on the ground for them?
Another question for which I haven't the slightest answer. Frankly, in all the times I've spent looking at red-tails in the wild, I've never seen one pick up a stick off the ground, or even snap one off a tree, living or dead. And not many others have either – except you folks in Central Park where everything is concentrated in a small area, with lots of observant eyes. For me, the vast majority of the nests we studied were nests already in place, ones being reused or refurbished. Consequently only a few top-dressing sticks and lining were being added. New nests were built by new pairs, but because we were watching the nests in an area of about 600 sq miles, a new nest could go up in two or three days before it was discovered. To this day, I don't know where Ohio red-tails procure their nest materials. I see them bringing stuff to the nest, but I didn't see them collect it. An Ohio nest typically has a territory about twice the size of all of Central Park, or more.
5. Seems there are a number of variables here. There is only a certain weight of stick that a given Red-tail can carry. Perhaps only a certain diameter of dead stick that a Hawk can break off due to beak gape.

No, red-tails can carry, should they want to, a stick of remarkable size and weight. They can carry a 3/4-lb. squirrel into a tree. Even a half-pound stick would be rather large. It’s neither a weight nor size thing. It’s a question of how effective a stick will be in the nest. Red-tails have a big mouth and even much larger feet.
I've always wondered if red-tails tend to select sticks that are rough, with lots of jagged projections so as to intertwine when placed in a heap. The missing factor in all of this may not be size or weight at all. It might be the roughness or frictional characteristics of the sticks that count the most. A nest made from densely-spiked hawthorn twigs isn't likely to go anywhere unless placed upon a glassy surface. So perhaps we all need to pay attention also to stick roughness. Remember, unlike robins, red-tails do not cement their nests together. They poke sticks together in only a rudimentary fashion. The pile has to stay together pretty much by itself.
6. In regards to weight, different diameters of stick would vary depending on species (density), on moisture content(deadness), and diameter. Therefore even a big stick that was more "dead" would weigh the same as a smaller fresh one and therefore conceivably blow off. Or is there truly something about diameter as opposed to weight that would make them work better in nest building?
Larger diameter sticks, even if lighter, have more surface area rubbing on the supporting substrate (on ledges), so it takes a bigger gust of wind to blown the nest off. But this may be a very small factor. This is a mechanical engineer’s question.
In retrospect, my reference to the successes of western cliff nests may not be valid, compared to the ledges used at Central Park. The buildings of New York are almost entirely of worked, even polished stone, with low frictional qualities. The western nest ledges were rough or coarse, providing a stronger grip.
Once again, good questions – and rather fabricated answers
Pipping is not so far off. How many will it be this year? The waiting is nearly done.
John A. Blakeman

Monday, April 11, 2005

12/13/04 -- Blakeman's Concerns about Various proposals

Subject: Re: Rooftop nestbox won't work (Ohio redtail biologist)

Date: 12/13/2004


A NY Times reporter, "Jenny," just called and aske me a fine list of important questions, mostly related to the nesting information I sent you yesterday.

I think the questions and answers were helpful in getting things back to where we want them, without any unforeseen untoward consequences (such as a nest box).

Let me know how I can help in any other way.


John A. Blakeman

Subject: Re: Rooftop nestbox won't work (Ohio redtail biologist)

Date: 12/12/2004


Here are some considered, experience-based answers.

I'd have no concern about the construction of some structural device to retain either nest sticks, or even dropped meal leavings (although that would quickly become clogged with the tremendous quantity of leavings they produce). The birds are attached to the ledge, no matter what's on it, so long as it stays open (no box).

The reinstallation of the pigeon-repelling pins would be ideal, as sticks would once again be lodged between them. Any similar arrangement would work.

And here's an important point -- one that I wouldn't want opponents to misrepresent. The destruction of the nest appears to be catastrophic, but if the stick-holding metal pins can be reinstalled, all of this activity, as destructive as it might appear, will actually have increased the pair bond. The forced rebuilding of the nest is actually a good thing, something we see in wild pairs.

You recall that I stated that Eastern redtails seldom use a wild rural nest in consecutive years. They build a nest in a tree, raise a broad there, and then in the next year they select an alternate site a quarter mile away. The first nest was in perfect shape and needed only a nest-bowl refurbishing with the mere addition of a layer of some small fine layer stuff at the bottom. But instead of doing a slight rearrangement (equivalent to merely getting a new set of sheets), the birds simply abandon the nest and go down the road a ways and build an entirely new nest.

This requires a great deal of effort, pulling off sticks, carrying them to the new tree crotch, and then trying to get them to balance until a sufficient, self-linking pile has been placed there. But the psychological effects of this inordinate new nest-building effort actually strengthens the birds' pair bonding (their mutual "love," as it were) and this also apparently really gets the breeding hormones flowing. In short, do not be concerned that the nest destruction, by itself, has been terribly disruptive. If the nest can be rebuilt, the birds will be quite OK. Nest rebuilding is normal and psychologically helpful. But the inability to complete the nest, making it ready for fine nest lining and sitting-in, as some many have seen the birds do, would be extremely disruptive.

If that happens, however; if the nest cannot be rebuilt, the pair may go a year without copulating or nesting. They may just float around Central Park, or may even drift over to Jersey or somewhere, and then return in the late winter of '05 and start things again. It is not uncommon for an old reliable redtail pair to take a year off. This is usually precipitated by the loss of a nest, when the tree blows down, for example. But the pair usually comes back the next year and goes at it again.

So, if the nest isn't rebuilt this year, not all is lost. The pair is likely to return, to search once again for a good nesting ledge in the area.

And while I'm thinking of it, take this thought into consideration. Pale Male is what, 10 or 11 years old? From nesting redtail population studies in Ohio I've been involved in it's estimated that the average longevity of nesting wild adults is about 7-10 years. But we have pairs that are known to have nested consecutively for nearly 25 years. We think now, in retrospect, that these long-lived pairs actually involved any number of birds. A member of the pair would die or drift off and bond with some other bird any number of miles away. The nest was always used, but the members of the attending pair probably varied. Of course, this is exactly what has happened with Pale Male. We know that it's almost always the male that selects a nest site and begins nest construction in an effort to lure in a female mate. Pale Male has been typical in this process.

Regrettably, I have not read your book. My apologies. But here's my take on the early pair's failure to produce young for two early seasons. We observed exactly this same thing in a major study of Northern Ohio redtails. We diligently observed 100 redtail nests in a county south of Toledo in the 1970s, carefully recording egg laying, eyass production, and number fledged. This went on for up to three years for many of the nests. Just as we thought from incidental field observations before the detailed field study, redtails more often than not fail in their first one or two nesting attempts.

I recall seeing some of these first nests, and I could actually look up through the sticks and see skylight when the bird was off the nest. There's no way such a shabbily built nest can keep eggs warm enough.

Other times, first year nesters would actually produce an eyass, but the parents would be incapable of properly feeding the young hawks and they died. In summary, many, perhaps most, redtails need to experiment for a year or two to learn how to be effective parents. The two nesting failures you observed were exactly what I would expect in a really tough area such as NYC. The normal redtail food preferences, voles and mice, simply aren't readily available. The pair spent two years trying to figure out what it was going to feed any young that might someday appear. When eyasses did get hatched, after the pair learned to build an adequate nest and learned how to properly sit on the eggs, they had figured out how to exploit the only plentiful and reliable protein source, the common pigeon.

And on that note, I'd really like to learn what these birds are eating. I'm sure pigeons are the major prey species, but are they pouncing on newly fledged squabs that are unable to fly well enough to avoid the big redtail? Or, has the pair honed some new pigeon-catching techniques that overcome the pigeon's innate hawk-avoiding abilities? Whatever, the pair has things all figured out. I'm very familiar with Toledo's and Cleveland's urban peregrine falcons, and they take full adult pigeons in full peregrine stoops (dives). Redtails can't match the peregrine's pigeon-taking dives.

Lastly, don't be concerned about the pair's inability to reconstruct a proper nest this year. Given the opportunity to put a nest back together, it will go up in an inordinately short period of time, seemingly over night. The pair may just putter around with sticks in December and January, but when the days begin to discernibly lengthen in February (the fact the causes the breeding and bonding hormones to exude), the pair will get a nest together so fast you'll wonder how they did it.

I've seen this in the wild. One day I'll see the male trying to drop sticks in the crotch of a big oak or ash. Most of the sticks just drop to the ground. Finally a few stay in the tree. By the end of the day it's apparent that this will be the real nest site. I drive home after marking the nest on my map. I come back two days later to note the progress, and find that the nest appears as though it has been there an entire year. In one or two days it's full sized. The pair may spend many hours in February and early March preparing the nest lining, but the big construction gets done in less than a week, usually in two days or less. This is especially true in old experienced pairs such as Pale Male and Lola.

Hope this info helps. Pass any of it on to anyone you choose.


John A. Blakeman, licensed falconer, redtail biologist

Blakeman on Egg Development

For everyone impatiently awaiting the hatching of this year's brood(s), let me make a few comments on what's happening in the eggs. (This is not any sort of treatise on the subject. Those have been written by the raptor researchers who perfected the captive breeding of peregrines, where copious numbers of eggs and eyasses were produced for placement in wild nests and release at hack boxes. But those are other stories.)
As observers have noted, the incubating adult will periodically rise above the eggs and carefully turn them with her bill. This keeps egg membranes and tissues from fusing together in inappropriate configurations. This is a crucial parental duty.
We are now late into the incubation period. In the first week or so, the embryo was tiny and consumed very little dissolved oxygen, and it produced very little waste carbon dioxide. But as the embryo matured, with completely functioning organ systems, the consumption of oxygen increased significantly, and the production of carbon dioxide increased commensurately . Carbon dioxide dissolved in blood and tissue fluids renders them acid, and increased acidity (decreased pH) warps ("denatures") proteins, especially delicately-configured enzymes, which control virtually all biochemical reactions. There is a delicate balance between oxygen consumption and carbon dioxide production. And excess of the later is certain death.
Once out of the egg, the little eyass can breath freely just like we do. But while in the egg, things are gaseously rather tight. By the third and forth week, it's getting critical in there, as ever more oxygen must diffuse into the egg, and equivalent quantities of carbon dioxide must exit. Where? There is no breathing hole.
When laid, the egg shell is thick and strong, but later, the shell chemically begins to thin out and become microscopically more porous as the embryo develops, allowing increasing volumes of gases to pass through. By now, gases are passing rapidly through the porous egg shell itself, and also across the membranes beneath.
This shell thinning will be crucial, as the little eyass would not be able to puncture the egg were it to retain its initial thickness and strength. As most know, the little eyass has an "egg tooth," a small projection on the tip or top of the beak that is used to puncture the egg. This process, called pipping, takes about 24 hours or more, and it allows the lungs to slowly become accustomed to the increased oxygen of the open atmosphere. But more importantly, it allows the lungs to slowly adapt to the drying of real air. Inside the egg there is 100% humidity, and to be thrust quickly into open air causes the eyass's lungs to rapidly dehydrate.
So don't be alarmed if ever a camera is placed above the nest and an egg is seen to be broken, but the little eyass doesn't emerge. It shouldn't, until at least 24 hours after the egg tooth makes its first, tentative puncture. We needn't wish for haste here, our mammalian, parental instincts notwithstanding. Incubation and hatching will proceed at their slow, ancestral pace. It's all controlled by humidity and gas concentrations within the egg.
Today, changes within the eggs are happening at a rapid rate. Much of the egg "white," a concentration of proteins provided by the mother's single fallopian tube during egg development, is being converted into muscles, skin, and feathers. The egg's "yolk,' a supply of energy-dense fats, is also being depleted as the eyass grows. The white provides the body-building materials, and the yolk provides the energy to power it all. At the end, only the little hawk and a thinned egg shell will be left, and then it will be time to start the pipping process.
In short, the eggs aren't sitting there just being warmed. Some remarkable, hidden physiology is taking place. We'll be seeing the results of this not long from now.
I'm beginning to get some anticipatory excitement. Who could decide to disregard all of this? I've watched it now for over 35 years, and it's still always a thrill. I'm glad so many in New York City will be able to once again watch the appearance and development of young red-tailed hawks . It's good for everyone to have some visible, personal connections to things in nature, and what could be more noble and inspiring than watching another family of red-tailed hawks be produced? What a privilege this is. I thank all of those in Central Park who are watching and describing these events for all of us.
John A. Blakeman

Sunday, April 10, 2005

Blakeman's Response to My Rejoinder

In response to my comment at the end of Blakemans letter published here under the heading of Blakeman on the proliferation of CP Redtails the raptor expert wrote:

Yes, I've discounted an essential role for the nest-supporting pigeon spikes. I've done this because Western red-tails frequently and successfully nest on bare rock ledges that offer no innate protection from winds and other nest-destroying natural forces. I still tend to believe that until a pair learns to properly line a nest, eggs can cool.
But this will be a wrong explanation when a New York ledge nest gets simply blown away. And your accurate noting of the older age of some of the other failed CP parents negates my contention. If I had to bet on which explanation holds, I think it's yours.
Here's a thought that just came to mind. Are NYC red-tail nests atypical because there are no dead trees from which larger branches can be snapped off and used to construct heavier nests? Looking back, it seems to me that the 927 nest has been constructed of twigs, the diameter of a pencil or less. I seem to recall that wild rural nest have a much larger component of sticks that approach the diameter of one's little finger or larger, real sticks not twigs.. Larger branches can't be snapped off a living, healthy tree by a red-tail. But large, heavy sticks can be easily broken off dead trees, where the bark is already falling away and the wood is brittle.
So here's another unique NYC factor to consider. Has good urban forestry, the removal of all dead standing trees, forced the birds to construct nests with smaller twigs from living branches, which are much less heavy than the larger sticks and branches used in rural nests? If this is so, the pigeon spikes become essential in keeping the twig nest from blowing away. It may be that NYC red-tail nests are not as heavy as wild rural ones because bigger, heavier branches that could be snapped off simply aren't available in Central Park.
Does the absence of dead trees indirectly dictate that NYC red-tails must nest on ledges with supporting pigeon spikes?
And here's a good one. Are pigeon spikes themselves the fundamental reason RTs have been able to invade and persist in NYC? Over the years, have a few errant red-tails attempted to construct building-ledge nests that quickly blew away, leaving the birds stranded (and unnoticed)? Was the introduction of stainless steel pigeon spikes sometime in the 1970s or 80s (I presume) the deciding factor in recent nest successes? Were pigeon spikes the thingamabob that did the job?
After leaf-out in May, look around Central Park and see if there are any dead trees from which larger branches could be snapped off next winter. This could be a major factor in our story. Western ledge nests don't get blown away, and I think they are made of much larger sticks. These may not be available in New York.
John A. Blakeman

Marie's answer:

Dear John,

We've got to get you to visit Central Park! We keep providing more info that you would see for yourself if you just took a walk in the Ramble.

In regard to the absence of dead trees: I'd say that there are plenty of dead trees in the park's woodlands. With the guidance of the Woodlands Advisory Board, of which I am a long-time member, the Ramble, the North Woods, and other small wooded areas are maintained for usefulness to wildlife as well as for public aesthetics. Thus many dead trees are left in place as nesting places for woodpeckers, chickadees etc., just so long as they don't pose a safety hazard to the public. Dead limbs hanging over public pathways are removed , of course. But there are brushpiles around, and many large limbs on the ground in the park's woodlands. There is a deliberate policy to avoid a manicured, horticultural look in these parts of the park.

As for the anti-pigeon spikes as an important factor: I only know of a single site where anti-pigeon spikes have led to a nesting success: Pale Male's ledge at 927 Fifth. I haven't seen any spikes anywhere else. That is the likely reason why all the other attempts of nest-building on various ledges on the periphery of the park have failed, year after year. No spikes.

I often thought that the doctors at Mt. Sinai Hospital should have installed spikes on the ledge where a pair of hawks made nesting attempts three years in a row. I know the doctors were intrigued by the hawks and wrote about them in some hospital newsletter. But those hawks only succeeded last year, when they finally wised up and built a nest in a tree due west of Sinai. [Of course we're not absolutely sure the Mt. Sinai pair are the same hawks as the one who nested in the tree in mid-park around 97th St. But the male we called Pale Male III was a very light-headed bird, and my own hunch is that it was the same pair.]

More {and more technical} info about Reverse Sexual Dimorphism from Steve Watson to John Blakeman

Sunday, April 10, 2005

Steve Watson, the Kestrelcam correspondent, sends in some results from his research into the reverse sexual dimorphism enigma -- that is, why female raptors are bigger than males of the various species. He intended his letter for Blakeman, but since he sent it along, I am posting it for those readers who studied statistics somewhere along the line.

Here's a challenge for Watson: for those who haven't taken a course in statistics, can you explain in ordinary language what a correlation coefficient is? [After all, I just explained in ordinary language what reverse sexual dimorphism is!] At least interpret the numbers in the final paragraph

Hi, Marie,

... here's some more info [about reverse sexual dimorphism in raptors]: .

In my research on kestrels (and specifically, dimorphism in their eggs), I found an article which summarized some average weights for males and females, by species, and their ratios, broken down by foraging type. This partially responded to my question about increasing dimorphism ratio as a function of increasing body mass, but the results appear less than conclusive to me (although, frankly, I'm not a statistician, so I'll defer proper analysis to the experts). Anyway, I've attached a chart I made from the data, just thought it might be of interest to John and perhaps others. The paper is Anderson, J. et al., Prey Size Influences Female Competitive Dominance in Nestling American Kestrels (Falco Sparverius), Ecology 74(2), 1993, pp. 367-376. The table summarizes Cade (1982) and Kemp (1987), all of which with I'm sure Dr. Blakeman is familiar.

As a bit of additional info for John, I did a quick correlation coefficient on these three groups (correlating female mass to sex ratio) and got -.209, .419 and .426 for vertebrate/invertebrate generalists, vertebrate generalists and bird/bat specialists, respectively.