Wednesday, March 23, 2005

ANOTHER THEORY ON REDTAIL FEMALE SIZE SHOT DOWN [gently]

Donna Browne keeps thinking and John Blakeman introduces some new facts.

Hi John,

How about this thought? With most species, when a
predator appears, the alert is given, and the birds
flock together to mob the predator. This doesn't
happen with Raptors. Therefore, particularly where
territory is larger than in the case of our city
hawks, the female is the only defense against the
predator until the male can return from hunting and
help defend the nest.

Sincerely,
Donna

Donna,

This may be the best explanation yet. Certainly seems plausible.

But as always, I have to play devil's advocate and see why this possibly wouldn't work, or might be an unlikely explanation.

When I stop to consider the predatory threats toward red-tail eggs and eyasses, only a few are prominent. Raccoons are a major concern. Many a nest has been cleaned out by a raiding raccoon the night after a biologist has climbed into a tree to band the young. Modern raccoons have learned to follow human scent through the forest, and almost inevitably it will lead to some sort of food (humans throw out a lot of raccoon-edible things).

Consequently, when I and my partners banded eyasses, or ever climbed into a nest, we always sprinkled mothball crystals at the base of the tree to disguise the scent trail we left. We never had a single raccoon problem. Others did, tainting their research results.

A male red-tail, in the eyes of a marauding raccoon, is essentially the same size as a female, especially at night when raccoons are on the prowl.

The other nest predator is the great-horned owl, and these brutes are so large and powerful that neither a male nor female red-tail would be able to dissuade such a bird from attacking either a parent or the eyasses. But of course, because great-horned owls are quite dependant on the availability of old red-tail nests upon which to raise their owlets, great-horns have been selected against outright red-tail predation. So big owls aren't a real concern, either.

That's just about it. I don't think that the slight (in this case) size difference between male and female adult red-tails offers much benefit in nest defense. The best defense is to get a nest way up there where it's difficult to get to, and red-tails do that well.

Very good thinking. Very plausible -- until the theory is measured against likely nest predators, and I think for our species they are only great-horned owls and raccoons, where the female's larger size is insignificant against either of these much larger predators.

But perhaps in the field (not the mind) this is the answer. Whatever it is, there has been natural selection for female incubation in all raptors. I've only pondered the red-tailed hawk. Perhaps size-dependent nest defense is a more significant factor in other species.

Again, very good thinking.

Sincerely,

John A. Blakeman

Monday, March 21, 2005

Great Horned Owl



After I posted the picture of the Great Horned Owl nesting outside of a science lab in Pasadena, [see below] John Blakeman sent the following response:

Marie,

The photo of the great-horned owl nesting in Pasadena brought back many memories. A few comments.

First, the owl didn't really build this nest. She (or probably her mate) selected it. Owls are famous for their inability to construct nests. The nest in this photo almost surely was built and used previously by a crow or raven. Great-horned owls simply don't bring sticks to a nest site. I don't think they even bring lining materials. They simply expropriate some other species' recent nest. This is a common observation in the wooded parts of the red-tail's continental range. Great-horns begin nesting before any other species, laying eggs often even in December. They are incubating in the absolute worst weather possible. But they bring off ample numbers of offspring (Eyasses? No, that word is reserved for baby hawks and falcons. They are "baby owls," or more properly, "owlets.").

Therefore, all red-tails commonly have to be prepared to construct a brand new nest each season, as last year's is commonly expropriated by an owl by the time sex hormones start to flow in January and February. This is a major reason I had no real concerns about Pale Male and Lola re-occupying their restored nest site, once the support structure went back up. This is a red-tail trait. Individuals that were put off by the loss of nests simply didn't breed. Those genetic traits were lost long ago. Our pair rebuilt just as they would have in a rural area after disruption by an owl pair, but this time back at the same site -- thank you everyone.

The ecological interplay and competition between great-horned owls and red-tailed hawks is complex, a story that I won't open here until great-horns colonize Central Park. When that happens, things are likely to change dramatically. But let's hope that's far in the future (or from my perspective, never occurs at all).

Sincerely,

John A. Blakeman<

Mai Stewart and Donna Browne wrote to ask more questions and offer theories
about the reverse sexual dimorphism problem.
Here are their letters, followed by John's response:

Dear Marie,

When I read John Blakeman's statement that there is
no known reason that female RTs are larger
than males, I wondered whether the reason might be
similar to the reason women
have larger hips/abdomens (in general)
than men -- simply because the women
are the ones who bear the children?
Could it be possible that female RTs
need to be larger to accommodate
her egg-producing procedures,
whereas male RTs (like men), don't need
space for that??

Also, is it usual for new twigs/nesting material
to be brought to the nest so late --
i.e., after we presume that eggs
have already been laid? Does this mean
that the nest is insufficiently warm,
and that an egg/s is/are in danger
(from cold air/wind/etc.)?

Best,

Mai Stewart


Donna Browne writes to John Blakeman

Hi John,

Interesting that there is no accepted theory as to
why female raptors are larger than males. I thought
perhaps it had to do with the ability to fly more
powerfully and therefore still fly well with the added
weight while gravid or to create larger eggs or that a
larger body could afford to be diminished by the
number of grams that are sometimes lost while on the
nest. Since these are obvious, I expect they've been
discounted. Fascinating, whatever could it be?

Sincerely,
Donna

John Blakeman answers:

Marie,

Glad to see others are pondering the sex size
and incubation question.

No, females aren't larger to accommodate the eggs.
The eggs aren't very big, just a bit larger than a chicken's egg, and they are produced one at a time.
The female is never "loaded" with two or three eggs. They are produced quickly, as the forming egg
descends along the single fallopian tube,
but there is no body size constraint. If there were, females of most other bird species
would be larger, too. But they aren't.
Males of most avian species are larger than the females. Not so with almost all raptors,
still for unknown reasons.


About the bringing of twigs to the nest,
now that incubation has irrevocably begun.
This is merely a continuation of the instinctive nest-building urges the birds had before
eggs appeared. The twigs are being
brought to the nest only on residual compulsion,
not with any real necessity. This behavior
is likely to subside as incubation proceeds.
Many birds, however, continue to bring
conifer sprigs to the nest for some time,
even after eyasses have hatched.
Again, these may help repel insects on the nest.

The bringing of twigs or lining material
to the nest at this stage does not indicate
nest porosity or air leakage. That was diligently attended to before the eggs were laid.
That's one reason both sexes spend many hours
sitting on the empty nest. If they get their bellies cooled in any way, more lining material
will be brought in. They were testing the nest
diligently, and it passed.

As I've noted, first-time nesters often don't get
any of this right, with a nest that is not air tight.

I've seen many red-tail nests where eggs were incubated that never hatched. Upon examination
of the nest, the coarse and loose construction
of the structure was obvious. The parents
didn't provide a warm incubation environment,
and the eggs died. This is rather common
for first-time nesting pairs. The following year,
however, the lessons will have been learned
and the nest will be tight, and eyasses will be produced.

Our pair has learned all the lessons
over several successful breeding seasons.
No concerns here whatsoever.

It's now just a matter of time.

Sincerely,

John A. Blakeman<

Blakeman on Sexual Dimorphism

Donna Browne writes to John Blakeman

Hi John,

Interesting that there is no accepted theory as to
why female raptors are larger than males. I thought
perhaps it had to do with the ability to fly more
powerfully and therefore still fly well with the added
weight while gravid or to create larger eggs or that a
larger body could afford to be diminished by the
number of grams that are sometimes lost while on the
nest. Since these are obvious, I expect they've been
discounted. Fascinating, whatever could it be?

Sincerely,
Donna

John Blakeman answers:

Marie,

Glad to see others are pondering the sex size
and incubation question.

No, females aren't larger to accommodate the eggs.
The eggs aren't very big, just a bit larger than a chicken's egg, and they are produced one at a time.
The female is never "loaded" with two or three eggs. They are produced quickly, as the forming egg
descends along the single fallopian tube,
but there is no body size constraint. If there were, females of most other bird species
would be larger, too. But they aren't.
Males of most avian species are larger than the females. Not so with almost all raptors,
still for unknown reasons.


About the bringing of twigs to the nest,
now that incubation has irrevocably begun.
This is merely a continuation of the instinctive nest-building urges the birds had before
eggs appeared. The twigs are being
brought to the nest only on residual compulsion,
not with any real necessity. This behavior
is likely to subside as incubation proceeds.
Many birds, however, continue to bring
conifer sprigs to the nest for some time,
even after eyasses have hatched.
Again, these may help repel insects on the nest.

The bringing of twigs or lining material
to the nest at this stage does not indicate
nest porosity or air leakage. That was diligently attended to before the eggs were laid.
That's one reason both sexes spend many hours
sitting on the empty nest. If they get their bellies cooled in any way, more lining material
will be brought in. They were testing the nest
diligently, and it passed.

As I've noted, first-time nesters often don't get
any of this right, with a nest that is not air tight.

I've seen many red-tail nests where eggs were incubated that never hatched. Upon examination
of the nest, the coarse and loose construction
of the structure was obvious. The parents
didn't provide a warm incubation environment,
and the eggs died. This is rather common
for first-time nesting pairs. The following year,
however, the lessons will have been learned
and the nest will be tight, and eyasses will be produced.

Our pair has learned all the lessons
over several successful breeding seasons.
No concerns here whatsoever.

It's now just a matter of time.

Sincerely,

John A. Blakeman<

Falconry, Sexual Dimorphism and other Mysteries

Hi Marie,

I have a question raised by John’s wonderful letter today -- we all know that his ‘Savanna’ is a female red-tail, or ‘hen,’ but he made other references to the fact that he has worked with several red-tailed hawks -- and they all were referred to by him as female. Are female hawks merely his preference (and why)? Or in training hawks for falconry, is the use of ‘hens’ preferred in general? And why, (of course)?I’m sure other people have noticed this and raised this question!!

Thanks Marie and have a wonderful day!

Shari

Here's Blakeman's answer to the question:

Marie,

Yes, I tend to work with red-tailed hawk females, "hens" for lack of a better term. This is for several reasons. The first is that more females are fledged than tiercels, for reasons no one knows. At least 60% of all fledged red-tails are females. Tiercels aren't equal in any way with the big females.Hen red-tails, as in virtually all other raptors, are markedly larger than the males. In fact, that's the origin of the term 'tiercel," an English corruption of a 16th-century French term meaning "one third." Peregrine tiercels are about one-third smaller than the falcons (the females).No one has come up with a good reason for the greater size of raptor females. In fact, it makes no sense whatsoever.

While big Lola is up there whiling away her many hours of motherly incubation, our patriarch Pale Male must be out on the hunt, not just to feed himself, but also his larger sedentary mate. His hunting prowess must more than double, although he is the smaller and less muscular of the pair.

The smaller mate should get the incubation task, but universally across all raptor species, the diminutive male is genetically assigned the daunting food provision task.And it gets even more difficult after eyasses hatch. The tiercel then must feed not just himself and his sitting mate, but also bring to the nest sufficient food for the rapidly developing eyasses. The tiercel has to spend virtually all daylight hours in late April trying to find, capture, kill, and transport food for everyone in the family -- a daunting task. (See why it's important to know the daily caloric requirements of adult hawks?)

Why the tiercels aren't the larger bird, more capable of attending to the demands of family food supplies during incubation and eyass brooding, is a complete mystery. I've utterly given up all thoughts on the matter. There are some ecological questions that may never have answers. This is one of them.

But why do I refer to every hawk except Pale Male with the feminine gender? It's an ancient convention of falconry, deriving from the abundance of hawk females in the wild, and their preference in the sport. Because hawk females are larger, falconers for centuries have preferred them over tiercels. Now don't misinterpret this. Tiercels can make exceptionally fine hunting birds, as with a remarkable red-tail tiercel captured and used by my current falconry apprentice here in Ohio. He has done notable things with this bird, and it holds its own against any prey it chooses to pursue, primarily cottontail rabbits. It just completed a very fine hunting year, and just "went up for the molt." It is being retired to the mews where it will sit contentedly and grow new feathers until autumn. (Incidentally the place falconers keep their hawks is called a mews. The Royal Mews of Buckingham Palace today houses the royal carriages. In former centuries, it housed the King's falcons.)From all of this, falconers simply refer to any hawk of unknown sex as a female. It is a matter of respect and honor.

Tiercels have their place in falconry, but the big females are most highly regarded. (And the greater presence of females is one reason that falconry has persisted for several millennia without reducing wild raptor populations. At the height of Renaissance falconry, in Shakespeare's time and the century following, there were 600 or so known peregrine eyries (cliff side nests) in Great Britain. One or two falcon eyasses (females) were removed from each of these every year for royal falconers for several centuries without diminishing the wild population. That's because an abundance of females is always produced, and just like the red-tails that have colonized Central Park, these excess falcons would have died without progeny in the wild, for lack of un-occupied nest sites after reaching adulthood.

For red-tails today, the same thing happens with the very few falconers in the U.S. We much prefer the giant females, and often refrain from taking a rarer male. European and American falconry has never had any impact on wild raptor populations except for the positive, as in raptor education and captive breeding of peregrines, etc.So, if you see a red-tail sitting in a Central Park tree, it's a she, unless you can surely tell otherwise. It's a matter both of convention and respect.

Sincerely,
John A. Blakeman