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:


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).


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.)?


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?


John Blakeman answers:


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.


John A. Blakeman<