Blakeman on Sexual Dimorphism
Donna Browne writes to John Blakeman
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<