Photo courtesy of palemale.com
Yes ladies and gentlemen, we are metaphorically looking up Octavia's skirts and having a rare glimpse of her brood patch. The bottom of the patch is quite obvious but scan right, over the side layer of white feathers which ordinarily mask the area from the side when she isn't using it to warm the eggs. The brood patch widens as it goes up and then comes back in again.
Why do hawks develop brood patches?
Feathers are really warm right?
Well yes they are warm for the wearer as they keep the heat in and the cold out. To brood though, heat must be allowed to get out and to the eggs or eyasses. Hence the bald patch.
Not only does a formel loose the down feathers in the brood patch area, but the area also becomes highly vascularized, filled with blood vessels.. And last but not least the patch becomes edematous. Fluid collects under the skin as well as having the naked spot and the increased blood flow.
Rather, I suppose like an internal hot water bottle on the wing.
Speaking of "on the wing" there are also other adaptations thought to be present to reduce reproductive weight. In almost all avian species the right ovary is permanently vestigial. (Why the right ovary, and not the left?) And the left ovary in the off season is small and only takes on heft during the reproductive season.
Plus of course the lightness advantage of laying eggs instead carrying around developing young inside is more aerodynamically friendly as well.
Tiercels also have modifications which we mammals don't have.
As we know sperm keels over and becomes ineffective if it gets too warm. Exactly the reason why male mammals carry their sperm for the most part outside their main trunk in a pouch.
(And why human males who wear tight underwear can have more trouble being fertile than guys who wear free and easy boxer shorts.)
But lets face it, hanging testicles just are not aerodynamic and they'd look pretty silly bobbling around in flight anyway.
Testicles in hawks lay tiny and dormant deep inside them in the off season.
Come reproductive season the hormones surge and their testicles can increase 200 t0 300 times their off season size.
In fact a sexually active duck's testes can comprise 10% of his weight. Good thing they are mostly on the inside.... walking could even become problematical.
Think of that ratio in a human. Say if a human guy weighed 200 pounds his testicles would weigh 20 pounds.
Now there's a mental image. Good grief.
Fine you say, but whatever size the bird testicle is, it is still inside therefore the sperm is still going to be too warm to do their jobs.
Enter the swollen cloacal protuberance.
Many male birds develop a swelling around the opening of their cloacae. In some cases the shape of the swelling looks a bit like a teeny volcano. And the cloacal protuberance contains part of the vas deferens which stores mature sperm who must be a little cooler to remain viable while waiting to go into action.
Some avian species also have a circle of feathers around the cloacal opening which during copulation, (the cloacal "kiss") help the sperm get to the female's cloaca where they can do some good.
And last but not least in some cases in sexually monomorphic species during breeding season, one can identify the sex of the bird by looking for a cloacal protuberance. A brood patch can also be used but do remember in some species the guys get them too.
Photo by Francois Portmann http://fotoportmann.com/birds/2014/04/rth-nestcam-tsp-ii-nyc/
The Thompkins Square pair, Christo and Dora on the nest with three eggs!
A photo by Murray Head of a Palm Warbler in Central Park courtesy of Marie Winn's Central Park Nature News http://mariewinnnaturenews.blogspot.com/
I love the expression on his face.
Though now that I've written the above material on swollen protuberant cloaci, I can't help looking between his legs.
Geez.... even if he had one I wouldn't be able to see it from here.