Birds do it, bees do it,
Even educated fleas do it.
Let’s do it, let’s fall in love.
You don’t have to be an astute observer to notice that something’s up in the avian world right now. It’s springtime and once the birds have recovered from the stress of migration or the burdens of winter, their raison d’etre becomes reproduction. The hormones from brain and adrenal glands rule the roost and result in both physical and behavioral changes, all focused on reproducing and preserving their species.
Claiming a territory is step one in this process. In my neck of the woods this is most noticeable with the boisterous Red-winged Blackbird, perched on the tallest reed, sporting his bright red and yellow epaulets, warning other males to stay clear and beckoning females to come and check him out. He hopes they are attracted by his beauty, health, and strength.
There are numerous avian signs of courtship, some common and others quite bizarre. The common include colorful breeding plumage, such as the striking migrating Warblers and their melodious songs, all evolved to attract a mate. The more bizarre include the Magnificent Frigatebird’s inflation of his giant red throat (which other jealous males attempt to puncture), the water ballet of courting Grebes performed in perfect unison, or the Baryshnikov-like leaps of the Sandhill Crane.
There are the gentle offerings of food and mutual grooming, or the spectacular flight and airshows of the Hawks, or the lower nighttime air dance of the Woodcocks. I don’t quite get the courtship practice of the male Parrot vomiting into the mouth of the female, allowing her to sample the prospective mate’s taste in food, nor do I condone the Mallards’ gang rape of a cornered female, so common with that specie. But these are all signs of spring, evolved over millions of years to recreate and preserve life.
As a Radiologist I have a special interest in the comparative anatomy of humans and birds. First for the male, the avian testes, the producers of sperm, are located high in the abdomen, near the upper poles or the kidneys–male birds have no scrotum. The sperm travel down through the deferent duct and seminal vesicles and empty into the cloaca at mating. As with mammals, sperm require lower than normal body-temperatures to mature. Birds solve this by lowering their body temperature at night and by storing sperm in the cooler seminal vesicles in the lower pelvis.
Most male birds have no penis. Mating occurs with “kissing cloaca”, the brief and often repeated contact of the male and female cloacae. But a few male birds including Ducks, Storks, Flamingos, and Ostriches do have an erectile penis arising in the cloaca. In the mating duck this is quite large and has a corkscrew configuration while the complementary female anatomy has a reverse corkscrew shape to accept it. Some have said that these birds, who often mate in water, use this penetrating copulation to prevent water from washing the sperm away.
The most interesting aspect of bird anatomy is that most female birds have only left-sided internal genitalia–the right sided structures are involuted or completely absent. There are a couple potential explanations for this. It may serve to reduce body weight and make flight easier, and it also prevents bilateral ovum forming simultaneously with opposing eggs obstructing each other in their passage to the lower genital tract.
The ovary is positioned high in the abdomen, similar to the testes, and its eggs pass to the cloaca via the oviduct, uterus and vagina. Grossly these three structures appear to be a continuous tortuous tube, but microscopic anatomy reveals differing functions. At ovulation the soft ovum enters the upper oviduct and becomes coated with albumin and keratin as it proceeds downward. The limey shell and egg coloration is added in the uterus and the finished egg is stored in the vagina. It appears that the fertilization with sperm occurs in the upper oviduct. The entire process from ovary to cloaca takes about 24 hours.
The structure and function of the avian egg is a fascinating topic by itself, but best left for a later posting. Just as a trailer however, consider that a bird’s egg must be strong enough to withstand the weight of the incubating parent but fragile enough to allow the hatching chick to escape. It must be a protective barrier, but also porous enough to allow oxygen transport and respiration. It must also contain all the nutritional and energy requirements of the developing embryo.
So as you head out this spring remember the raging hormones have expressed themselves in many ways, some visible and some unseen. The males are carrying testes that have increased their volume 100 times and are trying to find and impress a mate in any way possible. The female, her own genitalia markedly enlarged for the season is the egg producer, primed and ready, hoping to find a worthy mate. If successful, the real work will have just begun.