Longer days put birds in the mood

Singing loudly, a cardinal sits atop a bare tree across the street from our house. His voice — it's a male in courtship mode — is almost strident, not exactly romantic sounding, but, I assume, effective.

From the backyard comes the soft tapping of a downy woodpecker. It's his "song," his call for a mate.

Both birds are fueled by a boost of testosterone. One day in the very recent past the amount of daylight reached a very specific critical peak governing springtime production of that hormone for these birds.

Ten hours seems to be the minimum day length needed to stimulate the breeding changes, according to the Cornell Lab of Ornithology.

The eye retina is presumed to send neural impulses to the brain that eventually stimulate the hypothalamus, an area of the brain producing hormones that control body temperature, heart rate, hunger, mood, and, not least, hormones from the pituitary to put the sex drive into spring mode.

Under the influence of those hormones, the testes begin their cyclical change, for production of sperm and production and release of testosterone, according to the Cornell Handbook of Bird Biology.

"In many birds, especially migratory species, the gonads vary tremendously in size and secretory activity on an annual cycle," the book states. "During the breeding season, for example, the testes of small passerines [songbirds] may grow to several hundred times their nonbreeding volume and weight.

Preparation for spring is then complete.

Later in the spring cycle, the birds begin another series of changes, eventually leading to nonbreeding status.

The pituitary releases a hormone that inhibits further secretion of the other gonadal hormones, Cornell explains. This retards the activity of the ovary and the testes. Reduction of these hormones reduces the behaviors initiated by them.

"By the time eggs appear, a secretion from the adrenal gland has stimulated incubation behavior," the handbook explains.

For birds such as pigeons and doves, the production of "pigeons milk" in the crop is activated. (Pigeon or crop milk is a secretion from the lining of the crop of parent birds that is regurgitated to young birds.)

There are other triggers. Rainfall, for example, can stimulate breeding in arid regions. For other species, availability of food is a nesting determinant.

A vole reproduction cycle at its peak will support large snowy owl broods. The owls might not breed at all when the vole population is low.

"An important breeding cycle stimulus for some species is the presence or absence of others of their species," the handbook explains.

A captive female rock dove, for instance, will lay eggs readily in the presence of a male, and less readily when only another female is present, Cornell explains.

Curiously, if isolated from others of her kind, the rock dove will not lay at all — unless she has a mirror in her cage.

Brood patch formation

The embryo, developing in the egg, requires constant heat. Because feathers are poor conductors of heat, most incubating birds develop one or more incubation or brood patches on the breast that are without feathers.

Heavily suffused with blood vessels, these patches permit direct contact between the warm skin and the eggs.

Hormones control development of the incubation patch.

Before the female lays her first egg, her patch begins to develop as follows: The bird loses feathers, the outer layers of the skin thicken, blood vessels in the region increase in number and some enlarge, and the spaces between the cells under the skin fill with tissue fluid, remaining full during the incubation period.