Springing into the season for reproduction

Elisabeth Brown

There are rustlings and twitterings in the bushes, and I hear bird

song -- the males are tuning up. I even heard them singing in the

rain; on Laguna hillsides and canyons, the birds are thinking about

breeding.

Birds have to time their breeding to coincide with a time of

plentiful insects, so there will be plenty of food for hungry chicks.

That means they have to anticipate by many weeks when the insects

will be out. How do they know it’s time?

Also, because birds fly, they have to minimize every gram (1/28 of

an ounce) of extra weight. As every backpacker knows, everything

weighs something, and a lot of little items quickly add up to an

over-heavy pack. So it is with birds. When they’re finished breeding,

their reproductive system shuts down, and the ovaries and testes and

all the shell-making machinery shrivels up, saving a lot of weight.

But in the spring, something turns the reproductive system back

on. It’s not the warmer days; millions of birds breed on Arctic

tundra, making nests amid melting snow. Summer temperatures there

barely reach our winter temperatures, when our local birds are

definitely not breeding.

What else characterizes spring? The days are longer.

As it turns out, birds have a light meter in their brain -- a tiny

organ named the pineal gland -- that measures the day length and

regulates the reproductive cycle.

Buried deep in the brain, the pineal gland produces the hormone

melatonin -- but only in the dark. Melatonin, in turn, keeps the

reproductive system turned off. So, as the days get shorter, the

pineal gland produces more melatonin, and the reproductive organs go

into the deep freeze. In spring, as days get longer, less melatonin

is produced, and the bird’s reproductive system turns on again.

Other animals also have this light metering system. In lizards,

light reaches the pineal gland through a translucent section on the

top of the skull (sometimes called a “third eye”). In birds, a gland

around the eye registers the day length and passes the signal to the

pineal gland. We know it’s not the seeing part of the eye (the

retina) because even blind birds know what time of the year it is.

Humans measure light as well; some people become depressed in late

fall and winter, a condition known as seasonal affective disorder, or

SAD. Treatment includes sitting under banks of special lights (not

tanning booths!) for several hours a day.

Domestic chickens, bred for centuries to lay eggs year-round, are

still light-dependent. They gradually stop laying during the fall,

and restart in the spring. To keep them laying through the short days

of the year, chicken farmers use artificial light. In those

10,000-chicken sheds of the industrial egg farmers, the lights are on

all the time.

When I was an undergraduate at UC Berkeley, one of my professors

was an early researcher into the pineal gland, maybe because his own

day/night cycle was so unusual. If you happened to get to the biology

building very early, say, 5 a.m. or so, you would meet him coming

out, on his way home. He refused to teach an 8 a.m. class, saying, “I

don’t think I could stay up that late.”