On the Wing

It’s 3 a.m. and the lab is still, but the bird in the cage natters anxiously, flapping its small wings wildly in the dark. Ready for takeoff.

Ornithologists call this behavior Zugunruhe, German for nocturnal migratory restlessness. First noticed by 19th-century bird fanciers, it occurs every spring and again in fall, powerful evidence of the internal clock that tells the bird: It’s that time again: Time to fly.

“Biological clocks are found in any animal, from worms and flies up to humans,” says Paul Bartell, an assistant professor of avian biology at Penn State. Working in concert with daylight and other environmental cues, these innate regulators control not just patterns of sleeping and eating, but hormone releases and many other cell processes as well. In songbirds, they dictate the delicate timing of migration.

Not just the time of year, which is determined by a seasonal, or circannual, clock, notes Bartell. There’s also a daily, or circadian, component. “Songbirds migrate at night, to avoid thermal stress, water stress, and predators,” he explains. “At migration time, they have to become active at night.”

In recent years, researchers have determined that the circadian system in birds is made up of at least three independent but interacting clocks: one in the retina of the eye, another in the pineal gland, and a third in the hypothalamus. “They reinforce each other through a kind of negative feedback,” Bartell says.

But major questions remain, he adds. What’s controlling the switch from diurnal to nocturnal activity? What changes in protein and gene expression make these birds suddenly able to fly by night?

“Migration is physiologically demanding, and—if we can anthropomorphize a little bit—mentally demanding as well,” Bartell notes. “Yet these birds, when tested, don’t show decrements in physiological or mental functioning. They perform as well without sleep as they do with it. Understanding why this is might help us to understand how humans can perform without sleep. It also has implications for the ecology of migration.”

Paul Bartell
Laura Waldhier

Paul Bartell

As a postdoc, Bartell worked with the pioneering German ornithologist Eberhard Gwinner at the Max Planck Institute in Andechs, Bavaria. In a 2005 paper, Gwinner and Bartell reported the discovery of a distinct internal mechanism for controlling Zugunruhe in the garden warbler.

“There appears to be a separate biological clock which is only expressed at certain times during the year,” Bartell explains. “Its appearance for migration is likely due to interactions of other circadian clocks in the brain. As their relationship changes, it allows the migratory clock to be expressed.”

In his continuing research at Penn State, he says, “we’re trying to find where this clock is in the brain. Because birds put on fat right before migration, our hypothesis is that an impulse from adipose tissue is feeding into the brain. So we’re looking for areas of the brain which would be responsive to fluctuations in fatty tissue.”

At the same time, Bartell is exploring ways in which avian clocks control another vital function: reproduction. For a USDA-funded study involving both songbirds and chickens, he is looking at how clocks regulate the growth of ovarian follicles and the formation of eggshells.

Another study, with colleagues Margaret Voss at Penn State Erie, The Behrend College and Caren Cooper and David Winkler at Cornell, focuses on how the clocks regulate clutch size, i.e., the number of eggs that are laid at a given time. “As you increase geographical latitude, clutch size gets larger,” Bartell notes. “It also varies with laying date.”

“We know there’s a clock in the pituitary and a clock in the ovary involved,” he continues. “The clock in the pituitary puts out luteinizing hormone on a daily cycle. The one in the ovary is responsive to the hormone, but that responsiveness changes throughout the day. When these two clocks are in synch, there is maximum response. When they get out of phase, ovulation ceases and the clutch is terminated. So we know they interact. But how do these clocks communicate? How do they output and receive signals?

“That’s the kind of thing we’re still trying to figure out.”

Paul A. Bartell, Ph.D., is assistant professor of avian biology, pab43@psu.edu.

Last Updated March 22, 2012