For the Birds Radio Program: Keeping Time
How do birds know when to migrate? (4:05)
(Recording of an alarm clock)
That alarm clock means it’s now officially spring—at least for American birders and ornithologists. In order to standardize migration and distribution data over the continent, from the Florida Keys to the Pribilof Islands of Alaska, ornithologists set March first as the official first day of spring. Not that the birds take particular note of our human calendars. The swallows are supposed to return to Capistrano on St. Joseph’s feast day, but in reality they’re often off by a full week. And Turkey Vultures usually return to Hinkley, Ohio well before Buzzard Day—residents just ignore the birds that arrive before the official holiday.
We use clocks and calendars, but how do birds actually know what time it is? Most species have an internal clock. Scientists kept European finches called Chaffinches in enclosures, keeping track of perch-hopping activity over a long period of time. When the lights and temperatures in the enclosure were timed to coincide with the natural conditions of day and night, the birds showed a strong daytime activity rate. But when the birds were exposed to continuous light and a constant temperature, the rhythm of activity hardly changed at all, even after months of artificially constant conditions. The clock did seem to run a little fast, so that over time the birds became active earlier and went to sleep earlier, but the overall pattern was striking in its accuracy.
House Sparrows showed the same rhythm. In this species the actual clock seems to have been found. When scientists remove the pineal gland from the forebrain, the activity rhythm of House Sparrows is completely and immediately abolished. The rhythm returns if the pineal gland of another sparrow is transplanted, even into the anterior chamber of an eye instead of into the brain where it belongs–but now the sparrow lives in the donor sparrow’s time zone. Ornithologists presume that melatonin, a hormone produced by the pineal gland, is the essential substance that drives the clock. When Starlings are kept in continuous dim light after having their pineal glands removed, they can be synchronized to a normal pattern if they receive daily injections of melatonin.
Birds that live away from the equator are exposed to varying daylengths throughout the year. Right now, as the days grow longer in the Northland, birds are awaking earlier each day, and have more time to spend in ways other than feeding. Although day length is clearly one of the triggers for some annual rhythmic behaviors, like mating, nesting, and migrating, there is quite a bit of evidence that an internal clock is also involved in annual rhythms, much the was the pineal gland is involved in daily rhythms. For example, starlings that have been maintained in constant day-length conditions for 3 1/2 years still kept their normal physiological molt/breeding rhythms, so clearly day length was not the decisive factor.
Ornithologists will continue to shed light on the mysteries of the rhythmic ebb and flow of migration, but no matter how sophisticated our understanding of the mechanisms of bird biorhythms, we will always need poets to impress upon us just how wondrous avian clocks are. Rachel Field wrote:
Something told the wild geese
It was time to go.
Though the fields lay golden
Something whispered, –“Snow.”
Leaves were green and stirring,
But beneath warm feathers
Something cautioned, –“Frost.”
All the sagging orchards
Steamed with amber spice,
But each wild breast stiffened
At remembered ice.
Something told the wild geese
It was time to fly,–
Summer sun was on their wings,
Winter in their cry.
(Recording of a Canada Goose)
That was Rachel Field, This is Laura Erickson, and this program has been “For the Birds.”