"The real trick to life is not to be in the know, but to be in the mystery."
-Fred Alan Wolf

08 November 2016


"By three o'clock, large flocks of mallards began to leave the lake and prairies and headed east and southeast for the river and refuge from the stinging blasts of wind and snow. The bands of waterfowl sensed that this was the end for them at this latitude," wrote William C. Hazelton in Days Among the Ducks.

Actually, the ducks Hazelton described "sensed" key changes in their environment long before the December blast ultimately pushed them southward. While a dramatic change in weather was the immediate signal for the birds to find food and habitat elsewhere, the internal clock that determined their departure was set months before by the gradual lengthening of spring daylight.

Among waterfowl, regular seasonal movements (i.e., migration) are driven by changing photoperiod, the relative length of day and night during a 24-hour period. As a result, migration is physiologically "hard-wired" in waterfowl and other migratory birds. Beginning in spring, increasing day length affects hormone response and starts the clock ticking. Accumulation of fat, migration, breeding, and the molt follow in succession over the next weeks and months.

The prompt for fall migration is not as clear but is most likely related to the timing of reproductive events and molting. The result, however, is just as predictable. During a period of long days with gradually decreasing daylight, birds again accumulate fat reserves for migration and become restless—in a behavior known as zugunruhe—setting the stage for their departure south.


Thank you, Judson.

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