By Ken Wilson
Nearly all living things— trees, moles, humans— contain clocks attuned to daylength. This length of day is one substantial cue stimulating migration in birds. Yet at our latitude, today is only a few minutes longer than yesterday, implying a very fine-tuned clock. And while multitudes of songbirds are now departing northward from the tropics, at their departure latitude the change in daylength is even less –only 30 seconds a day. Clearly, other cues must also be programmed into their biological clocks.
One species pictured here is a Western Tanager at my backyard suet feeder. He is a one-year old. (Next year he’ll have sharper colors). As summer days shorten, his clock combined with his fantastic navigational systems, guide him to southern Mexico or Central America.The bird knew when and where to stop his southerly flightpath, even though this is his first migratory flight. The Tanager’s clock somehow dictated when to initiate his flight north to the Olympic Peninsula to breed, despite the near-constant daylength in southern latitudes. This individual has now flown a round trip distance of 3,000 miles. He hasn’t starved or flown into a window or been caught by a cat or a hawk. If he makes it another year, that would be another 3,000 miles. Like clockwork, at the end of April you will see the arrival of our Tanagers. And much more…
And when shall you look and listen for the arrival of these ‘neotropical’ birds, which make up more than half of our summer birdlife? Their arrival peaks towards the end of April through the first week or two of May. Birdsong will be everywhere. So, people – set your alarms for dawn!