What a difference a minute makes.
The winter solstice just passed, the shortest day of the year. On the human calendar, the solstice marks the beginning of winter. On the bird calendar, it’s different. For many of our resident birds, it marks the beginning of spring.
A few days after the solstice, as I walked the driveway to retrieve the newspaper, I heard singing. No, it wasn’t carolers that just couldn’t bear to hang up the mittens and scarf. It was bird song. It wasn’t much, but I heard a brief song of a black-capped chickadee from a neighbor’s yard. Next, I heard the song, also brief, of an oak titmouse from the yard of a neighbor on the other side. They were the first songs of either species that I had heard since last June. The next morning I heard the song of a collared dove.
They know. They sense the changing of the seasons. It is impressive just how finely tuned birds are to their surroundings.
The cue birds use to mark the passage of time is day length. A quick check of the Mail Tribune finds that between the winter solstice and the 27th of December, the length of daylight increased by one minute. One minute! And the birds marked that change.
How do they detect the subtle difference? The sensor for most, including both mammals and birds, is the retina in the eye. Besides detecting the rich array of patterns and colors that produces vision, the retina also marks changes in the duration of daylight. Other birds and mammals use a specific region of the brain directly, or more often the vestigial third eye, the pineal gland. These last two sensors only work on tiny animals where light can penetrate the feathers/fur, skin, skull and brain tissue to reach either the pineal gland or the light-sensitive region of the brain.
The pineal gland is particularly interesting. In humans the gland is associated with the production of melatonin and the regulation of sleep patterns. In many reptiles the pineal resides on the surface of the body. In the tuatara of New Zealand, a primitive lizard-like reptile, it is easy to see the tiny eye on the top of the head. This third eye even contains a lens and a retina, although poorly developed. You can see traces of the third eye in some of the reptiles here in Oregon. In mammals and birds, the pineal no longer reaches the surface of the body. It is now a tiny structure located between the two hemispheres of the brain. In some birds and mammals, it is still sensitive to light.
Many farmers note the change of day length with similar precision, especially those whose days are regimented by the milking of cows and other chores. I learned this when discussing the virtues of birds with a cousin. It’s just that most of us live in controlled surroundings where light is controlled more by the flip of a switch than sunrise.
This reminds me. It’s time to clean out the nesting boxes from last year. Spring is coming — or it’s already here, depending on whose calendar you use.
Stewart Janes is a biology professor at Southern Oregon University. He can be reached at firstname.lastname@example.org.