I feel a return to my roots today (I used to study birds)... Today is a beautiful day: the sun is shining, and the snow is glinting like trillions of tiny diamonds. It certainly doesn't look (or feel) like spring might be around the corner, but I know it's there, lurking, waiting to spring out at us (no pun intended). I know because the birds are singing.
I have noticed a marked uptick in the amount of activity and song among our feathered friends over the past week or two. They know spring is nearly here, despite the perpetually frozen ground and utter lack of anything growing. How is it that they know? The lengthening days.
We have finally entered that time of year when the amount of sunlight we get increases by about 4 minutes a day, which is enough that you notice it from one day to the next. Last week, it was still dark when we got up in the morning, now it's noticeably lighter. With 4 minutes a day, we add nearly half an hour a week to our daylength, and it is this increase in photoperiod (as scientists call it) that tells the birds spring is coming.
Photoperiod is incredibly powerful in changing both physiology and behavior of all kinds of organisms. So what, exactly is going on in the birds that they are suddenly so active? It turns out that their gonads (testes and ovaries) are increasing in size, and the resulting influx of testosterone in the males is stimulating them to sing. Sounds weird, I know, but in the Northern Hemisphere, at least, that's what happens (birds in the Southern Hemisphere do not experience this). Why on earth would birds experience such a bizarre physiological change? Other organisms don't... what makes birds different? Flight and migration are two important factors.
Have you ever considered the amount of energy required to fly long distances? Yes, airplanes burn a lot of fuel, but imagine you were a bird, flying day and night to reach your overwintering site. You would need a lot of fuel to get there. And just as airlines limit the weight of your luggage to reduce the amount of fuel required for the trip, birds can reduce their weight in preparation for their journey. And so they do. As days get shorter at the end of summer, their gonads decrease in size, drastically reducing the amount of weight the bird needs to get off the ground for the trip. Those species that stick around through the cold, snowy winter undergo a similar change. If conditions aren't right for breeding, why waste energy on organs you simply cannot use?
Then comes this time of year, when the one and only consistently reliable cue that the seasons are about to change is the increase in photoperiod. Birds must be ready to breed as soon as conditions are right, so their gonads begin to grow, returning to a functional state. Gonads produce hormones, hormones stimulate physical activity and, in males, song. And we humans get to enjoy the show.