I wonder what exactly inspires an individual chickadee to sing its spring song. It doesn't seem to have much to do with actual spring. It can't be the length of day, because I've heard them sing in pre-solstice December, and it can't be the temperature because I've heard them sing in sub-zero conditions. And it's not the available light, because today it was cloudy and overcast, and yet this morning while I was waiting for the bus, about half a block away there was a chickadee whistling a soft, clear fee-beee, fee-beee. It must be nice to be that optimistic about spring.
Any birders out there want to offer any insights or speculations?