Have you ever held to your ear a living sparrow, or finch, or warbler and heard its heart beat? Have you ever felt the almost terrifying lightness of those creatures, who, out there at the feeder or among the trees, seem substantial enough? Everything changes once you've done it, as I have done it. The warmth in the feathers could only signify life; but I stilled the bird between fingers whose power I had never imagined before that moment. Each finger weighed orders of magnitude more than the bird, and any one of them might have killed it. I was terrified that in my awkwardness I would kill it. Holding a thing so wild and wildly different helpless in my hand, I felt disoriented and awed. That first time I listened to a warbler's heartbeat my heart in hiding stirred for the bird. I couldn't guess how many times a second its heart beat, but it seemed that no thing, even in my imagination, could live at so intense a pace. And although the rapping, almost whirring, of the heart was slightly muffled by the layer of delicate feathers, probably heavier than the bird's bone structure, its sound came to me as a roar of energy out of control, more rapid than any machine. The energy was life itself; the bird was overwhelmingly alive.

I've since learned that a house sparrow's heart at rest beats at the rate of 460 a minute.