I knew it wasn't a rattlesnake that I'd found basking on the Nashua River Rail Trail last evening. It couldn't be.

Timber rattlers are extremely rare in New Hampshire and eastern Massachusetts (they are plentiful in parts of the Berkshires, however). Besides, the little guy I spotted, soaking up the last of the macadam's warmth, didn't look like a rattler. Its head was too small, for one, and its body too narrow, its greatest girth little more than an inch despite being roughly 30 inches long.

These are rough estimates, mind you. I lacked both a tape measure and inclination to get quite that close. It did have some regular, patterned markings, and the tip of its tail quivered menacingly as I stomped the ground nearby, trying to persuade the snake that it would be safer off in the grass. I'd nearly run it over myself. I eventually persuaded it to move along, though not without being startled (twice) when it turned to feint a strike at my front tire.

Home safe and sound later that evening, I consulted the U-Mass Amherst's excellent online guide to the snakes of New England, and identified my little friend as a common milk snake.

Both the milk snake and another common snake with a much cooler name, the black racer, will shake their tails when threatened, the guide notes. In dry leaves, where they often are found, they can sound a lot like rattlers.

So which came first, the rattle or the shake? At first guess, I assume these lesser serpents mimick the more fearsome rattlers, but who knows? Perhaps snakes found it useful, over the eons, to make a rustling noise to warn off or confuse predators, and the rattlesnake found a way to do it better than others. Someone with actual knowledge about snakes probably knows, but not me.

Another excellent wildlife identification guide, BTW, is the the turtle species guide on the Turtle Atlas website. It's great to have pictures and descriptions of every turtle you're likely to encounter, all in one spot.