The whippoorwills used to drive me mad at night. Or lull me to sleep. I can’t remember anything in between, it was so long ago. In bed I heard them repeat their name over and over in the dark, relentlessly, one phrase: Whip-poor-will. Whip-poor-will. Whip-poor-will. Whip-poor-will. Whip-poor-will. Whip-poor-will. Whip-poor-will.

It’s a three-note song. In poetic meter, three syllables pronounced together in this pattern are called a cretic, or amphimacer – an unstressed syllable between two stressed syllables. (English lends itself to iambs – two-syllable patterns of one unstressed and then one stressed. The word “ago” is iambic. The iamb is the unit in that famous but tortuously enigmatic phrase, “iambic pentameter.”) The whippoorwill’s cretic song is unforgettable. When I called up an audio file of it from the Internet, it sounded exactly as I remember from summer nights four decades ago on the coast of southern Maine.

But I can’t remember hearing a whippoorwill since then. Part of the reason, obviously, is that I spent much of the 1970s and ’80s living in brick-and-pavement Portland, where the whippoorwills don’t go. They live in woods near open fields, and lay their eggs directly on the ground, usually among leaves. The birds and eggs are so intricately camouflaged they’re practically invisible. The city compared to the woods and fields is bug-free, while whippoorwills make their living snatching insects out of the dusk in their huge maws, like their cousins the nighthawks.

I’ve never actually seen a whippoorwill. But when they’re there, they jar the night loud and clear. One researcher counted a whippoorwill sing its name 1,088 times in a row. That was long after it had been named “vociferus” (genus Caprimulgus).

But when I escaped the expansion of Portland and moved to Waldo County, I still did not hear a whippoorwill. What is going on?

The short answer is: No one knows for sure. But a National Audubon Society study recently reported that whippoorwills are one of 20 common birds (including also evening grosbeaks, chickadees, grackles, meadowlarks and several kinds of sparrows) whose numbers have declined by as much as half in the last four decades. The researchers theorize that suburban sprawl, climate change and invasive species are reshaping whippoorwill habitat. The Massachusetts Audubon Society is conducting a survey to try to find out what’s happened to them – they’ve all but disappeared down there.

Here in Maine, whippoorwills can still be heard. Or at least, I’ve heard that people have heard them. I don’t know. Even stranger than the long, strange trip from the nightjar summer evenings of southern Maine to Troy, is how clear the whippoorwills echo in memory. As if the meter and pitch are burned permanently into the disk. I’d give almost anything to be driven mad by that song again.