Male Nighthawks are distinguished by a white sub-terminal tail band which is absent in females and juveniles.
The birds are often flying with their huge mouth agape when hawking flying insects, but I only caught two of them doing so.
I apologize for these blurry shots, just using them to illustrate this behavior.
Here's a link to a video featuring recordings - the peenting nasal call and the boom made by the wings when the bird is diving - narrated by Macaulay Library curator Greg Budney. Great photos too, much better than mine.
Finally on the last day, just when the numbers of passing Nighthawks had tapered to almost nothing, we were treated to two Bald Eagles circling above.
Happy Fall Birding!