Thanks for the photo, which is of a flock of Common NIghthawks, Marcia.
A field mark that is useful is the shape of the triangle formed
by the white bar and both edges of the wing tip. In the photo
that triangle is isosceles, as Common NIghthawks show), vs the equilateral triangle that Lesser Nighthawks
show. That can be hard to see in the field, so the photo helps a lot.
Plus Lesser Nighthawks occur in CO as singletons (up to two at favored sites
like the Nucla Sewage Ponds, where they are almost annual); the east slope flock of twenty you saw is a common occurrence
for migrating Common NIghthawks.
Those photos are actually good at sorting out the two species.
Joe Roller, Denver