Those of us in the east and midwest really have only one Chordeiles to worry about most of the time, the Common Nighthawk. In the deserts of the southwest, however, a second species, the Lesser Nighthawk, flies into the identification ointment.
Common Nighthawks molt their flight feathers on the wintering grounds, far south of the US. Adult Lessers, in contrast, are shedding primaries this month, resulting in strange (but symmetrical) gaps and patches in the outer wing, as shown by the blurry bird in the photo. Neat, huh?