Eastern Whip-poor-will
Antrostomus vociferus
This medium-sized nightjar measures 22–27 cm (8+1⁄2–10+1⁄2 in) in length, spans 45–50 cm (17+1⁄2–19+1⁄2 in) across the wings and weighs 42–69 g (1+1⁄2–2+7⁄16 oz). Further standard measurements are a wing chord of 14.7 to 16.9 cm (5+13⁄16 to 6+5⁄8 in), a tail of 10.5 to 12.8 cm (4+1⁄8 to 5+1⁄16 in), a bill of 1 to 1.4 cm (3⁄8 to 9⁄16 in) and a tarsus of 1.5 to 1.8 cm (9⁄16 to 11⁄16 in). Adults have mottled plumage: the upperparts are grey, black and brown; the lower parts are grey and black. They have a very short bill and a black throat.
ID text from Wikipedia, CC BY-SA 4.0

Mexican Whip-poor-will
Antrostomus arizonae
The Mexican whip-poor-will is 23 to 24 cm (9.1 to 9.4 in) long and weighs 45 to 50 g (1.6 to 1.8 oz). In general its upperparts are grayish brown with blackish brown streaks; the crown has wide blackish brown stripes. The throat and breast are blackish with a thin white band on the lower throat between them. The belly is buff with brown bars. The male's outermost three pairs of tail feathers have broad white tips; the female's are narrower and buffy. The wings are brown with tawny and buff spots and speckles. The subspecies differ somewhat. Voice: The Mexican whip-poor-will is more often heard than seen. Its song is an onomatopoeic whip-poor-will like that of its eastern congener, but lower in pitch and "less 'snappy' and slightly 'lazier'". It also makes a variety of other sounds including "a single, low, mellow quirt or queerp note", a growl, and a "growl-chuck".
ID text from Wikipedia, CC BY-SA 4.0

Where you might meet both
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Play today's Daily BirdPhotos and recordings are credited to their authors under the licence shown. Full profiles: Eastern Whip-poor-will and Mexican Whip-poor-will.