The Puerto Rican Nightjar (Guabairo de Puerto Rico) is a small (22 cm) cryptically plumaged caprimulgid endemic to Puerto Rico. Adult plumage is mottled grey, brown and black (Kepler and Kepler 1973). Males have a black throat bordered by a white band. Similar to other caprimulgids, male nightjars have pronounced white in the outer tail feathers. Females have buff colored throat bands and tips of outer tail feathers (Raffaele et al. 1998). Semiprecocial chicks have reddish cinammon down at hatching. Immature nightjars exhibit adult plumage pattern by the time they are capable of sustained flight.
The Antillean Nighthawk (Chordeiles gundlachi) is the only other resident caprimulgid in the region of Puerto Rico and the Virgin Islands. The Antillean Nighthawk is a seasonal breeding resident and has a broad distribution in Puerto Rico. It is found in open areas throughout the coast, foraging in towns, agricultural fields, pastures, and coastal to mid-elevation forests. The nighthawk has pointed wings, unlike the more rounded wing shape of the nighthjar, and shows a prominent white band across the primaries, which is visible even at rest (Raffaele et al. 1998, Delannoy 2005). The Chuck-will's-widow (Caprimulgus carolinensis) is a Nearctic caprimulgid that migrates to and winters in Puerto Rico and other Caribbean islands, namely the Greater Antilles and Bahama islands. The Chuck-will's-widow has some resemblance to the Puerto Rican Nightjar; however, it is larger and has mottled reddish brown plumage, in contrast to the darker mottled gray, brown, and black pattern of the Puerto Rican Nightjar (Raffaele et al. 1998). Moreover, male Puerto Rican Nightjars have greater amount of white in the tail feathers. In Puerto Rico, the Chuck-will's-widow is found in woodlands along the coast to forest at mid-elevation. They are a common winter resident in the moist karst forests of northern Puerto Rico (F.J. Vilella, pers. obs.) where the nightjar has not occurred in approximately a century.
Adults (sexes alike) are considerably darker and smaller than the Whip-poor-will. Puerto Rican Nightjars have black upperparts mixed with gray and brown. The crown has a dark reddish brown shade, ear coverts and hindneck ochraceous-tawny to a paler buff. Dorsally, nightjars also exhibit dark plumage coloration with vertical, light wing patches. Throat, upper breast and base of the wings are reddish brown. Upper rectrices are grayish brown with black bands. Underside of the rectrices has white markings in males and buff tips in females. Amount of white on the tail of male Puerto Rican Nightjars is less than for the Whip-poor-will (Wetmore 1919, Reynard 1961).
Very little information available. Replacement of natal down starts approximately the first week post-hatching. The sheaths of the flight feathers start to appear and replace the cinnamon colored down that covers the chicks when they hatch. Developing rictal bristles also begin to appear at this time. Between 7 - 14 days of age, nightjar chicks assume an awkward appearance as their feather sheaths continue to develop (Kepler and Kepler 1973, Vilella 1989). Juveniles may have a partial pre-formative (post-juvenile) molt after the breeding season. Like other congeners of the New World adults likely have a post-breeding complete molt (Cink 2002).
Information on bare parts (Wetmore 1919, Reynard 1962) follows holotype described by Wetmore (Field Museum of Natural History FMNH 42099) and a specimen collected by Reynard and deposited at the U.S. National Museum of Natural History (USNM 476241):
Iris: Large, very dark black
Bill: Dark gray, with lighter gray tomia; long rictal bristles
Tarsi and toes: Gray; pads of feet grayish
Measurements are based on a combined data set of the two original specimens at the Field Museum of Natural History and the U. S. National Museum of Natural History (Smithsonian Institution), two males captured by Vilella (1989), and seven males captured at Guánica Forest and documented by Arendt et al. (2004). The only data for females comes from the holotype specimen described by Wetmore (1919). Data on weight of chicks and young is from Vilella (1995). Summary statistics include mean (standard deviation) and range. Mass is expressed in grams (g) and length of various body parts in millimeters (mm).
Mass: 36.11 (3.07), 33.8 - 38.1; 36.07 female
Mass of chicks and young: 4.13 (0.02) at hatching; 21.1 (5.1) after first week; 36.6 (2.5) after 14 days.
Wing: 133.1 (3.07), 128.9 - 135.8; 135.1 female
Culmen: 11.6 (1.65), 10.35 - 13.93; N/A for female
Nares: 6.91 (0.32), 6.57 - 7.33; N/A for female
Culmen depth: 2.78 (0.21), 2.57 - 3.07; N/A for female
Culmen width: 4.84 (0.30), 4.47 - 5.13; N/A for female
Primary (9th): 100.45 (2.65), 98.9 - 104.4; N/A for female
Tarsus: 17.95 (1.47), 15.86 - 19.21; 16.3 female
Tail: 108.66, 103.8 - 116.1; 112 female