Mid-story forest, aerial sallying. Vilella (1989) reported a limited number of foraging observations for the Puerto Rican Nightjar at Guánica Forest. Aerial insects are captured at foliage heights of 1.5 to 5 m by sallying from favored perches. Nightjars perch on branches approximately 2.5 m above the forest floor from where they sally after insects before returning to the same branch. Frequently, nightjars will return to their perch with a captured insect in its bill. These are usually large insects, mostly moths or beetles. After landing, nightjars hold their head upright, shake and swallow the prey. At Guánica Forest, Puerto Rican Nightjars also feed on insects attracted to artificial light sources. Nightjars will sally out keeping their bill open as they fly through clouds of small insects. Larger scarabaeid beetles (Phyllophaga spp.) often will flutter to the ground. Nightjars land and with outstretched wings, pick the beetles from the ground. Juvenile nightjars frequently sit on the ground before making short sallies to capture flying insects.
Puerto Rican Nightjar males maintain territories throughout the year. As calling rate increases during the latter half of December, territorial encounters between neighboring males become more frequent (Vilella 1995). The least intense encounters involves bouts in which two or more males sing near each other. They engage in these singing matches for 10-15 minutes with short, quiet intervals.
The more intense encounters usually involves a pair of neighboring males. In one type, two males will sing from branches less than 10 m apart. After prolonged singing, males may fly up above the canopy uttering a hoarse call and clasp bills in midair. These encounters will last approximately 5-10 seconds, after which the birds release their grip and fly back to favored singing perches. The most intense encounters involve males who after clasping bills in midair, will flutter down to the forest floor and roll in the leaf litter, all the time emitting a loud growling sound. These intense encounters become less frequent as the breeding season progresses.
The size of two nightjar home ranges, estimated from radiomarked males, were 4.8 ha and 5.6 ha (Vilella 1995; see Reproduction).
Monogamy. Like other caprimulgids, Puerto Rican Nightjars are believed to be monogamous. Vilella (1995) reports two instances of courtship behavior at Guánica Forest. One pair was observed courting 2-3 days before the first egg was laid, while the other was observed courting approximately 5-7 days before the nest was discovered. On both occasions courtship behavior occurred during early night hours (1900-2100) and within 30 m of where the nest was subsequently located. During courtship, both members of the pair perched horizontally facing each other less than a meter apart. The male sang repeatedly for about 30 seconds. After terminating the song, he spread his tail, drooped his wings, and vibrated his body as the female watched. While holding his tail spread and wings drooped, he would walk slowly towards the female. During the male's approach, both nightjars emit soft, clucking sounds. After approaching to within 50 cm of the female, the male would fly approximately 2 m away and resume singing. Approximately 3 days before egg laying begins, females roost during the day on the forest floor within 10 m of where the eggs are subsequently deposited.
Social and interspecific behavior
Territorial. Puerto Rican Nightjars do not aggregate in groups nor interact significantly with other conspecifics outside the breeding season.
Predation of breeding nightjars and their nests by exotic mammals has been documented (Vilella 1995). The remains of an incubating nightjar male and the eggs were discovered crushed and consumed. The predator responsible was likely a mongoose since a feral cat (Felis catus) probably would not have consumed the eggs. Even if a feral cat had killed the adult and left the eggs exposed for an avian predator, the remains of the eggs had been crushed, not pecked. Mongoose abundance is low at higher elevations in Guánica Forest (Vilella and Zwank 1993b). However, the depredated nest was located relatively close to the reserve headquarters. Mongoose are known to actively seek recreation areas where fresh water and refuse are readily available (Coblentz and Coblentz 1985).
Avian predators have been reported to take eggs from Puerto Rican Nightjar nests (Vilella 1995). At Guánica Forest, the Pearly-Eyed Thrasher (Margarops fuscatus) is an active ground forager and nest predator that commonly take eggs and young of smaller passerines (Raffaele et al. 1998). During August and following the conclusion of the breeding season, juvenile Puerto Rican Nightjars are frequently encountered sitting near the forest edge along the trails of Guánica Biosphere Reserve. A juvenile nightjar was observed flying across a trail when a Short-eared Owl (Asio flammeus) flew out and swooped at the young nightjar (Vilella 1989). The Short-eared Owl is a permanent breeding resident at Guánica Forest, where they roost in natural forest openings and nest on the base of dead Puerto Rican century plants (Furcraea tuberosa). Ants can also overwhelm nightjar chicks while hatching. Several species of carnivorous ants are found in Guánica Forest (Torres et al. 2001). While inspecting a nest located in semi-deciduous forest, a male nightjar was found brooding a 2-day old chick approximately 80 cm from the nest site. At the nest, a partly pipped egg was found completely covered with ants. The chick apparently had been killed by the ants as it was attempting to emerge from the egg. The male moved the surviving chick from the ant's path, avoiding the loss of his entire brood (Vilella 1989).