Demography and Populations

As with many other caprimulgids, estimates of life history parameters for the Puerto Rican Nightjar are lacking. Nightjar nesting success at Guánica Forest was estimated at 87% (Vilella 1995). Vilella and Zwank (1993a) estimated a minimum population size of approximately 1,400 – 2,000 individuals distributed among three separate areas of southwest Puerto Rico. However, updated estimates of abundance suggest nightjars may be more numerous than previously reported (González 2010). These new estimates relied on empirical models of detectability, not index counts, to generate nightjar density estimates (with precision) for Guánica Forest, Susúa Forest, and a small private reserve in the Guayanilla Hills (González 2010). This is a marked improvement from the methods employed previously (Kepler and Kepler 1973, Vilella and Zwank 1993a) as index counts rely on assumptions concerning detectability that are often nearly impossible to meet in most field situations (Rosenstock et al. 2002).

At Guánica, densities of 1.93 nightjar/ha were estimated in mixed evergreen-plantation forest and 1.71 nightjar/ha in semi-deciduous forest (González 2010).  These nightjar densities were higher than estimates previously reported (Vilella and Zwank 1993a). Also, the population density of the Puerto Rican Nightjar is higher than densities reported for continental caprimulgids (Conway et al. 2007, Wilson and Watts 2008). This may reflect a pattern of insular fauna known as compensation, whereby species in oceanic islands exhibit relatively higher densities compared with mainland forms (Brown and Lomolino 1998). Landscape habitat models predicted considerable more nightjar habitat exists than was previously estimated and highlighted several new areas of importance for the species (González 2010). More importantly, the amount of predicted nightjar habitat outside protected areas was considerably greater than previously reported (Vilella and Zwank 1993a). Predicted nightjar habitat included a number of new sites along the southern and southeastern coast of Puerto Rico.