SPECIES RICHNESS, RELATIVE ABUNDANCE, AND HABITAT ASSOCIATIONS OF NOCTURNAL BIRDS ALONG THE RIO GRANDE IN SOUTHERN TEXAS

The segment of the Rio Grande between International Falcon Reservoir and Del Rio, Texas (distance ca. 350 km), remains largely unexplored ornithologically. We surveyed nocturnal birds monthly during February June 1998 at. 19 stations along the Rio Grande (n = 6) and at upland stock ponds (n = 13) in Webb County, Texas. We conducted 10-min point counts (n = 89) after sunset and before moonset. Four species of owls and five species of nightjars were detected, Nightjars, as a group, were nearly five times more abundant (mean number/count = 2.63) than owls (mean number = 0.55). The most common owl, the great horned owl (Bubo virginianus), had a mean number of 0.25/point count. The mean for elf cowls (Micrathene whitneyi) was 0.16/point count. The most common nightjars were the common poorwill (Phalaenoptilus nuttallir; 1.21/point count) and lesser nighthawk (Chordeiles acutipennis, 1.16/point count). Survey sites on the river supported more species (mean = 2.2) than did upland stock ponds (mean = 1.4). However, only one species (common pauraque, Nyctidromus albicollis) showed a preference for the river sites. Our results establish this segment of the Rio Grande in southern Texas as all area of high diversity of nightjars in the United States, matched (in numbers of specics) only by southeastern Arizona and southwestern New Mexico.