Habitat Use by Clapper Rails in the Lower Colorado River Valley

-Densities of the Clapper Rail (Rallus longirostris yumanensis) were determined in marshy situations in the lower Colorado River valley in all seasons. We conducted a study to quantify environmental variables important to rails within censused areas along the 450 km of the lower Colorado River north of Mexico in order to learn more about their year-round habitat requirements. Quantified vegetation variables from 40 marsh areas were subjected to principal components analysis; four principal components collectively accounted for 75% of the variance. Habitat breadths of rails were broadest in summer and narrowest in winter. Marshes with the highest rail densities in one season tended to have large rail densities year-round. The converse was true for marshes with low densities. In the first of two analyses, 27 of the marshes (each censused monthly for two years) were used to determine rail associations with the vegetation principal components (PCs). Spearman rank correlations of rail densities with PCs revealed that rails were associated primarily with dense marsh vegetation (PC I) at all seasons. This outcome was tested and confirmed with data from 13 marshes censused during summer 1976 that were not included in the first analysis. One reed (Phrugmites austrulis) and two cattail (Typha domingensis) marshes of moderate foliage density consistently had more rails than expected. One dense cattail marsh consistently had fewer rails than expected. Size of marsh (2-29 ha) and bank slope into the water were apparently unrelated to density of rails per 10 ha. Censuses from this and unpublished recovery team studies suggest a rail population of about 750 birds for the lower Colorado River north of Mexico. The Yuma Clapper Rail (Rallus longirostris yumanensis), restricted to the lower Colorado River drainage system and the Salton Sea in the Imperial Valley of California, is considered an endangered species by the U.S. Fish and Wildlife Service (USFWS). Its numbers have been closely monitored because marsh habitat required by this species is only moderately abundant. In 1972, the USFWS established the Yuma Clapper Rail Recovery Team, which has since annually censused the breeding population along the lower Colorado River. These censuses did not include any quantification of the environmental variables associated with rail densities. Knowledge of the habitat requirements of rails comes largely from detailed, very local, single-year studies with emphasis on the breeding season. Such studies were conducted at Topock Marsh near the northern limit of the range of the Clapper Rail (Smith 1975) and at the western edge of the Salton Sea (Bennett and Ohmart 1978). Studying the habitat of the Clapper Rail over a large portion of its range is important because habitat relationships found in local areas can differ substantially from those over much larger (regional) areas (Wiens 198 1). Habitat relationships found by Bennett and Ohmart (1978) differed markedly from those found by Smith (1975). The rails in those studies inhabited stands of bulrush (Scirpus spp.) or cattail (Typha spp.) in differing degrees. We know of no studies about their use of other vegetation associated with marshes, such as reed (Phragmites australis), loosestrife (Lythrium californicum), and various grasses. In view of the endangered status of the Clapper Rail, data concerning habitat relationships over the entire area are necessary to properly manage the population. Since relationships between various bird species and the vegetation often change seasonally and annually (Rice et al. 1980; Laurenzi et al. 1982; Meents et al. 1982, 1983), data from as many seasons and years as possible should be included when attempting to clarify the relationship between a species and its habitat. In our study we examined Clapper Rail use of marsh habitat along the lower Colorado River for several seasons over a period of three