Distribution and Density of Black Rail Populations Along the Lower Colorado River

DISTRIBUTION AND DENSITY OF BLACK RAIL POPULATIONS ALONG THE LOWER COLORADO RIVER CHARLES F. REPKING AND ROBERT D. OHMART Black Rail (Laterallus jamaicensis) popula- marsh areas and stopping every 40 m to play the tions appear to be declining in the west be- recordings. The recorder was played at maximum volume for three to five min. Censusing was halted cause of habitat destruction (Wilbur 1974). when the wind velocity exceeded approximately 18 The discovery of an inland population near kmph. While the tape was playing, the observer Yuma, Arizona was, therefore, of great inter- moved about 20 m away from the recorder in order est. Black Rails were first verified by Snider to hear responses more clearly. After the tape was (1969) in California near Imperial Dam and stopped, we waited two more minutes for delayed responses. The recorder was then moved about 40 later were seen by R. L. Todd (pers. comm.) m and the process repeated. and by R. E. Tomlinson ( 1970, 1971) , who re- We used a boat for access to backwaters and fast ported a “small population” at Mittry Lake, water areas upriver where censusing on foot was not Arizona, also near Imperial Dam. Previously possible. In strong currents the tapes had to be the species was listed in Arizona as hypotheti- stopped frequently in order to hear responses before drifting out of range. cal (Phillips et al. 1964). Interested persons When a response was heard, the birds location had gotten responses to taped recordings, but was plotted on maps or aerial photos. The time of little was known about the biology of the spe- day, type of response, and description of the habitat cies on the lower Colorado River. were also recorded. We present data on numbers and distribu- RESULTS tion of Black Rails from an intensive survey in 1973-74 along the lower Colorado River The number of individual responses increased from Davis Dam south to the Mexican border. in five areas from summer 1973 to summer Our study of the habitats used by Black Rails 1974, decreased in six areas, and remained is in progress. constant in one (Table 1). During the winter census, the birds did not respond as readily STUDY AREA AND METHODS as in the breeding season. For all three census periods, the rails were most numerous in Following a spring 1973 reconnaissance of the Colo- seepage areas below Imperial Dam. In the rado River from Davis Dam S to the International Boundary and discovering no birds north of Imperial spring of 1973, densities were 1.14 rails per Wildlife Refuge, we selected a primary study area ha; in 1974, 1.58 rails; and in the winter of consisting of a 30-km section of river banks, marshes, 1973-74, 0.73 rails. and backwaters bordered on the south by Laguna The 236 rails that responded during the Dam (18 km NE of Yuma, Arizona) and on the three census periods were a mean distance of north by Clear Lake, about 48 km N of Yuma, Ari- zona (Fig. 1). Approximately 2,060 ha of marsh and 23 m from a permanent edge such as a road, marsh-related habitats are in this area, of which 1,732 desert or open water (range l-92 m). The ha are north and 326 ha are south of Imperial Dam. birds appeared to use the 04 m perimeter Other areas censused included marshes near Yuma of the marsh habitat extensively. Individually Territorial Prison; confluence of the Gila and Colo- rado Rivers; marshes and backwaters near Poston, calling birds were slightly closer together in Arizona; Blankenship Bend area; and parts of Topock the spring of 1974 than in the spring of 1973. Marsh (322 km of the lower Colorado River). Rails readily answered tape-recorded calls during DISCUSSION the breeding season and were located with such calls (primarily the kit-kit-keer and grr) on a cassette The Black Rail population on the lower Colo- recorder. Censusing was begun approximately one- rado River fluctuated among areas from sum- half hour before sunrise during the two spring-summer to summer, but the total for the two years mer census periods and at sunrise during the winter. was similar (Table 1). The majority of the During the summer, we obtained responses at al- most any time of day or night; however, they were birds were within a 6.5 km radius of Imperial more frequent and easily obtained during the cooler Dam; the most preferred habitat was immedi- early morning hours. After 10:00 winds usually ex- ately below the dam. Total numbers obtained ceeded 18 kmph and calling was curtailed. During in the winter months were probably below the the winter rails were more responsive between sunrise and early to mid-afternoon. true density because the birds did not call We censused by walking the periphery of the as loudly or often. Our data show habitat [4@31 The Condor 79:486489, 1977 BLACK RAILS ALONG COLORADO RIVER 487 .:. Lake ..::;:‘ Marsh Martinez Lake Headgate Rock Dam ftpO.../ izona Channel Senator Wash -Idlife All American Canal Lagum FIGURE 1. Map of the lower Colorado River. Inserts are enlargements of areas containing Black Rails. occupancy year round which suggests a resi- the spring of 1974, water flow was greater dent population. than previously at both Parker and Imperial Fluctuation in water level appears to be a Dams. At Parker Dam, water flow increased critical factor, especially in the spring months 1,220 cubic feet per second (CFS) in April, (when the males are calling vigorously) and 1,568 in May and 2,087 in June. Imperial possibly the breeding season. Rail densities Dam had increases of 844 CFS in April, 895 in habitats above Imperial Dam may be ex- in May and 1,672 in June. The large amount plained partly by water fluctuations. During of water impounded behind Imperial Dam, 488 CHARLES F. REPKING AND ROBERT D. OHMART TABLE 1. Distribution and density of Black Rail each sex on the East Coast. He noted the kic- populations along the lower Colorado River. kit-kerr as being the call of the male, and a cuckoo-like croo-croo-croo as that of the fe- No. of birds recorded male, the latter being rarely heard. Wilbur Winter Lwzntion ?lG3um 1973-74 spGY (1974) speculated that the kit-kit-km was the call of the male along the Colorado River. California We only heard the croo call on three separate West Pond 16* 11 22 Imperial Reservoir 2 1 1 occasions in the early summer months. Squaw Lake 10 4 6 If only males give the kit-kit-kerr call Senator Wash 21 2 10 (Reynard 1974), then males were the more Small Island : 0 50 vocal of the sexes throughout the year. This Ferguson Lake would then indicate that our population esti- Unnamed areas 5 2 10 mates were based solely on male responses, TOTAL 60 20 54 yielding minimum population estimates. How- ever, definite information on breeding is lack- Arizona ing for the Colorado River population. The Mittry Lake 9* 8 10 paucity of croo calls recorded during the study Gila Main Gravity Canal seepage 9* 4 12 does not yet warrant enlarging the population Imperial Reservoir 14 2 9 estimate. Large Island 14 0 3 We found Black Rails in close association Imperial National with three-square bulrush ( Scirpus ohqii) , Wildlife Refuge 0 0 8 which in pure or mixed stands occupies about TOTAL % 14 42 150 ha in our study area. Three-square bul- GRAND TOTAL 106 34 96 rush is most dense ( stems/m2) in shallow water or saturated soil moisture situations * Mean from three surveys. on gentle slopes. The area below Imperial Dam provides shallow marshes which sup- port the highest densities of three-square bul- combined with the extensive amount of shore- rush and Black Rails. Habitat supporting the line provided by numerous backwaters, yields highest Black Rail densities was found around a stable water level and much potentially West Pond, the edges of the north end of suitable habitat for Black Rails. Normal water Mittry Lake, the slough north of Mittry Lake, fluctuations above Imperial Dam are within Senator Wash and Ferguson Lake. The 40 about 0.1 m, but fluctuations were greater ha of habitat which appear suitable at the during the spring of 1974 as flows were in- farm area of the Imperial National Wildlife creased. The small stature (7-8 cm) of the Refuge are managed for waterfowl, and tim- Black Rail, combined with a strong preference ing of controlled water level fluctuations ap- for habitats with only moist surfaces or very pears to be the primary reason for the ab- shallow water, indicate that high water levels sence of Black Rails in this area. We found would probably drive the birds into less suit- rails in adjacent habitats in 1974 but not in able habitats. When water levels were high the waterfowl management block. above Imperial Dam (spring 1974), some Three-square bulrush stands, shallow wa- birds may have moved farther upriver, as ter depth, gently sloping shorelines, and mini- indicated by the increased population in the mum water fluctuations appear to be nec- Imperial National Wildlife Refuge area, or essary components for supporting Black Rail downriver to more stable marsh situations. populations along the lower Colorado River. Below Parker Dam the habitat is periodi- cally inundated to supply water and power, SUMMARY and water level fluctuations may be >l m in a 24 hour cycle. This 1.0-1.5 m daily fluc- Censusing Black Rails through the use of tuation appears to discourage use of this habi- recorded calls proved effective in locating tat by Black Rails.

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