A test of water depth niche partitioning by Western Grebe color morphs

Darkand light-phase Western Grebes (Aechmophorus occidentalis) have been described by Storer (1965) and Ratti (1981). Ratti (1979) and Nuechterlein (1981) reported on reproductive separation and isolating mechanisms between the color morphs. Data from Ratti (1979) indicate that "darkand light-phase birds represent reproductively independent populations" and that the morphs are separate species. Ratti (1979) stated that "morphologically and ecologically, darkand light-phase Western Grebes are practically indistinguishable" and may represent "one of the best avian examples of an exception to the competitive exclusion principle" (Grinnell 1904). However, Nuechterlein (1981) reported that light-phase birds tend to feed farther from shore than dark-phase birds. He also noted a preponderance of "level dives" by dark-phase birds and a preponderance of "springing dives" by light-phase birds. The light-phase diving posture is more perpendicular to the water surface and may facilitate diving to greater depths. These findings led to Nuechterlein's (1981) hypothesis that "the two color phases may be segregating behaviorally into two subtly different ecological forms specialized for feeding at different depths." The objective of this research was to test Nuechterlein's (1981) hypotheses. Data were gathered at Goose Lake, Lower Klamath National Wildlife Refuge (NWR), and Tule Lake NWR, California; Upper Klamath Lake, Oregon; and Bear River NWR, Utah. Goose and Upper Klamath lakes are natural water basins; Lower Klamath, Tule Lake, and Bear River NWRs are artificial water impoundments. All surveys were initiated at sunrise, when most birds were actively feeding, and usually terminated by midmorning. Survey routes were determined from previous field work (Ratti 1979). One survey was conducted at each study area and, thus, all data are statistically independent. Birds were classified as dark-phase or light-phase (Ratti 1981); rare intermediates were not recorded. Each bird observed was recorded into 1 of 3 categories, based on an ocular estimation of distance from shore; A = 75 m. The location, number of birds, and color phase also were recorded on a map of the survey route. After completion of bird surveys, I returned to bird observation sites and conducted a minimum of 3 water-depth transects. Transect lines were 100 m apart, parallel, and generally perpendicular to the shoreline of the observation site. I traversed the transect lines from a canoe, and recorded water depth from ocular estimated distances of 15, 50, and 100 m from shore to correspond with the distance categories of bird observations. Throughout the study, distances were measured periodically from shore to verify my ocular estimations. Data from bird observations were tested using Chi-square analysis and mean depths were compared by t-test. Between 15 June and 20 July 1981, I observed 1,132 dark-phase and 596 light-phase Western Grebes and measured water depth from 106 transects. Variation in the number of water-depth transects measured at each location was a function of the distribution of birds along the survey route. Distance from shore.-Observations of the color morphs in categories of distance from shore were not random (Table 1). There was a clear tendency for