Seasonal Population Fluctuation of White-throated Swifts at Roost Sites in Southern California

In southern California, White-throated Swift (Aeronautes saxatalis) roosts contain many more individuals during the winter months than during the breeding season.Dates of the high and low counts at these roosts fluctuate from site to site and from year to year.Evidence suggests that some individuals remain and nest at or near the roost; whether others migrate north to breeding areas remains unknown.The White-throated Swift (Aeronautes saxatalis) is widespread in much of western North America, where its bold patterning, loud vocalizations, and aerial habits make it easily detected and identified.It occurs year round in southern California as well as from central California, central Arizona, southern New Mexico, and southwestern Texas south to Oaxaca, Mexico (AOU 1998).It is a breeding summer resident only northward along the Pacific coast to northern California and inland to eastern Oregon, southern interior British Columbia, and southern Alberta (AOU 1998, Ryan and Collins 2000).Little detailed information about White-throated Swift migration (Hughes 1998, Ryan and Collins 2000)is available.The species is absent from the more northerly parts of its range for many months, in British Columbia, for example, from mid August to early April (Campbell et al. 1990, Goward et al. 1995).Grinnell and Miller (1944) and Small (1994) suggested that in California the White-throated Swift is more common and widespread in summer.Garrett and Dunn (1981) stated that in southern California, "this species appears more irregularly and in lesser numbers in most areas in winter, although large concentrations may still be found in some coastal and desert areas."White-throated Swifts nest and roost in narrow horizontal or vertical cracks and crevices in rocky cliffs (including sea cliffs) and rock quarries (Hanna 1909, 1917, Bent 1940, Anderson 1943).They also adopt manmade structures such as buildings (Yocum 1966, Collins and Johnson 1982), freeway overpasses, and bridges (Ryan and Collins 2000).Little is known, however, about their specific requirements for roost sites or nest sites.In this study we examine seasonal fluctuations in the populations of Whitethroated Swifts at roost sites in southern California by describing the attendance of swifts at three year-round roosts and making comparisons between breeding and winter periods. STUDY AREA AND METHODS As part of a broader study of swift behavior from late February

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