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Reproductive Success of Belding's Savannah Sparrows
in a Highly Fragmented Landscape

Discussion


Numbers of Belding's Savannah Sparrows increase with size of wetland area (see Herkert 1994 for similar results in migratory Savannah Sparrows), but only four extant wetlands in southern California are larger than 100 ha. Marshes smaller than 10 ha were less likely to support breeding sparrows.

Size and isolation of habitat of marshes in San Diego Bay appeared to be related to reproductive success, but our sample size warrants further study. Although F Street Marsh was larger in size than D Street Marsh, it was isolated from other marsh habitats and surrounded completely by an urban landscape. This small site supported breeding sparrows but produced no offspring during our study. In addition, no movements of banded birds were observed between F Street and other marshes in the area, indicating that immigration may be limited. Future investigations should examine reproductive success and dispersal across a broader landscape of remnant marshes than we studied.

Other species of salt marsh-nesting sparrows, such as the Seaside Sparrow (Ammodramus maritimus), suffer nest loss during high tides (Marshall and Reinert 1990). Nesting habitat in drier portions of the marsh often is not limited, unlike marshes in southern California. Belding's Savannah Sparrows established territories throughout salt marsh habitats, but were associated with drier sites that had tall, dense vegetation. High tides (> 1.9 m) inundated all of our study plots each month from April to August, but it was unknown whether any nests were lost during these events. The fact that Belding's Savannah Sparrows established territories even in wetter sites, however, suggested that habitat was limited.

In southern California, development surrounding coastal marshes has influenced the high-marsh habitat because it typically is dry most of the year and may not have been delineated as "wetland" prior to the establishment of recent guidelines. Thus, Belding's Savannah Sparrow habitat has been restricted not only in overall availability, but also within the high-marsh. This creates a tradeoff for the sparrows. Nests that are located away from potential tidal inundation are closer to the wetland edge, where the effects of predation and human disturbance may be higher. The edges of most salt marsh remnants in this region are "hard", abutting urban landscape features such as roads, flood-control channels, airport runways, and residential lawns.

Reproductive success has not been linked to habitat characteristics in a convincing manner in migratory Savannah Sparrows (Ross 1980a, Bédard and LaPointe 1984, LaPointe and Bédard 1985, Wheelwright and Rising 1993). Although variation in reproductive success of Savannah Sparrows in northern climates has been attributed to variation in weather (Welsh 1975, Ross 1980a, Bédard and LaPointe 1985), no severe weather occurred during the breeding season in 1995, which is typical of the Mediterranean climate in southern California. Predation on adults, eggs, and young, however, may be related to habitat features, with dense vegetation providing more escape cover than do open areas (Ross 1980b, Bédard and LaPointe 1984, LaPointe and Bédard 1986, Watts 1991). Our study showed that high-success territories were associated with tall, dense vegetation within large marsh fragments.


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