We studied the relationships among weather, reproductive success, and population density over 21 years (1980-2000) in a resident Song Sparrow (Melospiza melodia) population in coastal scrub habitat. Our goals were to test potential relationships between annual variation in weather and reproductive success, to evaluate whether reproductive success is density dependent, and to explore the effects of weather and population density on population dynamics. We analyzed the following components of reproductive success: clutch size, hatching success, fledging success, number of young fledged per successful nest, nestling weight, number of fledglings produced per female, number of broods per female, and probability of nest survival. During the study period, several very wet El Niño-Southern Oscillation (ENSO) events as well as an eight-year period of below-average rainfall occurred. Annual reproductive success and population density showed strong positive relationships to annual rainfall levels. Song Sparrow reproductive success increased with increasing rainfall and peaked at above-average rainfall levels. However, no component of reproductive success was related to an index of variation in the ENSO (i.e. Southern Oscillation Index). The components of reproductive success most strongly associated with rainfall were probability of nest survival and length of the breeding season. Nest predation rates were lower in wetter years. Song Sparrows nested over a longer period in years with higher rainfall and lower summer temperatures, and there was an increase in the number of successful broods per female in wetter, cooler years. Nestling weight was not associated with annual rainfall. Neither clutch size nor the onset of laying was associated with spring temperatures. Warmer summer temperatures were associated with a decrease in number of young fledged per successful nest. We found no evidence for decreased reproductive success in years of higher population density, which suggests that reproductive success was limited primarily by density-independent factors, such as weather. Changes in population density from one year to the next were positively related to rainfall-associated variation in reproductive success and negatively related to population density in the previous year. Rainfall in the previous year explained at least 49% of the annual variation in population density. Thus, population dynamics appeared to be driven both by density-independent and density-dependent factors.
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Vol. 122 • No. 2