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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 Nino-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. Received 29 January 2004, accepted 15 November 2004. Key words: breeding phenology, California coastal scrub, density dependence, density independence, El Nino-Southern Oscillation, Melospiza melodia, nest predation, nest success, population density, population dynamics, rainfall, reproductive success, Song Sparrow, temperature, weather.

Understanding nest survival is critical to bird conservation and to studies of avian life history. Nest survival likely varies with nest age and date, but until recently researchers had only limited tools to efficiently address those sources of variability. Beginning with Mayfield (1961), many researchers have averaged survival rates within time-specific categories (e.g. egg and nestling stages; early and late nesting dates). However, Mayfield's estimator assumes constant survival within categories, and violations of that assumption can lead to biased estimates. We used the logistic-exposure method to examine nest survival as a function of nest age and date in Clay-colored Sparrows (Spizella pallida) and Vesper Sparrows (Pooecetes gramineus) breeding in north-central North Dakota. Daily survival rates increased during egg laying, decreased during incubation to a low shortly after hatch, and then increased during brood rearing in both species. Variation in survival with nest age suggests that traditional categorical averaging using Mayfield's or similar methods would have been inappropriate for this study; similar variation may bias results of other studies. Nest survival also varied with date. For both species, survival was high during the peak of nest initiations in late May and early June and declined throughout the remainder of the nesting season. On the basis of our results, we encourage researchers to consider models of nest survival that involve continuous time-specific explanatory variables (e.g. nest age or date). We also encourage researchers to document nest age as precisely as possible (e.g. by candling eggs) to facilitate age-specific analyses. Models of nest survival that incorporate time-specific information may provide insights that are unavailable from averaged data. Determining time-specific patterns in nest survival may improve our understanding of predator-prey interactions, evolution of avian life histories, and aspects of population dynamics that are critical to bird conservation. Received 11 May 2004, accepted 29 November 2004. Key words: Clay-colored Sparrow, logistic exposure, nest survival, Pooecetes gramineus, Spizella pallida, Vesper Sparrow.