Abstract
It has been hypothesized that clutch size in bird species occurring in arid habitats is influenced by annual rainfall. We propose an alternative hypothesis that avian clutch size in resident species should be more strongly associated with rainfall in the short-term (during egg-formation) than during the long-term (cumulative across the season). We tested this hypothesis with museum egg-sets for California Gnatcatcher (Polioptila califomica) nests taken in southern California combined with rainfall data from the geographically closest weather station to nest collection site. Clutch size was independent of laying date. Seasonal rainfall was not a good predictor of clutch size; rainfall during egg formation was a better predictor. Using isotonic regression, we detected a strong positive trend in the association between clutch size and cumulative rainfall, with cumulative rainfall across 1 month prior to the estimated month of clutch completion having the strongest positive association. These data support the hypothesis that smaller clutches result from more immediate conditions, not from the wet-year/dry-year dichotomy.
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