We examined the effects of environmental and ecological factors associated with seasonal fecundity on spatial and temporal patterns of productivity in Sagebrush Brewer's Sparrow (Spizella breweri breweri) in British Columbia, the northwestern edge of its breeding range. This study is the first examination of seasonal fecundity in this species. Seasonal fecundity of females varied from 1998 to 2000. It was highest in 2000, when nest predation was lowest and number of clutches per female was intermediate, and lowest in 1998, when nest predation and number of clutches per female were the highest and warm El Niño conditions led to early breeding. Potential fecundity gains from early breeding were diminished by the interaction of shifting predation rates and variable effects of weather at different elevations. Early breeding in 1998 proved an advantage only at the low-elevation site, because an early spring storm destroyed 43% and 20% of first nests at two high-elevation sites. High seasonal fecundity varied between the sites, such that the best site in 1998 became the least productive in 1999 and vice versa. The overriding factor driving spatiotemporal variation was shifting rates of nest predation, though the elevation-related storm effects and variation in number of clutches were partly responsible. To maintain high productivity for Sagebrush Brewer's Sparrow at the northern edge of its range, where conditions are unpredictable and where there is no consistently best or worst site in terms of productivity, managers must protect sites from habitat loss or alteration across a range of elevations and conditions.
Fécondité Saisonnière de Spizella Breweri Breweri à la Limite Nord de Son Aire de Reproduction