Weather and Reproductive Variation in Shrubsteppe Sparrows: A Hierarchical Analysis

We used a hierarchial approach to investigate the relationships among within— and between—year variation in precipitation and temperature in the northern Great Basin and components of reproductive success in two species of common shrubsteppe sparrows. During a 5—yr period we monitored clutch sizes, brood sizes, and fledging success in Sage (Amphispiza belli) and Brewer's (Spizella breweri) Sparrows in shrubsteppe habitat in central Oregon. During the 5 yr of our study, the Pacific Northwest experienced precipitation extremes, with the driest bioyear (October—April) in 65 yr of records (1976—1977) followed by one of the wettest. Within each breeding season daily temperature and precipitation fluctuated widely and in most cases unpredictably over a scale of a few days. Both species achieved significantly greater final reproductive success (as measured by number of fledglings produced) in wetter, presumably more biologically productive years. In Brewer's Sparrows this was achieved through higher clutch sizes in wetter years, whereas Sage Sparrows had higher hatching rates. For Sage Sparrows, within—year weather variables accounted for a significant amount of variation in reproductive parameters beyond that statistically explained by bioyear precipitation alone. However, in Brewer's Sparrows addition of within—year weather variables was not significant, and thus this species appeared buffered from the effects of short—term variation in abiotic conditions. The observed patterns suggested that clutch—size variation in Brewer's Sparrows may represent adaptive flexibility, the ability to assess potentially favorable environmental conditions and adjust reproductive investment accordingly. In contrast, in Sage Sparrows the association between bioyear precipitation and final reproductive success seemed most likely to be a proximate consequence of resource availability at the time of oogenesis, and to be further modified by the effects of weather that each nest experienced.