Reproductive Biology of Shrubsteppe Passerine Birds: Geographical and Temporal Variation in Clutch Size, Brood Size, and Fledging Success

We investigated geographical and temporal variation in clutch size, brood size, and fledging success in three common passerine species in shrubsteppe habitats of the northern Great Basin of western North America. Over a 2-year period we surveyed three vegetationally similar sites spanning a latitudinal gradient of 650 km (the geographical component), one of which we sampled for five consecutive years (the temporal component). Clutch sizes of Sage (Amphispiza belli) and Brewer's (Spizella breweri) sparrows did not differ significantly among sites, but brood sizes and (for Brewer's Sparrow) fledging success did. These geographical differences were associated with site-specific variation in nest-predation rates and were attributed to differences in the abundances of snakes. Nests at a site in central Oregon that lacked snakes suffered virtually no predation during the 2-year geographical sampling, whereas nests in southeastern Washington and northern Nevada suffered significantly higher rates (40% and 93%, respectively). During the 5-year temporal sampling at the central Oregon site, all measured reproductive parameters of Sage and Brewer's sparrows and clutch size in Sage Thrashers (Oreoscoptes montanus) varied significantly. Although the region experienced extremes of precipitation during this period, in Sage Sparrows only brood size was significantly correlated with rainfall during the previous winter. The principal source of variation in Sage Sparrow final reproductive success was predation, which peaked during an outbreak of Townsend's ground squirrels (Spermophilus townsendi) in 1979. This ground squirrel irruption appeared to be triggered by a pattern of a drought year followed by two relatively wet years, beginning in winter 1976-1977. Although Brewer's Sparrows also experienced considerable predation pressure in 1979, clutch size, brood size, and fledging success were significantly correlated with winter precipitation. No component of Sage Thrasher reproduction was associated with climatic variation. The relationship between avian reproductive success and precipitation in arid regions is more complex than the traditional view of desert birds responding directly to increased primary and secondary productivity; such relationships are indirectly influenced by the effects of rainfall on predator populations, and are further complicated by time lags (up to 2 years) in predator response.