Landscape-scale patterns of black-throated sparrow (Amphispiza bilineata) abundance and nest success

Analyses of avian demographic patterns across entire, contiguous landscapes are rare, but such analyses are important for understanding population dynamics. We selected the Black-throated Sparrow in the northern Chihuahuan Desert as a model to test patterns of abundance and nest success across a landscape. We integrated abundance, nest density, and nesting success measured on sampling plots with a classified satellite map of the distribution of seven habitat types to analyze spatial and temporal patterns contributing to the population dynamics of this species. Adult relative abundance ranged from ,1 bird/100 ha in pinyon–juniper habitat to 24– 39 birds/100 ha in shrubland habitats. Nest density was consistently high in mesquite, moderate to high in creosotebush, and low in black grama grassland; this value exhibited more temporal variability than relative abundance of adults. Nest success rates exhibited a strong habitat effect and ranged from 8% in mesquite to 47% in black grama grassland; overall population nest success was 0.266. In all three years, nest success in mesquite was significantly lower than in all other habitat types (P , 0.01). There was no correlation between nest success and adult relative abundance. While mesquite habitat contained about one-third of all adults in the three years of the study, it contributed as little as 10% of successful nests. In creosotebush, the relative contribution to both adult abundance and successful nests was relatively high. Mesa grassland contained relatively few adults, but up to 44% of successful nests. We discuss how habitat selection theory suggests mechanisms for the observed patterns. Mesquite appears to be a population sink for Black-throated Sparrows and may be an ecological trap. While we do not propose that there is cause for conservation concern for this widespread species, our results underscore the pitfalls associated with using adult abundance as an indicator of habitat quality. The method presented here is applicable for many species and ecosystems and, thus, may be an important tool for conservation and management, as well as a new avenue for scientific investigation of landscape-level population dynamics.