A quantitative investigation examining the effect of changes in wetland habitat on reproductive output of Western and Clark's grebes (Aechmophorus occidentalis and A. clarkii, respectively) was conducted. Earlier studies examined local factors influencing nest success. This study supplements earlier work by seeking to determine which landscape-level habitat elements best predict annual landscape-level reproductive output of grebes. Western and Clark's grebes were monitored during the breeding season from 14 September 1998 to 20 September 2010 at Eagle Lake, California, USA. Remotely-sensed images were used to develop habitat indices and quantify changes in wetland availability and quality. The effect of these indices, and lake level, on annual reproductive output was analyzed using an information theoretic approach. Indices of habitat availability (βi = 0.20, 95% CI = 0.099–0.31) and habitat quality (βi = 0.28, 95% CI = 0.012–0.45) best predicted and had a positive effect on annual reproductive output. Lake level had little direct predictive power but was highly correlated with the other habitat covariates (Pearson's r > 0.80), indicating its importance as an indirect predictor of reproductive output. Thresholds of habitat availability and habitat quality were found, below which steep declines in reproductive output were observed.
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Vol. 41 • No. 3