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Abstract:
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When it is not possible to obtain direct survey estimates needed, it can be possible to obtain robust estimates using meta-analysis. As part of a larger effort to update and improve demographic parameters used in eagle population modeling efforts, Brennan and Millsap (2016) compiled a dataset of contemporary productivity information for bald and golden eagles, Haliaeetus leucocephalus and Aguila chrysaetos respectively, across the U.S. from 1995–2014. As in many ecological studies, individual surveys are done over multiple areas and/or multiple years. To obtain a representative estimate over areas and years, the variation over areas and years must be accounted for within the individual studies. Centered random effects accounted for separate spatial and temporal effects within studies so that overall estimates are apart from the individual study area and year variation. A random-effects meta-analysis model estimated the predictive distributions for bald eagle and golden eagle productivity. Differences between models were accounted for by differences in AIC. Bald eagle productivity differed by region with lower productivity in the Southwest (mean = 0.77, SE = 0.249) than in the rest of the continental U.S. (mean = 1.15, SE = 0.252), whereas golden eagle productivity did not differ by region (mean = 0.55, SE = 0.087). Apart from the fixed stratum differences for bald eagles, the best-supported models included standard errors for the random effects for study, area (bald eagles only), year given study, and overdispersion; the extent to which the random effect credible intervals overlapped zero varied by species.
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