Continent-wide patterns of song variation predicted by classical rules of biogeography.

We caution, however, that by including observer as a random factor for consistency with other models violated diagnostic tests, and there was a weakly significant negative association of beak length with elevation in the best fit model ( p = 0.049), which included observer as a fixed factor, and met model assumptions (Table S4 ), with longer beaks relative to body size at lower latitudes.

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