Fig. 6. Logistic regression of the presence of a breeding Wood Thrush on the log-transformed mass of calcium-rich prey, excluding slugs. The green circles represent the actual presence (1) or absence (0) of a breeding thrush. The blue diamonds represent the predicted probability of a breeding thrush presence, given the biomass of calcium-rich prey, i.e., snails, isopods and myriapods. The relationship is marginally nonsignificant (P = 0.0559), but predicts the presence of Wood Thrushes relatively well, correctly classifying 79% sites in which a breeding Wood Thrush was present (Table 2). The dashed line represents the cutoff or threshold value: points above or to the right of the line are predicted presences. The green dot at ~ -7.5 represents several sites in which no calcium-rich prey were found.