Predicting chick survival and productivity of roseate terns from data on early growth

Early growth of Roseate Tern (Sterna dougallii) chicks is a strong predicto r of chick survival and hence of productivity. Using data from a ten-lear s tudy at Falkner Island, Connecticut, we developed discriminant functions to predict chick survival from body-masses measured during the first three da ys of life. A discriminant function using only the mass of the B-chick (sec ond-hatched in each brood) on day 2 of life yielded Predictions of survival (83% correct classification) almost as good as any other. The correct clas sification rates were consistent across each of the 10 yr of the study. The simplest discriminant function gave good results (84% correct classificati on) in a limited test with a data set from another site. We derive a formul a to estimate the total number of B-chicks raised to fledging from the numb er of individuals for which the discriminant function predicts survival. Pr oductivity is estimated by assuming that almost all A-chicks (first-hatched in each brood) survive to fledging, and using the discriminant function an d the formula to predict the number of B-chicks that survive, in the absenc e of predation, the resulting estimates of Roseate Tem chick survival and p roductivity are almost as good as those obtained by more intensive methods, but require less effort and less disturbance. This approach might be usefu l for other seabird species in which chick survival is determined primarily by parental performance.