Predicting Chick Survival and Productivity of Roseate Terns from Data on Early Growth

• Published in: Colonial waterbirds 1999-01, Vol.22 (1), p.90-97
• Main Authors: Ian C. T. Nisbet, Hatfield, Jeff S., Link, William A., Spendelow, Jeffrey A.
• Format: Article
• Language: English
• Subjects: Bird nesting; Chicks; Confidence interval; Datasets; Discriminants; Estimation methods; Hatching; Marine; Predation; Productivity; Sterna dougallii; Waterfowl
• ISSN: 1524-4695; 0738-6028; 1938-5390
• DOI: 10.2307/1521997

Early growth of Roseate Tern (Sterna dougallii) chicks is a strong predictor of chick survival and hence of productivity. Using data from a ten-year study at Falkner Island, Connecticut, we developed discriminant functions to predict chick survival from body-masses measured during the first three days of life. A discriminant function using only the mass of the B-chick (second-hatched in each brood) on day 2 of life yielded predictions of survival (83% correct classification) almost as good as any other. The correct classification rates were consistent across each of the 10 yr of the study. The simplest discriminant function gave good results (84% correct classification) in a limited test with a data set from another site. We derive a formula to estimate the total number of B-chicks raised to fledging from the number of individuals for which the discriminant function predicts survival. Productivity is estimated by assuming that almost all A-chicks (first-hatched in each brood) survive to fledging, and using the discriminant function and the formula to predict the number of B-chicks that survive. In the absence of predation, the resulting estimates of Roseate Tern chick survival and productivity are almost as good as those obtained by more intensive methods, but require less effort and less disturbance. This approach might be useful for other seabird species in which chick survival is determined primarily by parental performance.