3.3. Logistic regression analysis of factors affecting starfish activity
Hierarchal logistic regression models using both feeding vs. non-feeding and moving vs. stationary activities as the response variable were fitted with tests of significant effects based on the mean deviance. For feeding vs. non-feeding activities, the probability of feeding was explained by quadratic regressions in size for each combination of time of day and depth; the fitted curves together with 95% approximate confidence intervals and the observed proportions are shown in Figs. 2(a) and (b). The interaction between time of day and size of starfish is extremely strong for both depths, with small starfish tending to feed predominantly at night, whereas larger starfish tend to feed more frequently during the day. For the largest starfish (.50 cm) time of day does not appear to influence feeding behaviour, though few observations were available as reflected in the comparatively wide confidence intervals. Though there were significant interactions between depth and both time of day and the quadratic polynomials in size, the pattern of size and time of day effects is of similar form for both depths though more pronounced for shallow depths (,3 m). Pooling over depths results in the quadratic polynomials shown in Fig. 2(c).