Fig. 2.

Boxplot of coefficients of determination (R²) for determining the minimum length of a drift fragment. Smooth splines were fitted to the time traces of vertical speeds using the data in each subsample (see Materials and methods). The dive fragments in each subsample were longer than a pre-determined length (x min) and were derived from one of the 11 individuals. For each fitted spline, R² was calculated as R²=1−(residual sum of squares/total sum of squares). This boxplot summarises the splines' goodness of fit. The fit improves starkly for splines fitted to vertical speed time series of fragments longer than 8 min.