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Fig. 2. Choice of prey as a function of visual occlusion (experiment I). Each data
point is the proportion (mean ± s.e.m.; N=6 birds; 9 trials
per bird per day over 2 days) of choice of prey side (`correct choice'). The
prey (live or a dead fish, 12–15 cm in length) was presented behind a
transparent Perspex door (open squares) on days 1, 2, 3 and 9, or behind a
visually opaque fabric door (solid squares) on days 4 to 8. Targets were 1.4 m
from the Y-junction. Broken line indicates the critical value for significant
choice (0.78).