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First published online February 29, 2008
Journal of Experimental Biology 211, 866-872 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.014324

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Prey detection by great cormorant (Phalacrocorax carbo sinensis) in clear and in turbid water

Tamir Strod¹, Ido Izhaki^2,3, Zeev Arad¹ and Gadi Katzir^3,*

¹ Department of Biology, Technion – Israel Institute of Technology, Haifa 32000, Israel
² Department of Evolutionary and Environmental Biology, University of Haifa, Haifa 31905, Israel
³ Department of Biology, University of Haifa at Oranim, Tivon 36006, Israel

View larger version (6K): [in this window] [in a new window]   Fig. 1. Experimental setup. (1) Pre-test pool, (2) test pool, (3) underwater trapdoor, (4) Y-maze tunnel, (5) stimulus boxes, (6) video camera. Arrows depict the swimming paths of the cormorants. The position of the Y-junction could be moved towards or away from the targets.

View larger version (4K): [in this window] [in a new window]   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).

View larger version (5K): [in this window] [in a new window]   Fig. 3. Choice of prey as a function of prey distance (experiment II). Each data point represents the overall mean proportion of choices for the prey side (mean of means ± s.e.m.; N=6 birds; 9 trials per bird per day over 2 test days). Values above data points are corresponding subtending angles of the prey's height. Broken line indicates the critical value for significant choice of the prey side (0.75). Prey (dead fish) were 9 cm in length at all distances except 2.8 m, at which they were 10 cm in length.

View larger version (5K): [in this window] [in a new window]   Fig. 4. Choice of prey as a function of prey size (experiment III). Each data point represents the mean individual proportion of choices for the prey side (mean of means ± s.e.m.; 9 trials per bird per day; 2 test days; target distance to Y-junction 3.1 m). For a prey length of 5 cm, the results for three birds are for nine trials only. Broken line indicates critical value for significant choice of the prey side (0.75).

View larger version (10K): [in this window] [in a new window]   Fig. 5. Choice of prey as a function of water turbidity and distance (experiment IV). Each data point is the individual proportion of choices (mean ± s.e.m.; 9 trials per bird per day; N=5 birds) of choosing the prey side. Target distance to Y-junction is given in parentheses; solid triangles, controls (prey in transparent cylindrical boxes). Prey in all tests were Tilapia species, length 9 cm. Broken line indicates critical value for significant choice (0.77).

View larger version (5K): [in this window] [in a new window]   Fig. 6. Maximal distance of prey detection as a function of water turbidity. The results are the mean cut-off points (±s.e.m.; N=5) from Fig. 5. Solid triangle, control test (no prey in boxes).