Foerageergedrag en prooikeuze van een overwinterende IJsduiker Gavia immer

Great Northern Divers in The Netherlands are passage migrants and wintering birds in very small numbers (Oct-Feb). Within the Delta area, wintering numbers have slightly increased since the 1970s, and some birds were found to stay several months. In February 2010, for one of those “resident” birds, detailed information on the foraging and resting behaviour were obtained. The duration of 79 recorded dives amounted to 27.2 ± 9.7 seconds (range 8-53 sec), handling time with prey at the surface averaged 12.0 ± 8.1 seconds (2- 43, n= 25), surface time in a resting position between dives averaged 12.0 ± 5.1 seconds (5-35, n= 76). The individual surfaced 18 times with visible prey (15x crab, 3x fish), but small prey may have been swallowed during dives and will thus be overlooked. The importance of crustacean prey is probably overestimated when only visible prey is evaluated, under the assumption that these crabs are more difficult to swallow than small fish and are therefore more likely to be brought to the surface. The carapax width of each of the crabs was less than one bill length (i.e. <70mm). Legs and claws were discarded by vigorously shaking or biting them off.