Head-bobbing and non-bobbing walking of black-headed gulls (Larus ridibundus).

Head-bobbing walking (HBW) and non-bobbing walking (NBW) of black-headed gulls were compared from kinematic and behavioral/environmental viewpoints. The birds walked with a longer stride length and lower stride frequency during the HBW than during the NBW. With respect to these two parameters, the HBW of black-headed gulls was similar to that of other head-bobbers, and the NBW was similar to that of other non-bobbers. The stride length and the amplitude of head bobbing were correlated. These results suggest that the head-bobbing and gait parameters are related. From a behavioral viewpoint, HBW was observed during seeking-type foraging by wading, and NBW was observed during waiting-type foraging on a flat substrate. The type of foraging behavior and/or substrate condition probably determines whether the birds walk with or without head bobbing.