Changes in the frequency of prospecting fly-overs by Marsh Harriers Circus aeruginosus in relation to short-term fluctuations in dabbling duck abundance

Wintering waterfowl gathering in large flocks during the day may be attractive to avian predators in a way similar to that of bird colonies. The response of Marsh Harriers to changes in dabbling duck abundance was monitored on four freshwater sites in the marshes of western France, a major wintering region for wildfowl in the country. The number of harriers present at each site remained stable throughout the winter. However, the frequency of prospecting fly-overs by harriers, i.e. search effort, increased with duck abundance. Frequency of fly-overs was also affected by the number of conspecifics present on the site, wind velocity and time of the day, and varied between months. All these factors are discussed in relation to prey availability, competition, and minimization of energy expenses while searching. ‘Female morph’ harriers appeared to scout more over duck flocks than ‘male morph’ harriers, which is consistent with the fact that sexual segregation in feeding habits is often recorded in dimorphic raptors. Direct predation was rarely seen but ducks always reacted to the fly-overs of harriers. The raptors were responsible for up to 130 disturbances per day and the number of disturbances was correlated to the number of ducks present. The activity of harriers may therefore affect the behaviour of dabbling ducks, hence the quality of their wintering quarters

Fritz H., Guillemain M. & Guerin S., ARDEA 88 (1): 9-16.