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

Wintering waterfowl gathering in large flocks during the day may be attract ive to avian predators in a way similar to that of bird colonies. The respo nse of Marsh Harriers to changes in dabbling duck abundance was monitored o n 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 pros pecting fly-overs by harriers, i.e. search effort, increased with duck abun dance. Frequency of fly-overs was also affected by the number of conspecifi cs present on the site, wind velocity and time of the day, and varied betwe en 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 feed ing habits is often recorded in dimorphic raptors. Direct predation was rar ely 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 dist urbances was correlated to the number of ducks present. The activity of har riers may therefore affect the behaviour of dabbling ducks, hence the quali ty of their wintering quarters.