Food niches of sympatric Marsh Harriers Circus aeruginosas and Hen Harriers C. cyaneus on the Dutch coast in winter

Abstract

Analysis of pellets collected from adjacent communal winter roosts of Marsh Harriers Circus aeruginosus and Hen Harriers C. cyaneus on an extensive saltmarsh in the southwest Netherlands showed highly significant differences between the diets of the two species. Marsh Harrier diet showed no change throughout January, February and March. They specialized on ducks, which were about half of their prey numerically and more important by weight. Although Marsh Harriers sampled other prey, this remained at a low level and showed no seasonal response to the availability of young lagomorphs. Hen Harriers occupied the niche of a generalist predator, having a broader diet and responding to the presumed changes in availability of prey with diet shifts. In November, about half of their prey items were passerines but these declined in importance in December, and small mammals rose. The proportions of both small mammals and birds fell in February and March, due to increasing dependence on young lagomorphs. Diet overlap between the two species was greatest in January and declined in February and March.

Habitat preferences of breeding Water RailRallus aquaticus

“…(Clarke et al 1993) and Foxes Vulpes vulpes (Cotgreave 1995). However, it is more likely that predation pressure will be greatest on the eggs and young chicks.…”

Factors affecting the vulnerability of farmland birds to predation by Hen HarriersCircus cyaneusin winter

“…A total of 93 harrier pellets were collected between 10 January and 30 March 2005, and analysed according to Clarke et al (1993Clarke et al ( , 1997. More than one individual of a given species was credited to a single pellet only where two gizzards, hind claws (nonpaired), primary or alula feathers, or bill parts were found in the pellet (Clarke et al 1997).…”

“…More than one individual of a given species was credited to a single pellet only where two gizzards, hind claws (nonpaired), primary or alula feathers, or bill parts were found in the pellet (Clarke et al 1997). Frequency of prey items in pellets was determined by counting the total number of individuals, rather than the number of pellets in which particular species occurred (Clarke et al 1993).…”

Marsh HarriersCircus aeruginosustarget TealsAnas creccaat roosts according to potential availability of vulnerable prey

“…This was because Marsh Harriers often 'test' ducks by forcing them to take flight, and then attack the wounded or sick flightless individuals (Schipper 1973, Schipper et al 1975, Clarke et al 1993. The probability of a raptor finding a flightless duck is therefore likely to increase with the total number of ducks present.…”

“…The probability of a raptor finding a flightless duck is therefore likely to increase with the total number of ducks present. However, Marsh Harriers also rely on surprise as a hunting technique (Schipper et al 1975, Clarke et al 1993, and attempt to put ducks on the wing by flying over them. The ducks that do not see the raptor coming from a distance will take flight or be attacked, while those that see the harrier will swim away and not be pursued.…”