Breeding observations of Marsh Harriers, Circus aeruginosus, in the Rantum Basin (Sylt) from 1966

Abstract

The landscape formation of the Rantum Basin (Sylt) allows a view of a 570-hectare treeless marshland from the dyke. Under these conditions, it was possible to observe 140 prey transfers from two male marsh harriers to eight females, whose nests were found except for one. Two females had already mated when I arrived on 20 April and began breeding in the first week of May. I first observed six harriers in juvenile plumage on 23 May. They were probably courted by both males and laid their eggs at the end of May.

I was only able to obtain information about possible polygamy from corn and harriers. For example, WOLTERS (mdl.) reports from a Holstein hunting ground that fourteen harrier nests located in a relatively small area were visited by a maximum of six males. 

I received a very interesting report from Dr. F. HAMERSTROM on polygamy in the North American harrier species C. cyaneus hudsonius. In one case, one male flew to two nests, another to three. Dr. HAMERSTROM also reports a copulation and that another male later flew to this harrier's nest.

Based on my observations, I believe it is reasonable to question whether the sex ratio among harriers is actually 1:1. In any case, marsh harriers elsewhere should also be studied for signs of polygamous behaviour. Due to the strong attachment of females to their nests during the first phase of primary moulting, the presence of several harriers in a small area is less noticeable than one might generally assume.