Even before the Bird Observatory started, (from 1954) there were counts of breeding pairs of Hen and Marsh Harriers within the reserve. The breeding of Hen Harriers ended after the year 1962 but the number of pairs of the Marsh Harrier has developed positively. Over the years there have been many studies including territoriality, breeding biology and population monitoring.
Territory Studies of Marsh Harrier have shown that birds distinguish a “nest territory”, a small area adjacent to the nest being defended against all intruders such as other harriers, eagles, foxes, minks and crows, and the hunting territory, which is just defended against other harriers. Territory boundaries are established early in the breeding season, and later contact between the territory holders, mostly males, are relatively peaceful.
“Escort” is a term that was defined during these studies, and that can be used to describe when a territorial male chases off a trespassing male by following him at a certain distance until outside of his hunting grounds. This behavior is highly characteristic and easy to observe. It looks so peaceful that you might think that two males together fly away to hunt.
The emergence of special nesting and hunting territories has probably to do with the availability of suitable nesting habitat (any field of reeds) in many places this is limited. Then it may be appropriate for birds to nest closer together. A sparse colony can also be good for nestlings, if they want to defend themselves from predation by, for example, sea eagles.
Breeding Biology Studies 1965-1996
A lot of breeding biology studies have been done on the Marsh Harrier. On the 19th-20th June 1972 the breeding activities of two nests were followed continuously during the day. The nestlings were then about twelve days old. In one nest there were four, and the other just one nestling. The males were getting all of the prey. 17 prey items were delivered to the clutch with four nestlings and eight to the second nest, with a lone nestling. The highest hunting activity was in the afternoon between 13:00-17:00. Females guarded the nest against intruders and delivered the prey to their nests. Their remaining time was occupied mainly through nest improvements. The female with the most offspring was noted to bring material to the nest 38 times, mostly between 06:00 and 08:00.
In 1992, we studied relationships and territorial exploitation of Kvismaren. As many as 14 nests were found, and in the now relatively dense population polygamy found in three cases. In each case a male had two females. Early arrival in spring and well developed plumage were common denominators for the polygamous males. At least two of the secondary females were able to choose another mate when more males arrived, but they stayed with the first selected. An absolute difference in breeding performance was found to exist between primary and secondary females in this year. In total 37 nestlings were ringed that year, this is very high productivity for Kvismaren.
In 1995 the hunting of the Marsh Harriers was studied in more detail. This year there were two polygamous males identified. This year was unusual with a relatively cold and rainy spring including snow in mid-May. The results showed that the harriers can use a wide range of prey items from voles as their staple food to birds, chicks (of Coot) and fish. Even a nearly full-grown young of Lapwing was noted to be Marsh Harrier food. A logical, but still interesting result was the finding that females of polygamous males shared their hunting areas. In 1996, the studies of harriers continued, now with the focus on females. They claimed the same territory as males and began to hunt more frequently in July.