The continued presence of a Marsh Harrier at Pitsford Reservoir over the past few days has provided a good excuse for a snapshot of the status of this species in Northamptonshire. Seen only briefly in Walgrave Bay by Neil & Eleanor McMahon and Neil Hasdell on the morning of Saturday 28th, I caught up with it in the same spot equally briefly the following morning and, after being on view for only a couple of minutes, it promptly disappeared for the remainder of the day. Fortunately (for the yearlisters!) it was relocated the following morning when it was seen in, and between, both Scaldwell and Walgrave Bays around midday. It remains in the area this evening.
Marsh Harrier is an uncommon, though regular, local passage migrant. Considering it breeds in some numbers in neighbouring Cambridgeshire shouldn’t we expect to see it more frequently in Northants? Especially in the east? Over the ten years 2000-2009 there was an average of 14 records per year, with a record 35 in 2010. The best time to catch up with one is in April-May or in August-September, although birds in spring tend not to linger. The best place is the Nene valley but the reservoirs have produced numerous records in autumn and, in theory, one could turn up almost anywhere. One wintered in the Nene valley between December 2001 and April 2002, being seen predominantly at Stanwick Gravel Pits. And the highest number together? Three juveniles at Pitsford Reservoir on 25th August 2002.
There are few photos of Marsh Harriers in Northants so Neil Hasdell did well to get the long-distance shots of the Pitsford bird shown here. This individual is a typical juvenile, aged by the plain upper wing coverts with fresh, pale tips to the greater and primary coverts and all flight feathers, forming a neat, narrow band across the upperwing and a thin, pale trailing edge. Additionally there is a pale crescentic base to the primaries on the underwing, barely visible in the photographs. Some juveniles also show an extensive creamy area on the upper forewing like adult females, while others lack the pale tips to the coverts and flight feathers altogether and so appear virtually identical to adult females.
If this isn’t confusing enough, and to make matters worse, adult males can appear identical to adult females as revealed by Sternalski, Mougeot & Bretagnolle last year in recent studies on populations in south-west France, where 40% of adult males exhibited plumage identical to adult females. In further analysis, Bavoux, Burneleau and Bretagnolle found the only sure way of sexing these individuals is by weight (females weighing more than males) and bill length. Assuming equal numbers of each sex in the population, then, 70% of all adults will therefore exhibit adult female plumage and, when you throw the immatures into the mix, this explains why the overwhelming majority of Marsh Harriers we see are simply chocolate-brown, cream-crowned birds! Bottom line: with many individuals you cannot be certain you are watching an adult male, an adult female or a dark juvenile of either sex!