Southbound harrier musings

On Sunday I was lucky enough to see a ringtail Hen Harrier fly over Milford. The bird was relatively high and heading south-west when I saw it at roughly 14:20. Some 70 minutes later, at approximately 15:30, the first Hen Harrier of the winter appeared at Pulborough Brooks, where it has been loitering since. Coincidence? I don't think so – I suspect it's the same bird.

Hen Harrier is a rare Surrey bird, averaging three or four records a year, almost exclusively on passage. Occasionally, though less so in recent years, a bird or two will winter on the western heaths. Directly south of Surrey, in West Sussex, the only properly regular wintering site for this species is the Arun Valley and the adjacent South Downs (also extending east towards Worthing). So, for any southbound harrier passing through Surrey, it's no surprise they should then rock up somewhere like Pulborough Brooks.

Harrier flightlines: the August Marsh on the left and the recent Hen on the right.

A similar thing happened on 16 August. I had a juvenile Marsh Harrier going south over Shackleford at 08:45; surely the same bird was seen heading south over Thursley Common at 11:00 by Paul O'Mara. Lo and behold, at roughly 17:00 that day, the first juvenile Marsh Harrier of the season was seen at Pulborough Brooks by Matt. Surely the same bird. The species remains a rare (albeit not as much as Hen) and strictly passage bird in Surrey, and, again, the Arun Valley is the closest haunt to this county. So, any southbound bird would see Pulborough as one of the first suitable spots at which to properly pitch up.

The difference in time taken to reach Pulborough can be explained, too. The Marsh Harrier was a leisurely bird, even seen hunting at Shackleford, and generally flying low into a warm southerly headwind. The Hen Harrier, however, was high and backed by a north-easterly tailwind; the weather was also inclement, meaning the bird would have been keen to finish the journey as quickly and efficiently as possible. This highlights the great significance of wind speed and direction, as well as general weather, in bird migration.

This post doesn't really unveil much, but merely attempts to join a small stretch of dots in an otherwise intriguing – and largely unknown – line. Where had these harriers originated? At what point did they enter Surrey? Do they have favoured flightlines that take them to their preferred (or instinctive) wintering grounds, or is it entirely random? We will never know – and that's why bird migration remains a constant fascination.