I took the early shift this morning, getting to the hide at about 8-15am and must have left shortly before Richard arrived. Shortly afterwards the female Marsh Harrier patrolled up and down the far side before disappearing east. As she did so the Pallid Harrier appeared, making a quick pass in front of the hide before crossing the Corse, ascending the hill opposite and disappearing towards Roscrowther. By the time I left I had not seen it return. 2 Buzzards and a Merlin completed the raptor count for me and a flock of c90 Stock Doves on the far side was notable.
What a stunning bird the Pallid Harrier is! All the more fascinating for the presence of the 5th primary finger on the right wing – a feature not usually seen in pure-bred Pallid Harriers. Could it be a cross with a Hen Harrier or is something else going on? I finally managed to get a photo of the right wing where the crossover from secondary to primary feathers appears well enough defined to shed some light on this.
The two photos below compare the left and right wings. The left wing has the expected 10 primary feathers. The right wing, however, seems to have an additional, 11th, primary. It looks as if the additional feather is one of the primary fingers, a misaligned feather lying between the P6 and P8 in this bird (or P6 and P7 in a normal bird).
How common is this feature amongst raptors? Unsurprisingly, perhaps, not common at all but also not unknown. In a study of c11000 raptors, Clark et al. (1988) found nine instances of extra primary feathers and eight of additional tail feathers. Intriguingly, in 89% of the cases where an extra feather was found it occurred on the right side, the same as for our bird.
So, we may well have a very rare bird here – not a hybrid but a Pallid Harrier which has the aberrant growth of an additional primary feather. It is made even more unusual by the fact that the extra feather is within the outer primaries – those found in the above study all lay between the P2 and P3 or P3 and P4 feathers and none of them occurred in any of the 385 harriers studied.
In my view the primary feathers appear distinct from the secondaries in the right wing photograph in a couple of respects:
1) the tips of the primary feathers are convex and rounded in shape whereas the tips of the secondaries are broadly flat and more angular.
2) There is a distinct colour and pattern difference between the primaries and secondaries. The primaries are slightly lighter brown and plain apart from a very thin light band at their tip. The secondaries are noticeably darker and the first 4 secondaries (S1, 2, etc) have a distinct white band across them.
In addition to this there is a subtle (in my photo) but distinct change in the angle of the trailing edge of the wing between the set of secondaries and the set of primaries at the point where they meet.
The sum of these features suggests to me a clear break in the character of the feathers between the one I’ve labelled as the first primary P1 and the preceding secondary feather S1. The same features in the left wing mark the break between secondaries and primaries.
The differences between S1 and P1, though clear in the photo above, are subtle enough to be difficult to distinguish in most photos of this bird. They are, however, seen also in Alan Merritt's photo in a preceding log post, where the 11 primaries are also visible.
Regarding where the P1 feather joins the skeleton? My feeling is this will be difficult to determine with any confidence without either a near-perpendicular photo of an extended wing or an X-ray.