I picked up Martin Elliott on 22/04/15 to go to Stanwick in Northants. and look for Steve Fisher’s Ad. Glaucous Gull which had been present for some time. We were both interested in this bird and I may return to the subject at some point. We failed to see it as I had another of my episodes of catastrophic decision making. Some ten minutes away from Stanwick we received news that Gary Pullen had found yet another Ring-billed Gull, this time a 1st S at Davetnry Res. No problem, I was resolute, I would not turn round now and we would continue to Stanwick. Steve then called to inform us the Glaucous Gull had left. Now there seemed no point in not going after the RbG and I turned the car around. About fifteen minutes from Daventry came the news that the RbG had flown out high to the NW, shortly followed by the inevitable and final kick in the teeth, the Glauc was back at Stanwick. We tried the Draycote roost for the RbG and needless to say it didn’t fetch up.
On 23/04 we surveyed one of my HLS farms at Long Compton on the edge of the Cotswolds in S Warks. Martin found a Greenland type Wheatear which prompted a discussion we have had several times previously about the fact that most of the Wheatears we both see each spring appear on structure to be of this type, only the very earliest ones being apparent nominate birds. A Redwing singing from the top of Harrow Hill was a real surprise representing a late record for the species in Warwickshire. Shortly after we parted company to do the final hill I looked up to see a Western Marsh Harrier above me at a reasonable altitude. I cursed not having my camera (which I don’t routinely carry on surveys preferring to ditch the weight) but Martin who was a little way below me did manage to grab a few shots. We concluded the bird was a female on the basis of structure and under wing pattern.
After work whilst birding some hills we got calls to tell us that there were 30 Little Gulls at Daventry Res. and then that the RbG was also back there. Again I abandoned my plans on the premise that if nothing else we would hopefully see 30 Little Gulls. This transpired to be the predictable case, the RbG again hoofing off high to the NW after a short time but the now 33 Little Gulls remaining long enough for us to soak them up for an hour or so.
Ad. Little Gull Daventry Res.
And I thought I was done with larid for the season.
That evening MIke Alibone contacted me via Twitter and put me in the picture regarding some French work which appears to render the ageing and sexing of Western Marsh Harrier in the field largely impossible. This fascinating research can be found here http://rsbl.royalsocietypublishing.org/content/8/2/167.abstract whilst the paper from British Birds here http://www.cebc.cnrs.fr/publipdf/2013/BBB106_2013.pdf is probably of greater value to most birders (thanks to Tom Lowe for sending me this link).
In short the research has found adult male Western Marsh Harriers occurring in a particular population in central France with plumage which mimics that of females. Males with typical plumage occur in the population as do males with female like plumage and a whole range of individuals with plumage somewhere in between. Studies with decoys showed ‘typical males’ attacked female like male decoys less frequently than ‘typical male’ decoys and so female like plumage in males may be adaptive, increasing the possibility that an individual will be tolerated. This hypothesis would appear to be supported by the fact that there is evidence the female type males tend to occur on territories adjacent to those of ‘typical males’. This had me viewing the putative adaptation as a phenotypic equivalent of the ‘sneaky male’ behavioural strategy employed by some primates in which low ranking animals literally sneak off behind the bushes to mate with receptive females which even supress vocalisations during ‘the act’ to reduce the risk of being caught ‘in flagrante’ by a dominant male. Then comes the bombshell that males with female like plumage showed reduced aggression towards male decoys of all types but directed aggression towards females and thus appear to behave like females as well as look like them! I am tempted to suggest that the female like males fail to undergo hormonal surges which would otherwise trigger both plumage change and increasingly aggressive behaviour towards other males but the authors are clear that the female like male plumage is a permanent one and does not represent delayed plumage maturity.
The putative adaptation may result from a ‘genetic bottleneck’ and whilst the authors do contend that there are examples of males with female like plumage from elsewhere within the species range they appear to be extremely rare. The authors also state that ‘females are heavier than males and their silhouette, as well as their type of flight, more closely resembles a buzzard than a harrier – females are more stubby than males with a wider base to the wing and a shorter tail’.
Western Marsh Harrier Long Compton photo by Martin Elliott
The Long Compton bird accords well with this description and showed very dark underwing coverts with pale carpal crescents as described by Forsman. Photo No. 14 in Plate 96 of the BB paper is the only one I can see which shows a male with an under wing pattern approaching the Long Compton bird and given the combination of structure and plumage I still think it was a likely to have been a female.
Work was productive for migrants again the following day at Toft Farm where I found M and F Common Redstart, M and F Whinchat and four more Greenland type Wheatears. Surveys on the Upton Estate over 26-27/04 produced the odd combination of a singing Grasshopper Warbler and a second late Redwing. I never saw the singing Redwing and wondered if it might have been an Icelandic one, tying in with the movement of leucorhoa type Wheatears but this one gave good views and appeared to be a typical Scandanavian bird.