The last few weeks have seen various numbers of Crossbills found at six localities, one of which is the stand of larches immediately adjacent to the car park at Wakerley Great Wood. For many veteran county birders, this area will no doubt bring back fond memories of seeing Northamptonshire’s first Parrot Crossbill, a female, there in November 1990, along with a male Two-barred Crossbill with up to fifty-five Crossbills present at the same time.
At least three Crossbills were there on 1st March and up to twelve were present on 3rd-4th March but they have been mobile and not always on show, with no further sightings until 18th March, when seven or eight were seen by James Underwood.
Among these was a rather large-billed male with a deep, hefty bill, of which James managed to capture a couple of images. At no time was it suggested it was a Parrot Crossbill but the photos portray a Crossbill with a super-large bill, the first image of which certainly sets the pulse racing.
The second image, however, perhaps depicts the true proportions of this individual, which enables a more balanced assessment to be made. The bill appears shallower (but still very large) and the head appears to be in ‘correct’ proportion with the body, i.e. it does not seem to have an overly large head like that of a Parrot Crossbill. Additionally, the visible depth of the lower mandible at its base does not appear to be deep enough for Parrot Crossbill. But these are simply two images and how accurately do they convey the true dimensions of the bird and its bill?
According to Shirihai and Svensson’s Handbook of Western Palearctic Birds (2018), the bill of Crossbill overlaps in size with that of the larger-billed Scottish Crossbill which, in the past, has been mooted as a race of Parrot Crossbill. Crossbill taxonomy is, however, far from straightforward. The IOC checklist (now adopted by the BOU) recognises nineteen races of Crossbill (Red Crossbill Loxia curvirostra) with the nominate race curvirostra occupying northern, western and central Europe, including the British Isles and Scottish Crossbill is retained in the list as a full species, Loxia scotica. Shirihai and Svensson, however, regard the latter as simply a race of Crossbill but recognise the British breeding population as an additional race ‘anglica’. So, during periodic Crossbill irruptions, are we seeing local movements of British birds ‘anglica’ or are they continental curvirostra – or both? Furthermore, does the supposedly sedentary scotica ever move south? And if so, how far? Only time (and DNA) will tell …