In recent years I have spent much time observing Black Woodpeckers Dryocopus martius at nesting and roosting cavities, mainly in Hungary but also elsewhere in central and eastern Europe. With a telescope, I have been able to obtain close and prolonged views of individual birds and was surprised to note that many birds seemed not to have a circular pupil, but rather a pear-shaped or teardrop-shaped one. Intrigued, I began to focus systematically upon the eyes of all the Black Woodpeckers I encountered, noting the colour and shape of the pupil and iris.
Gradually, it became apparent that the irregular shape of the pupil was actually created by dark marks on the iris, i.e. the pupil was circular but the dark marks created an impression that it was pear-shaped. Indeed, BWP (1985) mentions that adult Black Woodpeckers have a ‘black spot in front of the pupil’. Most of the birds I observed, however, showed rather more than a simple spot on the iris. In many cases there was a dark, elongated slit between the edge of the pupil and the bill base, sometimes stretching across the iris and being of rather variable shape, occasionally giving the impression of a ‘double pupil’. It soon became clear that such marks were the norm for both males and females, typically (as far as I could determine) found in both eyes. Black Woodpecker nestlings have bluish-black eyes, in which it is difficult to distinguish the pupil from the iris, but it is clear that many older, fully feathered chicks already show a dark mark on the iris as it becomes paler.
I examined over 100 close-up photographs of the eyes of adult male and female Black Woodpeckers, taken across the Palearctic range of the species, and including both races: nominate martius (most of range) and khamensis (southwest China, Tibet). Approximately 85% of Black Woodpeckers in these photographs had some form of dark mark on the iris while 15% did not. There was no evidence that such marks were confined to a particular population, region, race or gender, and there was no clear evidence of an irregularly shaped pupil.
Marks on the iris may not be unique to Black Woodpeckers, but seem to occur much more frequently in this species than in most other picids; they are much less common in the other (six) Dryocopus species. For example, I examined many photographs of both White-bellied D. javensis and Pileated Woodpeckers D. pileatus and found that very few White-bellied and no Pileated showed such marks.
What, if any, is the function of these dark marks on the iris? In some animals, irregular pupil shapes modulate the amount of light reaching different parts of the retina that may have different sensitivities or resolution – so the shape is adaptive for vision. Iris pigmentation patterns have little effect on vision but perhaps act as signals, perhaps in terms of fitness or mate recognition. In some species iris pattern indicates gender (for example American Black Oystercatchers Haematopus bachmani, in which the iris of adult females is patterned, but that of adults males is not; Guzzetti et al. 2008), but this is clearly not the case with Black Woodpeckers, nor is it likely that the marks are colobomas or other optical defects. Rather, the high proportion of birds showing dark iris marks could suggest that it confers an advantage of some sort and has been selected for. Indeed, the presence of two forms of iris shape within the same population may be an example of a polymorphism which is heritable; and it would be interesting to know whether this is the case. There is clearly more to find out about the function of these distinctive dark iris marks in the Black Woodpecker.
Reference Guzzetti, B. M., Talbot, S. L., Tessler, D. F., Gill, V. A., & Murphy, E. C. 2008. Secrets in the eyes of Black Oystercatchers: a new sexing technique. J. Field Orn. 79: 215-223.
Acknowledgments I thank Martin Collinson and Richard Chandler for commenting extensively on a first draft of this note.
Gerard Gorman, Budapest 1511, Pf: 4, Hungary; e-mail firstname.lastname@example.org
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