Whilst there is no doubt that the tone of adults and the white mirrors are on average subtly different to average nominate michahellis, my recent observations of gulls on Fuerteventura, would suggest that there is quite a lot of variability amongst adults, and that the plumage of young birds may actually show clearer differences from other forms. Plumage features of young birds have been described in the past, but this photo-essay concentrates on them, pointing out some of the ways in which they differ from most michahellis and also (perhaps even more difficult) the very similar Lesser Black-backed Gull L graellsii, which over-winters in fair numbers in the area.
There is also, of course, the possibility that some birds over-wintering in the Canary Islands are michahellis, though I found no real evidence of this. Most or all adults present appeared to be paired off, with some "on territory", whilst with immatures it is even more difficult to tell, the ‘first-winters’ were typically intermediate in plumage between graellsii and michahellis.
The following images of birds in first-basic (post juvenile) plumage show the basic plumage patterns, albeit with some variation. At the end, there a small number of images of second- and third-basic plumages, with brief discussions of plumage. I would refer readers to other photo-essays on Yellow-legged and Caspian Gulls at Surfbirds for images taken at the same time of year, and compare the state of moult and plumage.
(Click on the thumbnails to see larger images)
(Figure 1 and 2 above) Note how Great Black-backed-like the structure can seem, with a strong bill, distinct profile and white head. The tertials are largely old, but a good number of coverts are new. The tail of this bird was almost totally brown-black, with barring at the extreme base, but also a white sub-terminal band – this was noted on a few birds – and it can also be reminiscent of GBbG. The almost all black tail of some, and strong barring on the under-and upper-tail coverts, are reminiscent of American Herring Gull L argentatus smtihsonianus.
(Figure 3 to the right) The very extensive moult of tertials, and inner wing coverts is typical of many first-basic gulls, as is the pattern of the second-generation feathers – note how grey the new greater coverts are (aside from the inner ones). I tended to use the extent of moult as one of the first indicators of whether a bird was YLG or LBbG. Though many of the first-basic michahellis wintering in Britain show some moult in these areas, this is extreme and probably indicates the early breeding of YLGs on the Canaries – as does the wear on the retained juvenile coverts. Very strong legs are typical.
Figure 4 to the right - (same as Figure 2) Large size and attenuated rear end are typical of YLGs in general. A critical look at the scapulars shows that they are generally darker than michahellis and similar to some michahellis-like graellsii. Scapular patterns are variable and this possibly indicates different moulting times, as does the greyer fourth new greater covert.
The two flight images below (Figures 5 and 6) show a number of aspects of the plumage of first-basic YLGs in the Canary Islands that differ from the norm for michahellis. From below, solidly dark median, lesser and marginal under-wing coverts contrast with "silvery" greater under-wing coverts and remiges are as one would expect, as is the ‘window’ formed by pale inner primaries. Though from above the window is much less apparent, and is usually less obvious than on michahellis - this feature has been stressed as a distinctive feature of atlantis, but is variable, with many seeming to have evenly toned remiges from above, but the majority have slightly pale ‘windows’ on the upper wing. Another clear difference from michahellis is that on many the tail band is much broader (sometimes to the point of being virtually all dark), shows fairly extensive barring on the white tail base (when present), and a lot of barring on the upper- and under-tail coverts – very much like typical graellsii.
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