Hi Deb,
I don't believe your thinking is muddled at all, but I have to admit, I am finding it very difficult to get through the Voelker paper that you linked above, and this is not my first attempt to do so! There are a number of very disconcerting aspects to this paper, the principle concern I have being that so many of his conclusions are based entirely on analysis of skins, most of which (apart from juveniles and 2c-y with retained juvenile feathers) would have been of unknown age (though it might be safe to assume that all breeding plumaged adults at least in their 4 cy). In the outline of the methodology applied, under the sub-heading of Age identification (p403) the author states:
"I sorted all specimens into the following age classes (from Pyle et al 1987): hatching year (HY), birds in their first calendar-year; second year (SY), birds in their second calendar-year; third year (TY), birds in their third calendar-year; and finally after third year (ATY), any birds in at least their fourth calendar-year."
It is important to understand that "from Pyle" here means that he copied Pyle's age classification, rather than that he assigned SY and TY birds to their respective age categories based on information in Pyle [Pyle does not actually provide much information on this problem]. So, how did Voelker sort his specimens? We can only guess that he did so on the basis of the outer tail feather differences he goes on to describe. Maybe he is right, but I am not convinced that this isn't an example of circular reasoning.
The next concern is that so much of his presentation is based on extrapolation of moult regimes, backwards and forwards, from his interpretation of what he is seeing in skins. In addition to the already mentioned serious shortcoming of the whole paper, that there is absolutely no illustration/photographic content by which readers might get a visual idea of what is being described (I can't help wondering, for example, if his references to 'scaled' feather patterns always refer to juvenile feathers, as is assumed, or could he actually be seeing something similar in finely patterned post-juvenile feathers?). Throughout the paper Voelker gives us detailed information on the moult score of particular specimens (for example at the bottom of p406) but he does not provide the all-important date, or even the month when the specimen was collected.
If, as is likely, the complexity and extent of individual variation in the plumage development in Arctic Tern is anything like it has been shown to be in Common Tern, there is really no way that a true understanding of what is going on will be gained from examination of museum specimens, where all but juveniles, or birds with retained juvenile feathers, will be of unknown age. I believe a correct picture will only emerge when we are able to examine and analyse in detail the appearance of individual marked (ringed) birds of known age, as was the basis for the excellent White & Kehoe paper on Common Tern ageing that you provided a link to earlier in this discussion.
I am glad to be reminded of this interesting ageing question, and will avail of any opportunities I get in the coming weeks to take another look at any sub-adult Arctic or Common Terns I encounter, especially the Arctics that, curiously, have started to moult their inner primaries.
regards, Killian