E-mail fra Killian Mullarney,Irland 06-01-2006 (uddrag/extract):
Thanks for your email re the terns you photographed last September at Funchal and the Desertas. They present an interesting problem, but I hope you will not be too disappointed if I tell you that in my opinion, all are Common Terns!
One thing that has become clear in recent years is how unsafe it is to age 'sub-adult' Common Terns if they do not definitely have retained juvenile feathers of one sort or another (see an excellent paper in British Birds Vol 94: 268 - 277, June 2001 for more on this; it is based on detailed analysis of the plumage and bare-part colouration of over 1000 known-age (ringed) Common Terns in the UK). So, at the time of year you saw your birds I don't think it can be reliably determined what age they are, beyond "2 cal-year +". It is probably true also of Roseate Terns, though it seems very few sub-adult-type birds venture this far north in the summer months, so it is difficult to say for sure without a larger sample of well-studied and known-age sub-adults. This is a project I have been meaning to apply myself to for some years, maybe I'll manage it this summer?!
Anyway, coming back to the birds you photographed: My observation of a small number of apparent first-summer-type Roseate Terns suggests that second-generation primaries (and presumably all subsequent generations) have much the same pattern as those of adults. This provides one of the most reliable (and least subjective) means of identifying 'sub-adult type' Roseate and Common Terns. As can be easly seen in photos of breeding adult Roseates, especially when perched, the inner webs of all but the (two, three or four - seldom five) dark outermost primaries are broadly edged white, which continues right to the tip of each feather. In fact, even on the dark outermost primaries the white inner web continues, finely, all the way to the tip. This means that on Roseate you never see the broad dark trailing edge to the outer primary tips, when viewed from underneath. I have taken the liberty of adding a crudely 'retouched' outer wing to one of your photos to give you an idea of how I would expect the outer wing of a Roseate to look...
In addition to the primary markings, the head pattern (Roseates tend to have a little more dark in front of and below the eye than Common) the bill structure (Roseate often has a very slightly finer and more pointed bill than Common), the dark on the outer rectrices (I have never seen this in Roseate..) and the stronger grey shade of the upperparts all point to your birds, without exception, being Common Terns. I will attach a couple of poor quality video-grabs of (probable sub-adult) Roseate Terns to illustrate some of these points.
I hope this helps you decide what you have photographed!
While I am writing, I'd like to compliment you on the many wonderful photographs of yours I have enjoyed over the years - keep up the good work!
With best wishes,
Canon EOS-20D + Canon EF 100-400mm f/4.5-5.6L IS USM