Posted by Keith Regan (213.108.39.216) on August 07, 2000 at 05:27:50:
This is quite an interesting photograph. One of the earlier correspondants suggested that the compiler of this quiz liked to leave traps for the uncritical to fall in to. If this is so first perhaps we should take a good look at the picture to see exactly what is shown (I saved the image and looked at it up to x4). Its a picture of a tern (no bold leap there)in bright sun light almost directly above the bird. This does tricky things to colour perception and shadowing, well worth remembering later on. What is the bird doing? Its standing looking slightly towards us (bill foreshortening?). If you take a close look it has lifted the one leg you can see. It also has a dark pool of shadow under its body so that you can not judge exactly where the ground is in relation to the body. The back end of the bird looks odd as though the left wing has become folded under the right wing. We are looking slightly down on the bird.
Thats the essential but boringly pedantic bit out of the way. The next job should be to take a stab at the age of the bird. The white crown says its not a mature summer adult. The picture was taken in California so the bird is not on its wintering grounds. This should make you reasonably confident that it is not a Roseate as they nearly always appeared dark headed until after their migration and juveniles have darkish legs. It also helps to rule out Whiskered Tern as they moult their caps "all over" not forehead first(the underparts are too pale for an adult and the saddle too pale for juvenile).The bill colour at first glance looks as though it has an all red base making it look like it an late summer adult. The presence of worn juvenile type tertials does appear to make that unlikely or is it possible for 1st or even 2nd summer birds to show these characteristics? A magnified view of the picture helps here as this clearly shows a dark culmen and that the red is restricted mostly to the lower mandible more in keeping with juvenile plumage. How about the wing bar? This would be helpful if we could see it but we can't. The wing on this bird is well tucked under the scapulars and all but perhaps a tiny part of the bend of the wing is covered. The slight darkening that is present is shadow from the raised scapulars, remember the overhead light. The coverts do however show clean neat white edges, another feature leaning towards immaturity or at least new feathers. From what you can see of the flight feathers they appear uniforn but because they are folded or twisted we cannot see too much. The dark edges we can see may just be shadows so they do not help as much as they could.
So what does this add up to. This bird is in worn juvenile plumage. For it to be any older it would need to have retained its tertials and its juvenile bill colour. This does not seem likely but perhaps we still have more to learn. This bird is too pale and uniform for a Roseate Tern. To me they always look more like small Sandwich Terns with their heavily scalloped plumage and with ginger tones retained far longer than Arctic or Common. So how do we differentiate the remaining two. The photo does not help to judge leg lenght. We cannot see the wing bar. Both Common and Artic can lose their wing bar under their scapulars if their posture is right. It is more easy for Arctic to do this than Common as theirs is generally smaller. Whilst Arctic is generally paler than Common with thinner subterminal bars and less ginger, both become significantly paler with wear as the Summer proceeds. The bird in question is quite pale and uniform so we are left with the question can Common Terns wear to this level of paleness before they arrive at their wintering grounds. My answer would be probably not but I would not rule it out completely. The harder you look the more you find birds that have not read our field guides.
So I have managed to sit on the fence so far but nows the time to put up or shut up. If I was assesing this bird as a country first and the only information was this photograph I would have to come to the conclusion that it it was unidentifiable with absolute certainty. Fortunately I dont get to do that job and if I had glanced at this bird on my local foreshore I would not give it too much thought as it looks like so many of the young Arctic Terns I see in late summer. I am sure that in life this bird would not present huge dificulties as presumably it is not stuffed and we would be able to see all the other relavent features well with time. So on the balance of probabilities this bird is an worn juvenile Arctic Tern.
I have of course assumed that it hasn't taken a journey across the eguator to say hello to its American cousins in which case I cry foul and go off in a huff (whats a huff?).
Keith