The coloration of bare parts of birds vary a lot according to particular season. These colors are usually more bright in breeding season or just during a short period of courtship (like in Cattle Egret when for a week or so birds have deep red billl and even legs, comparing to usually yellow). More hormones in the blood more colors they have (and not only birds ;-). In additional that may vary with "bird's mood" and condition. Week and sick birds show tendency to have more dull coloration of bare parts as well as feathers. (contrary to some handsome and proud male...). So, I wouldn't be surprise to see Royal Tern (especially very macho male) with more reddish bill than the others. The color is described as "orange to orange-red, paler and yellower from July" (Olsen and Larson 1995). Personally I never use colors to correct ID of Gulls, Tern, Shorebirds etc., more important are structural features (size and proportions) together with behavioral ones, that what some called "jizz". So, if you see tern which is Royal in size and structure then it most probably IS Royal despite of strange bill/legs color. Caspian is "a size bigger creature" just slightly smaller than American Herring Gull, with much more bigger bill. In addition on underwing outer primaries are distincly darker than other flight feathers (large blackish patch on outer underwing) whereas Royal has distinc blackish (sometimes very diffuse) trailing edge to the outer primaries. However from above outer promaries are DARKER than the others in Royal and more or less same grey color in Caspian.So, my question is is there much bill color variation in Royal terns, with some of them inclined towards red rather than orange. Or is the red such a definite diagnosic mark that its presence (despite no size difference) would clinch this as a Caspian.
Again, colors are in most cases additional features for Terns-Gulls ID (being very important in immature Gulls especially but it is a different story). Gull-billed Tern comparing to Sandwich is longer legged, bigger (more rounded) headed (ones could say even bigger eyed bird) with distinctly shorter, heavier base bill. In flight Gull-billed is more heavy-bodied bird with distinct (heavy) bill (and all head actually). Flying Sandwich has slimmer body emphasized by long and narrow bill which make the bird looks more "very pointed nose silhouette". In addition Gull-billed has less slender wings and they are not so distinctly "banded or curved" in carpal join as in case of Sandwich. There are also good color features of flying birds. Gull-billed has primaries rather uniformly gray with diffuse blackish tips to ca 5 outermost primaries. That is seen as darker so diffuse trailing edge to the outer primaries. In Sandwich (due to different moulting strategy) 4-6 outer primaries are older than the others so they are more wearied out means they are DARKER (especially late Summer). That is well visible as contrast between lighter inner and darker outer primaries (seen well both sides upper and under and even against the sun). The other feature like gray rump of Gull-billed comparing to white of Sandwich are visible from rather close distance. However for me Sandwich looks more whitish whereas Gull-billed more grayish on mantle and upper wings.THe second question is regardng Sandwich vs. Gull Billed Tern. Yes, I know that Gull Billed Terns have a shorter and thicker bill; however, when one sees a lone tern with nothing to compare it to it is difficult to decide if the bill is thick enough or not and I have now several times decided that a tern was Gull billed only to then finally see the little yellow tip as it turns and catches the light. So is there any other clue besides the lenght and thicknessof the bill. If, for example, one sees a Gull Billed/Sandwich Tern over the ocean can one definitly rule out the GUll Billed since they are supposed to feed on mudflats and grassy fields?
-- ************************************************ Michal Skakuj Ph.D. 4600 University Drive apt. 1312 Durham, NC 27707 phone: 919 402 9961 mobile phone: 919 599 3040