BY JONATHAN DWIGHT, M. D.
In discussing the moults and plumages of the Glaucous Gull, a dozen years ago I took occasion to bury “Larus barrovianus” among the synonyms of Larus hyperboreus (then known as glaucus) because the alleged characters seemed to me to afford insufficient grounds for recognizing even a subspecies (Auk, XXIII, 1906, p. 29). Later, in the 1910 edition of the A. O. U. ‘Check-List,’ the Committee on Nomenclature and Classification adopted my view of the case and discarded “barrovianus”; but recently Dr. H. C. Oberholser has seen fit to dig it up and it is revived, somewhat impressively, as a subspecies of hyperboreus (Auk, XXXV, 1918, p. 472).
If it were not for certain aspects of the matter I would merely reaffirm my convictions of 1906; for it is a question whether Dr. Oberholser has added anything new to the original claims made by the describer, Mr. R. Ridgway (Auk, III, 1886, p. 330). This does not seem to be the case, for his diagnosis is virtually a restatement of Mr. Ridgway’s, except that a supposed character of the bill is discarded on evidence I submitted in 1906. My measurements had shown that this character, namely, “depth through the angle never less and usually decidedly greater than through the base,” was not diagnostic, but this was not my only “evident reason” then for rejecting “barrovianus” as Dr. Oberholser now wrongly assumes. What I said was that this form “is scarcely 3% smaller [than glaucus] in size and 4% smaller in bill” and furthermore, I said; “It is true that the largest specimens of barrovianus never quite reach the dimensions of the largest glaucus, but overlapping of size is so considerable even when careful comparison of sexes is made that without first reading the labels one cannot, except in a very few cases, tell whether a bird is from Greenland or Alaska. The variation in the size and shape of the bill in gulls is very great and a few millimeters difference in wings that are as long as one’s arm is hardly ground on which to rest a subspecies, much less a full species.”
These conclusions may be contrasted with Dr. Oberholser’s recent diagnosis which reads, “Similar to Larus hyperboreus hyperboreus, but smaller, the bill particularly so and relatively as well as actually more slender; mantle decidedly darker; and the line of demarcation between the white tips to the primaries and the pale grayish basal portions usually more evident.” I would here call attention to the fact that the “line of demarcation” is not a distinct character but a corollary of the preceding, for the color of the mantle in the Glaucous Gull regularly runs over, so to speak, into the wings, and a darker mantle would mean darker bases of the primaries and therefore greater contrast as a matter of course. Consequently, in the final analysis there are two characters and only two on which “barrovianus” rests,—(1) darker mantle and (2) smaller size, especially of the bill. I will invite attention to a new estimate of the value of these characters.
Fig. 1. Diagrams showing relative measurements in millimeters of 31 adult specimens of Larus hyperboreus and its alleged race. Top line shows actual length in largest birds, middle line shows average, and bottom line shows smallest of the series.
1. As for the color of the mantle, which Mr. Ridgway calls “somewhat” and Dr. Oberholser “decidedly” darker, I can only say that my series fails to support either of these statements. I find that if comparison of like stages of plumage be made, birds from Greenland are quite as dark as Alaska specimens and conversely Alaska birds are as pale as those from Greenland. It is, perhaps, a matter of more than passing interest that the majority of adult Greenland birds in the collections I have seen are in worn faded plumage while most of the Alaska material is in fresh dark plumage. One might easily get the impression that the darker birds represent a race unless due allowance is made.
It may not be generally known that the adult Glaucous Gull moults twice in the year, a complete postnuptial moult beginning toward the last of July and extending over nearly two months and a prenuptial in March and April which involves most of the body feathers but not the wings nor the tail. Between moults the mantle fades and looks even paler than it is in color because of the worn and whitened feather edges. There is some individual variation in the depth of color in freshly moulted specimens, whether from Greenland or Alaska, but both may be equally dark and they may become equally pale after the lapse of a few months. I have examined birds taken nearly every month in the year and I am at a loss to understand how Dr. Oberholser finds a “decidedly darker” race unless he has unwittingly compared birds of unlike stages of plumage.
2. As for size, this is a question of relative dimensions that permits some latitude of opinion, so that a new presentation of the facts seems desirable.