Weights.—The NAMRU2 party obtained specimens with the following weights: two adult males 256, 257; four females 147, 153, 210, 252 grams.
Specimens examined.—Total number, 13 (5 males, 6 females, 2 unsexed), from Mariana Islands, USNM—Guam (Jan. 29, May 8, June 19, 20, 23, 28, 30, July 14, 19, 23, Sept. 8).
Nesting.—A nest was found by McElroy of the NAMRU2 party at Guam on October 24, 1945, in dense grass on a hillside near Mount Santa Rosa. The nest contained three eggs, which the author (1948:48) describes as "white with a pinkish cast and a scattering of small spots of colors near 'russet' and near 'pear blue' which are concentrated at the large ends. They measure 37.5 by 29.1, 39.1 by 28.0, and 40.7 by 29.0." Downey, black chicks were found on April 1, May 16, and May 26. M. Dale Arvey found a chick on August 2, 1946, near Tumon Bay. A parent bird with young ones was seen near Merizo on October 2. A male taken on January 26 had enlarged gonads. Seale (1901:30) obtained a black chick in June or July. On the basis of the above observations it seems that the nesting season extends from spring to fall, although Marshall (1949:219) assumes that this rail breeds the year around.
Remarks.—The Guam Rail was first reported by Quoy and Gaimard who called it "Ralê tiklin," but was not described as new until 1895 by Rothschild. It appears to be equally at home in upland grassy areas and in jungle areas. The species was not seen frequently by the NAMRU2 party, although birds were occasionally observed crossing the roads. Few birds were shot; most of the specimens were taken in rat traps, which may be the most satisfactory method of obtaining them. Coultas took his specimens with the aid of a dog. On June 19, 1945, a small patch of woodland was being removed by a bulldozer. Four rails, which were hiding in this thicket, were surrounded and three were captured by hand. These birds tried to escape over the cleared ground by running with wings flapping but made no effort to fly. I am inclined to believe, as the natives do, that these birds are virtually incapable of actual flight.
The Guam Rail usually appeared to be a quiet bird, but at Tarague Point on July 12, 1945, I heard its loud penetrating cry; it was a series of rapid screeches. At the same time rapid movement made considerable noise in the undercover. The bird making the call suddenly appeared, either rapidly chasing, or being chased by, another rail. The birds had abandoned their usual skulking habits and had little concern for the observer. I took this to be breeding behavior, comparable to that of some of the North American rails during the mating period.
The Guam Rail is probably not in serious danger of extermination. It is utilized by the natives as food; they capture the bird, using dogs and trail snares. Its skulking habits and ability to inhabit most types of cover on the island should insure its existence for a long time to come.
Evolutionary history.—Rallus owstoni is endemic to the island of Guam with no closely related forms nearby. It is one of the several rails found in the Pacific which live on isolated islands. In comparison with other species in the region, it has some resemblance to both R. torquatus and R. philippensis. In general, the underparts of R. owstoni resemble those of the R. philippensis group, although the upper parts resemble somewhat those of R. torquatus. Certain specimens of R. owstoni have a slight indication of a pale pectoral band. The bill is shorter and heavier than that of R. torquatus, possibly more like that of R. philippensis. The short rounded wing is a distinctive character. The bird came from an ancestral stock possibly resembling R. philippensis and probably originated in the Philippine or Papuan areas. It may have invaded Micronesia at an early date and may have had a wider distribution in the islands in former times. Perhaps this same invasion resulted in the establishment of R. wakensis (Rothschild) at Wake. The supposed route of colonization is shown in [figure 9].
