565 THE SEX-RATIO IN THE WILD ANIMAL POPULATIONS OF THE NEW HEBRIDES. By A. J. Marshall and John R. Baker, D.Sc. [Read 27th November, 1940.] The chief biological object of the Oxford University Expedition to the New Hebrides (1933-4) was the study of the breeding seasons of animals in a remarkably unvarying climate, but we kept a careful record of all specimens obtained with a view to reporting on the sex-ratio. Our collections were made chiefly in eastern Espiritu Santo, though specimens were also taken in the west and in the small island of Gaua. We soon noticed that the great majority of our vertebrates showed a remarkable preponderance of males. The same peculiarity is shown by the natives of the islands, and those of the district called Sakau have one of the highest sex-ratios of any people in the world (see Baker, 1928).* In this paper we report on the birds and mammals. The sex-ratio is given as the percentage of males, and the standard error of the ratio has been calculated from the Imf formula -\/ > where m is the percentage of males, f that of females, and n the total \ n number of specimens counted. The birds will be considered first. All species of which we shot 40 or more specimens are considered in what follows. There are 20 such species and the total number of individuals shot of these species (of all ages) was 2,193. The sex-ratio of the whole group was 59 ± 1-0. Of the 20 species, only 3 had an excess of females, and in no case was such excess of females statistically significant. The highest sex-ratios were as follow: Cacomantis pyrrophanus (cuckoo) 84 ± 4-9 Myzoviela carclinalis (honey-eater) .. 81 ± 3-6 Rhipidura brenchleyi (flycatcher) 68 ± 5-8 Halcyon chloris (kingflsher) 67 ± ft^ . Turdus poliocephalus (blackbird) 65 ± 6-7 Of the total of 2,193 birds, 319 were juvenile. It is interesting to' notice that the sex-ratio of the juveniles was as high (60 ± 2-7) as that of the adults (58 ± 1-1). We did not obtain any young Cacomantis, but in Myzomela cardinalis the sex-ratio of young was 78 ±: 5-9 and of adults 83 ± 4-5. These flgures show that high sex-ratio is not to any significant extent a product of differential mortality of the sexes in the later stages of growth. The fact that sex-ratio is as high in young as in adults also shows that the preponderance of males is not due simply to the females being too much occupied with incubation to be shot. Other lines of evidence point in the same direction. Thus the highest sex-ratio of all (84 dz 4-9) is in a cuckoo, which does not incubate at all; and the flycatcher and kingflsher have high ratios, though in these families it is usual for both sexes to incubate. We do not think that our shooting was selective. Natives were nearly always paid the same price for specimens of each sex: when we were paying more for one sex, we neglected the birds received in our studies of sex-ratio. On the whole males have brighter plumage, but there is no direct correlation here with sex-ratio. Thus the male Myzomela is brilliantly-coloured and the female dull, and the species has a ratio of 81; but in Cacoviantis, Rhipidura brenchleyi, and Halcyon, * We particularly wish to acknowledge our indebtedness to T. H. Harrison. XX
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