Spring bird census in the Ryu Kyu Is. (1972)

Abstract

Winter (Kuroda 1969) and autumn (1970) bird censuses of the Ryu Kyu Is. have already been reported. The present paper is the result of the survey planned by the Ornithological Society of Japan, principally for analysis of the status of Sapheopipo noguchii of Okinawa I. This will be reported elsewhere.
The bird census was made on Okinawa I. 24-28 May in the northern mountain zone, on Ishigaki I. 31 May and 5 June and on Iriomote I. 1-4 June. The weather was fine except on 5 June.
In total, 51 species, 30 land, 16 water or waterside, and 5 sea birds, were recorded. The land bird species were 24 on Okinawa, 15 on each of Ishigaki and Iriomote. These land birds are residents and show the reduced number of breeding species in these subtropical islands where palearctic species (such as thrushes, flycatchers, warblers and tits, etc.) decrease or disappear and tropical elements are represented only by few species. The number of individuals is also generally low, especially in the montane zone in spite of the excellent and extensive forests. This may be due to the distributional periphery for both palearctic and tropical species.
The most generally abundant species was Hypsipetes amaurotis with the dominance of 21% in totalized avifauna, followed by Streptopelia orientalis of 18% of dominance. This species was particularly abundant on Iriomote where it gathered on a few small coastal islets (Hatopanare, Usagi (newly named islet), etc.) for breeding (and roosting). They nested on the ground under dense grass and all had laid two eggs, with the density of a true colony, and flew out to perch on rocks or dead shrubs by small flocks. The environmental safety and potential habit of oversea dispersion of the pigeons and doves may, among others, be attributed to this peculiar island concentration. Three birds were seen flying low over the sea surface from the main island to the offshore island of Hatomajima, km apart.
The next was Zosterops palpebrosa of the general dominance of 8%, but more may have been missed in the census. Cettia diphone was even more abundant with the dominance of 22% than Hypsipetes on Okinawa, but was not recorded on the other two islands. Next were dominant Passer montanus (common on Ishigaki but absent from Iriomote), Corvus macrorhynchos and Parus major and the subtropical nature of the avifauna of Ryu Kyu Is. was well characterized by such species with medium dominance, as Terpsiphone atrocaudata, Pericrocotus roseus, Parus varius (not encountered on Ishigaki and Iriomote this time), Cisticola juncidis, Halcyon coromanda, Otus scops, Sphenurus sieboldii and Turnix suscitator, etc.
Coastal and marsh birds were not plentiful, egrets and waders having already passed north, and only a few remained. On extensive saltflats small flocks of Tringa brevipes, Tringa nebularia and one Tringa totanus were recorded. An Ardea purpurea and Sterna hybrida were found on Iriomote at the same places where they were seen in 1970. Ixobrychus cinnamoneus was common on rice fields with Gallinula chloropus. Alcedo atthis is said to have decreased due to the recent use of insecticides, and only one was seen in a mangrobe of Iriomote.
Sea birds recorded were five species, Sterna sumatrana was arriving north to Okinawa and was seen in pairs. Its breeding places on small coastal rocky islets, Hatopanare off the north coast and other two islets off the western coast of Iriomote were first confirmed, but they were not laying eggs yet. Some flocks of Sterna fuscata were seen north of Iriomote, probably with the seasonal migration of the bonito to this sea area where schools of small fish were seen chased to the surface and a flying fish was observed.