Birds of Yururi and Moyururi Islands, eastern Hokkaido in summer

1. Cormorants and anserine birds

Abstract

Observations on birds were made on Yururi and Moyururi Islands, known as natural monument of sea bird colonies, off the coast of eastern Hokkaido during the summers of 1960, 1972 and 1973. Incidental observations were also made in the adjacent waters of the islets. Yururi Island occupying an area of 168 hectares has 7.5km of coast line. Moyururi Island is smaller than the former, occupying an area of 31 hectares with the coast line 3km. Both islets are surrounded by rocky cliffs reaching a height of 20 to 30m except for a few coves such as Kato-hama and Aburakoma, where there are beachs. There are a few isolated rocks, Futatsu-jima, Nanatsu-iwa and so on, along the coasts. Both islets are generally flat at altitude of 30 to 40m above sea level (Fig. 2). Five major habitat types can be recognized (Fig. 3). On the flat top of islets there are the grassland dominated by Sasa niponica and the peat bog for Yururi Island and the grassland dominated by Sasa niponica and Calamagrostis langsdorffi for Moyururi Island. Mean temperatures at Nemuro showed 7.1°C in May, 10.0°C in June, 14.3°C in July and 17.1°C in August.
During the study a total of 29 species of birds were recorded (Table 1). In addition, other 13 species of birds have been recorded from the study area in summer: Gavia arctica, Fulmarus glacialis, Puffinus tenuirostris, Phalacrocorax pelagicus, Haliaeetus albicilla, Falco peregrinus, Rallus aquatieus, Tringa brevipes, Numenius madagascariensis, Larus hyperboreus, Uria lomvia, Aethia psittacula and Fratercula corniculata. Of them P. pelagicus have been found breeding on the rock stacks, Kamo-jima.
Fairly large sea bird colonies were located on the inaccessible rocks, on the flat-tops of steep cliffs and on the steep seaward-facing slopes (Table 2 and Fig. 6). Two to for species of sea birds occurred together on nearly all colonies except for the rocks J and M of Yururi Island, where only P. urile nested (Figs 4 and 5).
Supplementing our data by those of Haga (1973) who made a survey on the both islets in late June of 1972, it is estimated that numbers of nests containing eggs or chicks were at least 64 on Yururi Island and 150 on Moyururi Island for P. filamentosus and 170 on the former and 11 on the later for P. urile. On other hand, both species of cormorants were scarcely encountered in the adjacent waters of the islets.
The P. filamentosus colonies on Moyururi Island increased from 50 in 1959 to 65 occupied nests in 1960, furthermore to 150 in 1972, enlarging nesting grounds. The situation is very different in the P. urile colonies of the islet. Numbers of occupied nests were 25 in 1959, 35 in 1960 and increased to 60 in 1965, but decreased to 11 in 1972. There were 20 to 30 occupied nests of P. urile on the west side cliffs of Yururi Island in 1957 and 72 on the northeast side between I to P inclusive in 1960. However, changes in whole numbers of occupied nests of P. filameutosus and P. urile on Yururi Island are not clear because of the lack of sufficient information for other years or for other parts of the islet, respectively.
The two species of cormorants distinctly differed in nest sites. P. filanentosus nested on flat broad parts on the tops of rocks or cliffs, but P. urile on the narrow ledge of steep cliffs (Figs. 7 and 9). In the case of two species nesting on the same rock, the former nested on the flat-ground just above the cliffs and the later on the lower ledges (Fig. 4). The shift of nest sites by year was characteristic for P. urile. For a detailed analysis of ecological separation in other aspects described by previous workers for cormorants, sufficient information was not obtained.