The clutch size and the breeding season of the Seven Is. Thrush

Two hundred nest-record-cards of the Seven Is. Thrush Turdus celaenops were obtained through pupils' assistances of a junior high school in Miyake-jima, Izu Is. off Honshiu in 1963-65. 167 of them could be utilized in this paper. Main results are: 1) Common clutch size was 3 and 4 (rarely 2 and 5). Annual mean clutch size was 3.5 in 1963, 3.7 in 1964 and 3.3 in 1965, and was roughly parallel to mean temperature in April. 2) The breeding season (date of first laying) was between March and June, rarely up to August. Most birds probably breed once a year. In 1964, the warmest of the three years, the earliest breeding was recorded and in 1965, when a cold wave swept Japan in spring, breeding became active later in the season. 3) Seasonal change of the clutch size was like that of the Blackbird in Great Britain. The commonest clutch size was 3 in April, 4 in May and 3 after June. The largest clutch size, 5 eggs, was recorded from two nests in May. It attracts attention that there was difference between the time of the largest clutch size (the end of May) and the height of breeding season (from the middle of April to the middle of May). 4) The obtained data were too poor to discuss correlation between clutch size and breeding success because many nests were broken by children.