Nest Records of the Song-Thrush

Abstract

UNDER its nest record card scheme, members of the British Trust for Ornithology are asked to complete a card for every nest of every species that they find, giving details of locality, situation, number of eggs and young and other particulars, together with the dates of recording. This method of inquiry represents a valuable advance in the technique of field ornithology, and, in a recent issue of British Birds, Miss E. T. Silva shows that much interesting material has been collected from 173 nest record cards for the song thrush (Turdus ericetorum) (42, No.. 4 ; April 1949). The collated information shows that the usual laying period for the song-thrush in Britain is from mid-March to mid-July, when it generally lays a clutch of four or five eggs. The average clutch-size in Britain increases from early spring to a peak in May, and then decreases again. A similar seasonal variation occurs in Holland and Finland ; but in the latter the peak occurs in June. The average clutch-size is larger in Holland than in Britain and larger still in Finland, at the same time of year. There is a significant variation in average brood-size in different years in Britain, and predation is the most usual cause of nesting failure. Nesting success does not vary appreciably with clutch-size or season. The song-thrush usually lays one egg each day until the clutch is complete ; a few exceptions have been recorded, but more observations are needed. The average incubation period is 13·4 days and the nestling period is 13·2 days. Both periods appear to be slightly longer in the early part of the season.