Abstract
Nemuro Plain is a mixture of woods, grazing pastures and wet grasslands, with very low human population (mostly stock-farmers). Fields are frozen with thick snow during November and April, but the breeding birds are abundant in number though very patchy in nesting-density. Some birds nest as early as in late March, the peak being from late June to August and the breeding is ended in early September.
The factors affecting their breeding success were analysed based on the 104 nests of 29 species observed in.1953 (over 200 nests of 55 species were recorded). Only 32 (30%) nests were successful and 72 (70%) failed, by reasons of:
Human factors 69.5% 1) Deforestation 2) Cutting of grass 3) Collection of marsh-birds' eggs and damage by boys.
Birds and mammals 19.5% Especially crows (detail of their damage is given).
Other reasons 11% Natural factors, winds, rains, etc. and inadequacy of nestsite or nesting technique, etc.
