Abstract
1. This report dealt only with changes in specific census effectivity during the breeding season from the middle of May to early July.
2. The seasonal changes of the census effectivity were calculated for 18 species in Takizawa area based on 9 years census data (Fig. 1, 2). In them, only four species, namely, Turdus chrysolaus, Hypsipetes amaurotis, Streptopelila orientalis and Aegithalos caudatus, showed significant changes of effectivity during the breeding season mentioned above. But, the changes of Streptopelia orientalis and Aegithalos caudatus were considered to be caused by the chage of their densities.
3. Finally, in 29 species surveyed until now in Japan, it was considered that only three species, namely, Turdus chrysolaus, Hypsipetes amaurotis and Certhia familiaris, showed a definite seasonal change of the census effectivity.
4. As the cause of stability of the census effectivity during the breeding season in almost of the species concerned, it was supposed that registration chances of birds do not change during the breeding season because various bird activities such as song, call and flight show compensatory changes in the course of the season.
5. To minimize an error of density estimation derived from seasonal change of the census effectivity, samples should be collected from broad areas and covering a long period during the breeding season.
