Breeding biology of the Varied Tit in the subtropical broad-leaved forests of Okinawa-jima Island

Abstract

The Varied Tit Sittiparus varius is a hole-nesting passerine bird widely distributed in the Japanese archipelago and the eastern part of the Eurasian continent. The species prefers nest boxes for reproduction, and its breeding biology is well studied in the central part of Japan (Honshu and the Izu Islands) but there are few reports in the southern part (such as Kyushu and the Ryukyu Archipelago). In this study, the breeding biology of the Varied Tit was investigated in subtropical broad-leaved forests on Okinawa-jima in the Ryukyu Archipelago during 2019-2022, using a total of 300 nest boxes. To identify the breeding status in nest boxes, the shortage of fieldwork was complemented with nest box cameras, simple photo recording devices customized from trail cameras, attached to the inside of nest boxes. The earliest record of egg laying date was February 18th, and the laying dates were mostly concentrated in early March. Both these indices were approximately one month earlier than those on Honshu. The average clutch size was 3.7 on Okinawa-jima, 2-3 eggs less than the records in Honshu. The results suggest the existence of latitudinal clines in these life history traits. The fledging success on Okinawa-jima was as low as 35.1% among 94 breeding nests. Nest box cameras had been installed in 36 of those 94 nest boxes, and day-by-day status was photo-recorded in 34 nests. Two nests experienced mechanical trouble with the cameras and were not recorded. Inexpensive trail cameras, with just some simple customizations, have been proven to work efficiently as nest box cameras recording life history traits of breeding birds.