LINKED PAPER Geographic variation in clutch size of the Varied Tit in East Asia: latitudinal gradient along the Japanese Islands. Seki, S.I., Yamaguchi, N., Fujita, K., Mizuta, T. & Higuchi H. (2026) Ornithological Science.VIEW
Clutch size is one of the most traditionally studied life-history traits in birds because of its importance to avian reproductive components, apparent variation, relative ease of data collection, and the striking latitudinal gradient commonly observed across various taxonomic levels. The ubiquitous increase in clutch size with latitude suggests a substantial contribution from environmental factors that change with latitude, and this latitudinal gradient has been regarded as one of the key phenomena in understanding the diversity of life-history traits. However, most species-level analyses on this clutch size variation have concentrated on European and North American species, because of the greater number of basic ecological records in those regions.
Figure 1. The Varied Tit nesting in a nest-box on Nakanoshima Island, southern Japan © Shin-Ichi Seki.
The Varied Tit (Sittiparus varius) is a hole-nesting Paridae species widely distributed in the Japanese archipelago and the eastern part of the Eurasian continent. It is one of the most common nest-box breeders in the region, and its breeding and behavioural ecology have been studied in various sites in Japan, Korea, and China. Its strictly sedentary life history, relatively short breeding season, and the low frequency of the second brood reduce the within-population variation of breeding traits, and the Varied Tit is assumed to be an ideal subject for studying geographic variation in life-history traits.
Figure 2. The Varied Tit distribution (areas shaded in green) and locations of study sites (yellow circles). After Seki et al., 2026.
In our recent paper, geographic variation in the clutch size of Varied Tit was reviewed and analysed using a generalized linear model based on a parametric bootstrap approach, and we investigated the relationship between clutch size and latitude, subspecific classification, and habitat size. We collected data from 33 sites ranging from 26.77°N to 42.66°N, and found an evident latitudinal increase in the local mean clutch size from 3.7 to 7.6 eggs. GLM analysis supported a latitudinal increase in clutch size, with an average of 0.184 eggs per degree. Meta-analyses on the life-history traits of nest-box breeders have primarily focused on European and North American species to date, and such a clear latitudinal gradient in the Varied Tit’s clutch size was the first confirmed case in Asian species.
Figure 3. Clutch size—latitude relationship of the Varied Tit modeled using a parametric bootstrap approach. The black line represents the mean predicted clutch size across the latitudes, and the grey lines represent the results of 1,000 bootstrap trials. After Seki et al., 2026.
Population isolation as a subspecies was identified as another influential factor in clutch size variation for one of the four subspecies examined, i.e., S.v.owstoni, which inhabits small islands far south of the Japanese main islands. However, the effect of habitat size (islands smaller than 100 km2) was not significant, although the clutch size of some small island populations tended to be smaller than that of nearby larger habitat populations.
At some islands’ sites in the southern part of the Varied Tit’s range, we complemented the shortage of fieldwork with nest-box cameras, simple photo recording devices self-customised from trail cameras, attached to the inside of nest boxes (Seki, 2023a). This camera system could be operated for approximately three months using four Alkaline AA cells, set to run for one hour each morning, recording 5MP images at 20-minute intervals. Inexpensive trail cameras with simple customisations have been proven to work efficiently as nest-box cameras to identify the breeding status of nest-box breeders (Seki, 2023a; Seki, 2023b).
Figure 4. Nest box camera using a self-customised trail camera. Top left: a nest-box camera unit customized from a trail camera, right: installation image (side view) of the camera, lower left: an example image recorded by the camera. Records of a nest-box camera are available from Seki (2023b) as a time-lapse video, and a brief self-customisation manual of trail cameras is available from Seki (2023a).
Our article is freely available in Ornithological Science, published by the Ornithological Society of Japan. This volume marks the journal’s transition to a trial open-access model, utilizing the Subscribe to Open (S2O) pilot program for three years, offered by BioOne and set to launch in 2026.
References
Seki, S.I. 2023a. Breeding biology of the Varied Tit in the subtropical broad-leaved forests of Okinawa-jima Island. Bird Research 19:51–61 (written in Japanese with English summary).VIEW
Seki, S.I. 2023b. Records of a nest box camera, self-customized from a trail camera, to monitor the breeding biology of the Varied Tit Sittiparus varius on Okinwa-jima, southern Japan. Japan Bird Research Association. VIEW
Image credit
Top right and featured image: Varied Tit (Sittiparus varius) © Takuma Watakabe
