Abstract: Nest-box size itself may influence clutch size. In this study, we analysed the variation in clutch size and brood size (number of fledglings at ringing) in relation to nest-box size in the White-throated Dipper. Nest-boxes were open on three sides (i.e. nesting shelves; n = 35). The internal base area of the nesting shelves varied from 237.5 to 587.5 cm². The shelves were located under the bridges along an elevation gradient of 500 m in the Levočské vrchy Mts. and Spišská Magura Mts., north-eastern Slovakia. Data were collected during the breeding seasons 2013–2023. Only
data from first clutches were considered. Mean clutch size was 5.3 eggs (SD=1.0, range 2–7, n=43 clutches), mean brood size was 4.9 fledglings (SD=1.0, range 2–6, n=52 clutches). There was no association between clutch size and the base area of the nesting shelf in dipper (simple linear regression: F=0.061, d. f.=1, 41, R²_adj.=-0.02, P=0.806, slope estimate (SE)=0.0004 (0.0015)). Similarly, dipper brood size was not associated with nesting shelf size (F=0.869, d.f.=1, 50, R²_adj.=-0.003, P=0.356, slope (SE)=0.0013 (0.0014)). Elevation had no effect on clutch or brood size. Observed pattern of variation in clutch and brood size in relation to nest-box size can be explained by the nest-box design, which does not impose physical limits on the size of the nest itself. Another nest box design – a front-open nest-box – could affect dipper clutch size by spatially restricting its nest.

Key words: clutch size, brood size, fledling number, breeding succes, nest-box size, altitude