The work was carried out in the villages of the łomianki commune near Warsaw, Poland (52°20′N, 20°50′E) in 1994 and 1995. From among 315 clutches of Tree Sparrows studied, 20 exhibited one-day interruptions in the laying of first and second broods, while 2 were characterised by two-day interruptions. Amongst third broods, there was just a single one-day interruption noted in each year of the study. Interruptions did not occur immediately prior to the laying of the last egg in a clutch. Only in the case of the first brood in 1995 could a period of cold account for interruptions; in the remaining cases, the phenomenon must have been influenced by non-meteorological factors. The Tree Sparrow resembles the House Sparrow Passer domesticus in having far fewer interruptions to laying than other small hole-nesting birds, such as tits Parus spp. This is probably a reflection of the genus Passer having evolved in dry areas, where the accumulation of body reserves in the female prior to laying is an adaptation reducing the length of the breeding period to match the time associated with the rainy season, when food is abundant.