445 eggs of the Collared Flycatcher from 82 clutches were measured during three breeding seasons (1997–1999). The mean length was 17.82 ± 0.80 mm, breadth — 3.45 ± 0.37 mm and volume — 1.65 ± 0.14 cm3. Egg dimensions were positively correlated. No significant differences in egg sizes during the three seasons were found. This suggests that the environmental conditions in the Białowieża Forest during the study period did not change or had no influence on egg size. Laying sequence had no influence on egg dimensions; only egg length depended on clutch size. Some characteristics of the females did affect the size of eggs: heavier birds and those with longer tarsi laid larger eggs. Older females did not lay significantly larger eggs than younger females. In conclusion, egg size in the Collared Flycatcher from the Białowieża Forest appears to be influenced more by the characteristics of the female than by environmental conditions.
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Vol. 36 • No. 1