Clutch Size of the Siberian Tit Parus cinctus Boddaert, 1783 and the Red-Throated Flycatcher Ficedula albicilla (Pallas, 1811) in the Central Yakutia
The study results on the geographic, biotopic, annual and seasonal variability of the clutch size of the Siberian tit Parus cinctus and the red-throated flycatcher Ficedula (parva) albicilla, obtained at two sites ( biotops) near Yakutsk (62° N, 129° E) and probable factors contributing to its variability are considered. The Siberian tit clutch size in nest-boxes is 7.83±0.17 (n = 55) eggs; in the tree hollows, 7.64±0.34 (n = 11); those for the red-throated flycatcher, 7.78±0.09 (n = 212), 7.64±0.33 (n = 22), respectively; no significant differences among them have been exposed. Comparative evaluation of the average and efficient size of clutches revealed that, of the two compared forest types, the optimal breeding spot for the Siberian tit is a larch forest with an admixture of deciduous species (birch, willow); for the red-throated flycatcher, a mixed pine-larch forest. Seasonal clutch size variability of the studied species, like with most monocyclic bird species, is characterized by its gradual reduction over the nesting period. Annual fluctuations of the average clutch size of the sedentary Siberian tit correlate with the average breeding date; for the red-throated flycatcher such dependence has not been revealed. Analysis of the obtained data and publications showed that the geographical variability of the Siberian tit clutch size probably results from different breeding times in different parts of the species distribution range: at higher latitudes it decreases, unlike with the migratory red-throated flycatcher, whose clutch size increases with its advancement to the north and the east.