Variation in egg dimensions of the Tree Sparrow [Passer montanus L.] in North-Western Croatia

A nest-box-breeding population of Tree Sparrow was studied during 2004–2005 in the rural area (in locality Mokrice) in northwestern Croatia. The mean egg length was 18.9 mm, egg breadth – 14.1 mm, egg volume – 1937.8 mm³ and egg shape index – 1.34. No significant correlation between egg volume and clutch size was found (P> 0.05) and there is no significant difference in those parameters between years of study (2004 and 2005) as well as between successive clutches in the given year. The repeatability of egg dimensions (r = 0–1) of females that bred in an area three times a year was estimated. The egg dimensions are highly repeatable r = 0.78, 0.63, 0.72 and 0.75 for length, breadth, volume and shape index, respectively. The results suggest that in Tree Sparrow population one should expect relatively high heritability of egg dimensions.