Seasonal change in basal bill colour and possibile sex discrimination based on exposed measurements of Eurasian Tree Sparrows in Hokkaido

Abstract

During ringing of Eurasian Tree Sparrows Passer montanus in Hokkaido, three adult birds were recaptured. The colour of the basal part of the bill was black in one individual and yellow in two. Field studies show that juveniles have yellow bills, but both adults and young have black bills during June and July. From September to December, all birds developed yellow on their bills indicating seasonal change in bill colour. Male sparrows were significantly heavier with longer natural wing and tail lengths than females. Sex can be distinguished by a natural wing length of 67.7 mm and the probability of correct discrimination was 90%.