Wood Nuthatch (Sitta europaea) Science Article 3
Bills of juvenile Nuthatches continue to increase in size (length and depth) for up to three months after fledging. Bills of individual full-grown birds increase in length from April to October with a temporary drop in June, and decrease throughout the winter season. Bill depth increases from April to May and mainly decreases throughout the rest of the year. Bill variation patterns are largely identical in males and females except for a slight size dimorphism, and a more strongly expressed seasonal variation in bill depth in males. The temporal pattern of bill length change and measurement on bill growth and abrasion support the hypothesis that seasonal variation in bill length is mainly affected by differential bill wear as a result of variation in foraging behavior and general activity, although the existence of seasonal variation in growth rate cannot be excluded with the present data.
Matthysen E., ARDEA 77 (1): 117-125.