van Balen J.H. (1967) The significance of variations in body weight and wing length in the Great Tit, Parus major. ARDEA 55 (1-2): 1-59

A study was made of the various factors contributing to variation in wing length and body weight in the Great Tit, with special reference to differences due to habitat. Many hundreds of data on weight and wing length were collected in nine areas during the winters of 1955-'56 to 1964-'65. Most of this information was obtained from two areas: the Liesbosch (oak wood) and the Hoge Ve1uwe (mainly pine wood). Special attention was given to the reliability of age determination and to the various methods of measuring wing length. The wing length of the males exceeds that of the females by about 4%. The sex difference in wing length is largest in deciduous woods. In these woods the wings of both male and female tits are somewhat longer than in coniferous woods. This applies to yearling and older birds. Yearlings have shorter wings than older birds. After the first complete moult (i.e. in their second summer) the wing length remains constant. Abrasion of the primaries is very slight during winter, but plays a role during the breeding season. The mean wing length in a population varied strongly from year to year probably influenced by feeding conditions and possibly by the weather during the moulting season. Male Great Tits are significantly heavier than females during winter. This holds true when males and females of equal age and wing length are compared. Males have more subcutaneous fat than females. In both sexes the highest mean weights occur in the deciduous habitats. In these habitats, where feeding conditions in winter are presumably better, the variability of body weight is larger than in less favourable habitats. In most cases there is a distinct positive correlation between weight and wing length. This correlation exists in all age classes, but is least pronounced among the second-year birds. In most tit populations body weight increases with age until the second or third winter and then decreases. In the Hoge Veluwe, where feeding conditions are probably worse than in the areas mentioned above, body weight decreases from the first to the fourth winter, presumably representing a gradually worsening condition of the birds. Observations on the times of rising and roosting show that in oak woods the birds need a slightly shorter period of activity for feeding than in pine woods. The rate of weight loss during the night is not constant but decreases in the course of the night. The nocturnal weight loss was strongly affected by the air temperature; the tits lose weight at a faster rate in cold than in mild nights. With temperatures below zero the birds in the oak woods increase their evening weight by means of an increase in food intake, and more than compensate for the increased nocturnal weight loss. The pine wood birds are apparently unable to do this, and consequently lose weight during periods of prolonged cold. Body weight fluctuates seasonally with a maximum in January and a minimum shortly before the start of the breeding season. Annual variation in weight is very small and is not clearly affected by the size of the beechmast crop or the density of the tits. Seasonal and annual fluctuations in body weight are related to air temperature, especially when feeding conditions are good.