Growth and heritability of size traits of Willow Tit Parus montanus nestlings-a supplementary feeding experiment

Abstract

Growth and final body size of passerine birds may be influenced by both genetic and environmental factors, growth often being constrained by food limitation . This study investigates whether the growth of nestling Willow Tits Parus montanus is constrained due to food shortage and if offspring resemble their parents more in body size when parents are given access to additional amount of food (maggots). This was studied by comparing nestling growth in food-supplemented and control broods. During the study year nestling period coincided very well with peak abundance of caterpillars, the main prey of Willow Tits . This resulted in a very good growth of nestlings in the control group. Of the morphometric measurements, only wing length at 13 days of age differed between nestlings in fed broods and unfed controls . To control for the influence of heritable variation in growth a cross-fostering experiment was carried out with six broods. When eliminating the genetic factor, additional food showed a tendency to affect body mass at 13 days. The heritability of tarsus length was very high (h2 = 0.89 ± 0.28) . It is proposed that fast growth rather than large body size per se, is a life history strategy enabling an early nest-leaving and early establishment as a member of a winter flock, which increases an individual's survival chances during the first winter .