Insulin-like growth factor 1 and growth rate in nestlings of a wild passerine bird

1Post-natal growth rate is an important life-history trait that is strongly associated with survival during the transition to independence and the overwinter period. In laboratory conditions, insulin-like growth factor 1 (IGF-1) has been shown to be one of the main hormones regulating growth and development in maturing vertebrates. However, the extent to which plasma IGF-1 enhances growth in free-living animals, particularly in birds, is still unclear. In this study, we used brood size manipulation in free-living great tits (Parus major) to investigate how changes in nestling nutritional condition and post-natal growth rate are related to nestling IGF-1 levels. Nestlings in reduced broods had increased post-natal growth rate, better body condition and also higher IGF-1 levels prior to fledging than those in control and enlarged broods. Plasma IGF-1 levels were higher in the middle of the nestling period, when growth rate is most rapid, than in the pre-fledging stage. Our results suggest that nestling plasma IGF-1 varies depending upon resource (e.g. food) availability and potentially plays an important role in regulating the post-natal growth and development of wild passerines in a time-specific manner.