Summary1. Post-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. 2. 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. 3. 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. 4. 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.

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1. Life history of animals is strongly linked with the variability in postnatal growth rates, because the ability to achieve necessary body mass early in life is directly connected with the ability to survive and reproduce later in life. Surprisingly, little is known about the physiological mechanisms connecting food availability with the variability of growth in wild animals. 2. We used a hormonal injection treatment in free-living nestlings of the pied flycatcher (Ficedula hypoleuca). In doing this, we aimed to see how their plasma insulin-like growth factor 1 (IGF-1) levels and its mediation of growth rate vary in response to administration of exogenous IGF-1 during the early period of postnatal development.3. We showed that the levels of IGF-1 as well as growth rate and body size, in terms of body mass and tarsus length, were significantly higher in IGF-1-injected nestlings, compared with those in the control group. These differences became especially pronounced from day 7 posthatch onwards during the 13-day nestling period. Additionally, the proportional investment into growth was more profoundly downregulated in IGF-1-injected nestlings at the end of the nestling period, which achieved fledging size earlier, compared with control nestlings. 4. Our results are the first to highlight IGF-1 as the causal link between growth conditions and variability of growth rate and body size in wild vertebrates. Interestingly, the change in IGF-1 levels also showed a plausible, distinct adaptive age-and physical maturity-dependent pattern in nestlings to match the benefits gained by them when investing in growth, with the cost endured during the early days of postnatal development to maximize their fitness. Therefore, this mechanism might be among those mediating the maintenance of genetic variability in growth rates in populations, presumably regardless of the presence of strong directional selection acting upon them.

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