Causal link between insulin-like growth factor 1 and growth in nestlings of a wild passerine bird

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.
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.
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 post-hatch onwa...