Genetic and environmental components of growth in nestling blue tits (Parus caeruleus)

We investigated the effect of brood-size mediated food availability on the genetic and environmental components of nestling growth in the blue tit (Pa rus caeruleus), using a cross-fostering technique. We found genetic variation for body size at most nestling ages, and for dur ation of mass increase, but not of tarsus growth. Hence, nestling growth in our study population seems to have the potential to evolve further. Furthe rmore, significant genotype-environment interactions indicated heritable va riation in reaction norms of growth rates and growth periods, i.e. that our study population had a heritable plasticity in the growth response to envi ronmental conditions. The decreasing phenotypic variance with nestling age indicated compensatory growth in all body traits. Furthermore, the period of weight increase was longer for nestlings growing up in enlarged broods, while there was no diff erence to reduced broods in the period of tarsus growth. At fledging, birds in enlarged broods had shorter tarsi and lower weights than birds in reduc ed broods, but there was no difference in wing length or body condition bet ween the two experimental groups. The observed flexibility in nestling grow th suggests that growing nestlings are able to respond adaptively to food c onstraint by protecting the growth of ecologically important traits.