Adaptive developmental plasticity in growing nestlings: sibling competition induces differential gape growth.

Abstract

Sibling competition has been shown to affect overall growth rates in birds. However, growth consists on thecoordinated development of a multitude of structures, and there is ample scope for developmentalplasticity and trade-offs among these structures. We would expect that the growth of structures that areused in sibling competition, such as the gape of altricial nestlings, should be prioritized under intensecompetition. We conducted an experiment in the spotless starling (Sturnus unicolor), cross-fosteringnestlings to nests with different levels of sibling competition. We predicted that nestlings subjected tohigher levels of sibling competition should develop larger gapes than control birds. We found that, halfwaythrough the nestling period, overall size (a composite index of mass, wing, tarsus and bill) was reduced innests with intense sibling competition, whereas gape width remained unaffected. At the end of the nestlingperiod, experimental nestlings had wider gapes than controls. Additionally, a correlative study showed thatnestling gape width increased when feeding conditions worsened and overall size decreased. Thesepatterns could either be due to increased growth of gape flanges or to delayed reabsorption of thisstructure. Our results show that birds can invest differentially in the development of organs during growth,and that the growth of organs used in sibling competition is prioritized over structural growth