Differential growth of body components among coexisting passerines in response to nest predation risk

Environmental sources of mortality can exert strong selection pressures on growth strategies across taxa. Studies of growth responses need to consider multiple body components because components can compete for resources during growth in an integrated growth strategy. However, such studies are lacking and little is known about the extent to which body components may differ in their growth responses to environmental selection pressures. Theory predicts that growth of body components with relatively higher advantages for survival should be prioritized. For example, increases in time dependent mortality, like nest predation risk in birds, should favor growth of body features that enhance the ability to leave nests earlier. We studied 12 coexisting species of passerines to specifically test predictions that species with higher nest predation rates would prioritize growth of locomotor components (e.g. tarsi and wings) at the expense of growth of body mass. We also tested the prediction that these altricial birds should develop endothermy earlier to facilitate their ability to leave the warm nest environment. We found species that experience higher nest predation rates exhibited relatively faster growth rates of wing chord, but not tarsus, compared with body mass. Furthermore, species with higher nest predation rates achieved adult-sized tarsi and 60% of adult wing-chord lengths at relatively smaller body mass, further demonstrating the prioritization of wing and tarsus development. Species with higher nest predation risk also developed endothermy earlier at relatively smaller body mass. Thus, our results suggest that growth responses among species to differences in nest predation risk include an integrated strategy across body components to facilitate an ability to escape a risky environment