Risk-taking restraints in a bird with reduced egg-hatching success

Risk taking, as is any other phenotypic and/or behavioural trait, is determ ined by proximate constraints related to time or resource availability and by evolutionary adaptive restraints related to the differences in the costs of risk taking and its benefits in terms of fitness. Because risk taking i s influenced by many confounding variables related to experimental design, environment, parents and offspring, few field studies have been reported wh ich unambiguously separate the effects of restraints from those of constrai nts. We compared parental risk taking in blue tits (Parus caeruleus) during brood defence towards a nest predator in broods with experimentally reduce d and natural egg-hatching success leaving the original number of eggs in t he nest. The experimentally reduced broods had more time or resources avail able and lower risk-taking benefits compared to the control broods. 'Constr aint' would predict more risk laking in broods having experimentally reduce d egg-hatching success, whereas 'restraint' would predict the opposite effe ct with more risk taking in broods with natural egg-hatching success. We re port, to our knowledge, the first field study experimentally demonstrating a brood defence restraint in response to reduced egg-hatching success. This demonstration was only possible after controlling for more than 20 potenti al confounding variables showing once more how complicated it is to separat e proximate from evolutionary levels of analyses in natural populations.