Heat dissipation rate constrains reproductive investment in a wild bird

The heat dissipation limit theory (HDL) posits that animals with higher capacity to dissipate metabolic heat can increase reproductive investment. This theory remains untested in the wild. We recently showed that increased workload in a small bird causally relates to maximum body temperature. Here, we have expanded this approach by experimentally facilitating sensible heat transfer rate in nestling-feeding blue titsa small bird with high resting- and work-induced body temperaturesthrough removal of ventral plumage. Feather-clipped parents did not increase work rate but sired larger, and sometimes heavier, nestlings while maintaining lower body temperature and losing less body mass than controls. Thus, when relieved of the demands to dissipate metabolic heat, parents could invest more into both current (nestling condition) and future (self-maintenance) reproduction. In accordance with the HDL theory, we conclude that constraints on heat dissipation rate could be a potent mediator of life-history trade-offs in wild animals.