Testing optimal body mass theory: Evidence for cost of fat in wintering birds

Optimal body mass models for small wintering birds are central to animal ecology, and offer insights into maximizing individual fitness in a complex environment. Such models assume both costs and benefits of fat deposition, and consider how they affect winter survival probability. Hypothesized mass‐dependent costs of elevated fat include increased wing load and subsequent reduced ability to avoid predators, as well as increased predator exposure...