Martin, T. E., B. Tobalske, M. M. Riordan, S. Case, and K. P. Dial. 2018. Age and performance at fledging is a cause and consequence of juvenile mortality between life stages. Science Advances 4: eaar1988.
Should they stay or should they leave? That is a critical question facing young birds in a nest as they mature. The answer has ramifications for juvenile mortality, an important influence on demography and fitness, but differs among species and the reasons why are unclear. The probability of young being eaten increases with each day they remain in the nest. We show that species at greater risk leave the nest at a younger age with less developed wings that cause poorer flight performance and greater subsequent mortality. Experimentally delayed fledging verifies that older age and better developed wings reduce juvenile mortality. Fitness benefits of staying in the nest to reduce fledgling mortality are opposed by nest predation costs, and parents and offspring conflict on the optimal resolution. Ultimately, fledging age and associated offspring development balance mortality in and out of the nest in a compromise between parents and offspring.