Effects of Food Availability and Parental Risk Taking on Nestling Period Duration: A Field ExperimenT on the Pied Flycatcher Ficedula hypoleuca

Summary. The timing of fledging is critical in altricial birds as it affects offspring survival probability and thereby both parental and offspring fitness. The high vulnerability of offspring to predation soon after fledging may induce a parent-offspring conflict with respect to the age of fledging, as parents would benefit more than offspring from reducing the period of whole-brood vulnerability before fledging. Parents also benefit from inducing early fledging through reduced commuting costs and a decreased exposure to predators. A reliable food resource near the nest could thus favour a relaxation of parental drives to induce fledging and enable an approach to the optimal fledging age from the offspring point of view. A population of a cavity nester, the European Pied Flycatcher Ficedula hypoleuca, was offered food supplements (live mealworms) at the nest-box to randomly selected pairs throughout the nestling period and fledging ages were compared with those at nests without supplements. Provisioning parents were also trapped at the nest-box to estimate their willingness to incur predation risks in terms of easiness of capture (trappability). Fledging ages ranged from 15 to 20 days. Fledging was delayed in food-supplemented nests (n = 21) compared with control nests (n = 20) by one day on average, a significant difference. This was not due to the size and mass of nestlings at 13 days, which were similar under both treatments. Moreover, fledging age was positively related to parental trappability. Parents with easy access to food near the nest and those more willing to risk predation were those whose nestlings fledged at older ages. Parents apparently adjust the timing of offspring fledging to their foraging costs.—Moreno, J. (2020). Effects of food availability and parental risk taking on nestling period duration: a field experiment on the Pied Flycatcher Ficedula hypoleuca. Ardeola, 67: 29-38.