Fitness, behavioral, and energetic trade-offs of different migratory strategies in a partially migratory species

Abstract

Alternative migratory strategies can coexist within animal populations and species. Anthropogenic impacts can shift the fitness balance between these strategies leading to changes in migratory behaviors. Yet some of the mecha- nisms that drive such changes remain poorly understood. Here we investigate the phenotypic differences, and the energetic, behavioral, and fitness trade-offs associated with four different movement strategies (long-distance and short-distance migration, and regional and local residency) in a population of white storks (Ciconia ciconia) that has shifted its migratory behavior over the last decades, from fully long-distance migration toward year-round residency. To do this, we tracked 75 adult storks fitted with GPS/GSM loggers with tri-axial acceleration sensors over 5 years, and estimated individual displace- ment, behavior, and overall dynamic body acceleration, a proxy for activity-related energy expenditure. Additionally, we monitored nesting colo- nies to assess individual survival and breeding success. We found that long-distance migrants traveled thousands of kilometers more throughout the year, spent more energy, and >10% less time resting compared with short-distance migrants and residents. Long-distance migrants also spent on average more energy per unit of time while foraging, and less energy per unit of time while soaring. Migratory individuals also occupied their nests later than resident ones, later occupation led to later laying dates and a lower num- ber of fledglings. However, we did not find significant differences in survival probability. Finally, we found phenotypic differences in the migratory proba- bility, as smaller sized individuals were more likely to migrate, and they might be incurring higher energetic and fitness costs than larger ones. Our results shed light on the shifting migratory strategies in a partially migratory popula- tion and highlight the nuances of anthropogenic impacts on species behavior, fitness, and evolutionary dynamics.