Dependencies in the timing of activities weaken over the annual cycle in a long-distance migratory bird

Abstract

Migrating birds have to incorporate migration into their annual cycle, next to breeding and moult. This presents the challenge to arrive at the right place at the right time at any given moment during the year to maximize fitness. Although many studies have investigated the timing of specific (life-history) activities of migrating birds, it is poorly studied how the timing and duration of these activities depend on each other and, ultimately, how they affect fitness. Therefore, we investigated variability and dependencies in the timing and duration of successive activities throughout the annual cycle and assessed their fitness consequences in hoopoes (Upupa epops), a long-distance migratory bird, using geolocator and breeding phenology data of five consecutive years. We found that the timing and duration of seasonal activities generally depended on the timing and duration of the preceding activity; yet, the strength of these dependencies and the degree of variability varied between activities. The strongest dependencies were found between the end of breeding and departure from the breeding grounds as well as between the arrival in the breeding grounds and the onset of breeding. We also found fitness consequences of timing and duration but only for specific activities: spring migration and particularly the duration of the pre-breeding period influenced the quality of the territory acquired as well as the total number of fledglings. Consequently, we suggest that our study species has the flexibility to adjust the timing and duration of activities but to varying degrees. This is a step forward in understanding the time-constraints that migratory animals face and in identifying their fitness consequences. To date, few studies have investigated the timing and dependencies of seasonal activities in migrating birds throughout the annual cycle and their fitness consequences. However, understanding temporal constraints and identifying sensitive—fitness-relevant—periods within the annual cycle is crucial to understand the population dynamics of migrating species. We investigated the timing and duration of life-history activities in a long-distance migratory bird and assessed their fitness consequences. Our results clearly show that seasonal activities were generally linked to the previous activities but that this dependency ceased relative to activities further in the past. We also found clear fitness consequences but only for the timing and duration of specific activities. Thus, our study shows that migrants can adjust the timing and duration of activities to a certain degree but also that this flexibility is more constrained for specific activities.