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

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 mechanisms 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 displacement, behavior, and overall dynamic body acceleration, a proxy for activity-related energy expenditure. Additionally, we monitored nesting colonies 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 number of fledglings. However, we did not find significant differences in survival probability. Finally, we found phenotypic differences in the migratory probability, 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 population and highlight the nuances of anthropogenic impacts on species behavior, fitness, and evolutionary dynamics.

Timing is critical: consequences of asynchronous migration for the performance and destination of a long-distance migrant.

BACKGROUND: Migration phenology is shifting for many long-distance migrants due to global climate change, however the timing and duration of migration may influence the environmental conditions individuals encounter, with potential fitness consequences. Species with asynchronous migrations, i.e., with variability in migration timing, provide an excellent opportunity to investigate how of the conditions individuals experience during migration can vary and affect the migratory performance, route, and destination of migrants. METHODS: Here, we use GPS tracking and accelerometer data to examine if timing of autumn migration influences the migratory performance (duration, distance, route straightness, energy expenditure) and migration destinations of a long-distance, asynchronous, migrant, the white stork (Ciconia ciconia). We also compare the weather conditions (wind speed, wind direction, and boundary layer height) encountered on migration and examine the influence of wind direction on storks' flight directions. RESULTS: From 2016 to 2020, we tracked 172 white storks and obtained 75 complete migrations from the breeding grounds in Europe to the sub-Saharan wintering areas. Autumn migration season spanned over a 3-month period (July-October) and arrival destinations covered a broad area of the Sahel, 2450 km apart, from Senegal to Niger. We found that timing of migration influenced both the performance and conditions individuals experienced: later storks spent fewer days on migration, adopted shorter and more direct routes in the Sahara Desert and consumed more energy when flying, as they were exposed to less supportive weather conditions. In the Desert, storks' flight directions were significantly influenced by wind direction, with later individuals facing stronger easterly winds (i.e., winds blowing to the west), hence being more likely to end their migration in western areas of the Sahel region. Contrastingly, early storks encountered more supportive weather conditions, spent less energy on migration and were exposed to westerly winds, thus being more likely to end migration in eastern Sahel. CONCLUSIONS: Our results show that the timing of migration influences the environmental conditions individuals face, the energetic costs of migration, and the wintering destinations, where birds may be exposed to different environmental conditions and distinct threats. These findings highlight that on-going changes in migration phenology, due to environmental change, may have critical fitness consequences for long-distance soaring migrants.

Performance of GPS/GPRS tracking devices improves with increased fix interval and is not affected by animal deployment.

The use of GPS tracking technologies has revolutionized the study of animal movement providing unprecedentedly detailed information. The characterization of GPS accuracy and precision under different conditions is essential to correctly identify the spatial and temporal resolution at which studies can be conducted. Here, we examined the influence of fix acquisition interval and device deployment on the performance of a new GPS/GSM solar powered device. Horizontal and vertical accuracy and precision of locations were obtained under different GPS fix acquisition intervals (1min, 20 min and 60 min) in a stationary test. The test devices were deployed on pre-fledgling white storks (Ciconia ciconia) and we quantified accuracy and precision after deployment while controlling for bias caused by variation in habitat, topography, and animal movement. We also assessed the performance of GPS-Error, a metric provided by the device, at identifying inaccurate locations (> 10 m). Average horizontal accuracy varied between 3.4 to 6.5 m, and vertical accuracy varied between 4.9 to 9.7 m, in high (1 min) and low frequency (60 min) GPS fix intervals. These values were similar after the deployment on white storks. Over 84% of GPS horizontal positions and 71% of vertical positions had less than 10m error in accuracy. Removing 3% of data with highest GPS-Error eliminated over 99% of inaccurate positions in high GPS frequency intervals, but this metric was not effective in the low frequency intervals. We confirmed the suitability of these devices for studies requiring horizontal and vertical accuracies of 5-10m. For higher accuracy data, intensive GPS fix intervals should be used, but this requires more sophisticated battery management, or larger batteries and devices.

Changes in surface water drive the movements of Shoebills.

Animal movement is mainly determined by spatial and temporal changes in resource availability. For wetland specialists, the seasonal availability of surface water may be a major determinant of their movement patterns. This study is the first to examine the movements of Shoebills (Balaeniceps rex), an iconic and vulnerable bird species. Using GPS transmitters deployed on six immature and one adult Shoebills over a 5-year period, during which four immatures matured into adults, we analyse their home ranges and distances moved in the Bangweulu Wetlands, Zambia. We relate their movements at the start of the rainy season (October to December) to changes in Normalized Difference Water Index (NDWI), a proxy for surface water. We show that Shoebills stay in the Bangweulu Wetlands all year round, moving less than 3 km per day on 81% of days. However, average annual home ranges were large, with high individual variability, but were similar between age classes. Immature and adult Shoebills responded differently to changes in surface water; sites that adults abandoned became drier, while sites abandoned by immatures became wetter. However, there were no differences in NDWI of areas used by Shoebills before abandonment and newly selected sites, suggesting that Shoebills select areas with similar surface water. We hypothesise that the different responses to changes in surface water by immature and adult Shoebills are related to age-specific optimal foraging conditions and fishing techniques. Our study highlights the need to understand the movements of Shoebills throughout their life cycle to design successful conservation actions for this emblematic, yet poorly known, species.

Flight altitudes of a soaring bird suggest landfill sites as power line collision hotspots.

Anthropogenic structures are increasingly encroaching wildlife habitats, creating conflicts between humans and animals. Scaling up renewable energy requires new infrastructures such as power lines, that cause high mortality among birds since they act as obstacles to flight and are used for perching and nesting, which can result in collisions or electrocutions. These interactions often endanger wildlife populations and may also result in high financial costs for companies. Flight behaviour plays a crucial role in collision risk, and the study of flight altitudes enables us to understand what drives birds to fly at collision risk altitudes. This allows the identification of high-risk areas, conditions and bird behaviours, and the implementation of mitigation measures by power line companies. In this study, we use boosted random tree modelling to identify drivers of white stork (Ciconia ciconia) flight altitudes and to investigate the factors that lead them to fly at collision risk altitudes. We found that the main drivers of flight altitude for this soaring bird species were time of day, distance to the nearest landfill site and cloud cover density. Bird age, habitat type and season were comparatively less important. Collision risk increases during crepuscular hours near landfill sites, also in days with high cloud cover density and during the breeding season. In recent years, hundreds to thousands of storks congregate daily at landfill sites to take advantage of the predictability and superabundance of anthropogenic food waste. Some of these sites have high density of power lines, becoming collision risk hotspots for storks and other landfill users. Despite being susceptible to collision, our results suggest that white storks can avoid power lines to a certain extent, by changing their flight altitude at ca. 80 m from these structures. This study shows that the implementation of mitigation measures for existing power lines should be prioritized in areas in the vicinity of landfill sites within white stork distribution ranges, and the projection of new lines should avoid those areas. These measures would benefit species vulnerable to mortality due to power line collision, and it would also reduce associated power outages and economic costs.

Second-growth and small forest clearings have little effect on the temporal activity patterns of Amazonian phyllostomid bats.

Secondary forests and human-made forest gaps are conspicuous features of tropical landscapes. Yet, behavioral responses to these aspects of anthropogenically modified forests remain poorly investigated. Here, we analyze the effects of small human-made clearings and secondary forests on tropical bats by examining the guild- and species-level activity patterns of phyllostomids sampled in the Central Amazon, Brazil. Specifically, we contrast the temporal activity patterns and degree of temporal overlap of 6 frugivorous and 4 gleaning animalivorous species in old-growth forest and second-growth forest and of 4 frugivores in old-growth forest and forest clearings. The activity patterns of frugivores and gleaning animalivores did not change between old-growth forest and second-growth, nor did the activity patterns of frugivores between old-growth forest and clearings. However, at the species level, we detected significant differences for Artibeus obscurus (old-growth forest vs. second-growth) and A. concolor (old-growth forest vs. clearings). The degree of temporal overlap was greater than random in all sampled habitats. However, for frugivorous species, the degree of temporal overlap was similar between old-growth forest and second-growth; whereas for gleaning animalivores, it was lower in second-growth than in old-growth forest. On the contrary, forest clearings were characterized by increased temporal overlap between frugivores. Changes in activity patterns and temporal overlap may result from differential foraging opportunities and dissimilar predation risks. Yet, our analyses suggest that activity patterns of bats in second-growth and small forest clearings, 2 of the most prominent habitats in humanized tropical landscapes, varies little from the activity patterns in old-growth forest.