Christiane Trierweiler

When and where does mortality occur in migratory birds? Direct evidence from long‐term satellite tracking of raptors

Information about when and where animals die is important to understand population regulation. In migratory animals, mortality might occur not only during the stationary periods (e.g. breeding and wintering) but also during the migration seasons. However, the relative importance of population limiting factors during different periods of the year remains poorly understood, and previous studies mainly relied on indirect evidence. Here, we provide direct evidence about when and where migrants die by identifying cases of confirmed and probable deaths in three species of long-distance migratory raptors tracked by satellite telemetry. We show that mortality rate was about six times higher during migration seasons than during stationary periods. However, total mortality was surprisingly similar between periods, which can be explained by the fact that risky migration periods are shorter than safer stationary periods. Nevertheless, more than half of the annual mortality occurred during migration. We also found spatiotemporal patterns in mortality: spring mortality occurred mainly in Africa in association with the crossing of the Sahara desert, while most mortality during autumn took place in Europe. Our results strongly suggest that events during the migration seasons have an important impact on the population dynamics of long-distance migrants. We speculate that mortality during spring migration may account for short-term annual variation in survival and population sizes, while mortality during autumn migration may be more important for long-term population regulation (through density-dependent effects).

Migratory connectivity and population-specific migration routes in a long-distance migratory bird

Knowledge about migratory connectivity, the degree to which individuals from the same breeding site migrate to the same wintering site, is essential to understand processes affecting populations of migrants throughout the annual cycle. Here, we study the migration system of a long-distance migratory bird, the Montagus harrier Circus pygargus, by tracking individuals from different breeding populations throughout northern Europe. We identified three main migration routes towards wintering areas in sub-Saharan Africa. Wintering areas and migration routes of different breeding populations overlapped, a pattern best described by ‘weak (diffuse) connectivity’. Migratory performance, i.e. timing, duration, distance and speed of migration, was surprisingly similar for the three routes despite differences in habitat characteristics. This study provides, to our knowledge, a first comprehensive overview of the migration system of a Palaearctic-African long-distance migrant. We emphasize the importance of spatial scale (e.g. distances between breeding populations) in defining patterns of connectivity and suggest that knowledge about fundamental aspects determining distribution patterns, such as the among-individual variation in mean migration directions, is required to ultimately understand migratory connectivity. Furthermore, we stress that for conservation purposes it is pivotal to consider wintering areas as well as migration routes and in particular stopover sites.

Satellite tracking of two Montagu's Harriers (Circus pygargus): Dual pathways during autumn migration

Autumn migration routes of two Dutch female Montagu’s Harriers (Circus pygargus) were documented for the first time using satellite telemetry. Both migrated to their African wintering area—one via the Straits of Gibraltar through the Mediterranean and the other via Italy/Tunisia. The rate of travel was comparable to values reported for larger raptor species.

Competitive abilities of oystercatchers (Haematopus ostralegus) occupying territories of different quality

In territorial species, habitat heterogeneity results in some individuals occupying poor quality sites while others occupy high quality sites. Floaters (mature nonbreeders) may accept a low quality territory, because it is the best they can get and defend (‘inferior phenotype hypothesis’), or because it is a strategic alternative for a high quality territory in the long run (‘queue hypothesis’). Oystercatcher Haematopus ostralegus territories differ consistently in the amount of offspring produced each year and this is related to the distance between the nesting and feeding territories. The inferior phenotype hypothesis was previously rejected on the basis of the absence of morphometric differences (assumed to indicate competitive abilities) among breeders. We investigated social dominance, in the field and in captivity, in relation to the quality of the breeding territory. In the field, birds with high-quality territories won more often compared to those occupying low-quality territories. However, this difference was not apparent in a small dataset of captive birds. These results are discussed in the framework of the long-term fitness prospects of settling in a high or low quality territory.