Sex-specific difference in migration schedule as a precursor of protandry in a long-distance migratory bird

The putative role of the habenula in animal migration

BACKGROUND

When an addicted animal seeks a specific substance, it is based on the perception of internal and external cues that strongly motivate to pursue the acquisition of that compound. In essence, a similar process acts out when an animal leaves its present area to begin its circannual migration. This review article examines the existence of scientific evidence for possible relatedness of migration and addiction by influencing Dorsal Diencephalic Conduction System (DDCS) including the habenula.

METHODS

For this review especially the databases of Pubmed and Embase were frequently and non-systematically searched.

RESULTS

The mechanisms of bird migration have been thoroughly investigated. Especially the mechanism of the circannual biorhythm and its associated endocrine regulation has been well elucidated. A typical behavior called "Zugunruhe" marks the moment of leaving in migratory birds. The role of magnetoreception in navigation has also been clarified in recent years. However, how bird migration is regulated at the neuronal level in the forebrain is not well understood. Among mammals, marine mammals are most similar to birds. They use terrestrial magnetic field when navigating and often bridge long distances between breeding and foraging areas. Population migration is further often seen among the large hoofed mammals in different parts of the world. Importantly, learning processes and social interactions with conspecifics play a major role in these ungulates. Considering the evolutionary development of the forebrain in vertebrates, it can be postulated that the DDCS plays a central role in regulating the readiness and intensity of essential (emotional) behaviors. There is manifold evidence that this DDCS plays an important role in relapse to abuse after prolonged periods of abstinence from addictive behavior. It is also possible that the DDCS plays a role in navigation.

CONCLUSIONS

The role of the DDCS in the neurobiological regulation of bird migration has hardly been investigated. The involvement of this system in relapse to addiction in mammals might suggest to change this. It is recommended that particularly during "Zugunruhe" the role of neuronal regulation via the DDCS will be further investigated.

Across the deserts and sea: inter-individual variation in migration routes of south-central European barn swallows (Hirundo rustica)

BACKGROUND

The spatiotemporal organization of migratory routes of long-distance migrants results from trade-offs between minimizing the journey length and en route risk of migration-related mortality, which may be reduced by avoiding crossing inhospitable ecological barriers. Despite flourishing avian migration research in recent decades, little is still known about inter-individual variability in migratory routes, as well as the carry-over effects of spatial and temporal features of migration on subsequent migration stages.

METHODS

We reconstructed post- and pre-breeding migration routes, barrier crossing behaviour and non-breeding movements of the largest sample (N = 85) analysed to date of individual barn swallows breeding in south-central Europe, which were tracked using light-level geolocators.

RESULTS

Most birds spent their non-breeding period in the Congo basin in a single stationary area, but a small fraction of itinerant individuals reaching South Africa was also observed. Birds generally followed a 'clockwise loop migration pattern', moving through the central Mediterranean and the Sahara Desert during post-breeding (north to south) migration yet switching to a more western route, along the Atlantic coast of Africa, Iberia and western Mediterranean during the pre-breeding (south to north) migration. Southward migration was straighter and less variable, while northward migration was significantly faster despite the broader detour along the Atlantic coast and Iberia. These patterns showed limited sex-related variability. The timing of different circannual events was tightly linked with previous migration stages, considerably affecting migration route and speed of subsequent movements. Indeed, individuals departing late from Africa performed straighter and faster pre-breeding migrations, partly compensating for the initial departure delays, but likely at the cost of performing riskier movements across ecological barriers.

CONCLUSIONS

Different spatiotemporal migration strategies during post- and pre-breeding migration suggest that conditions en route may differ seasonally and allow for more efficient travelling along different migration corridors in either season. While highlighting patterns of inter-individual variability, our results support increasing evidence for widespread loop migration patterns among Afro-Palearctic avian migrants. Also, they suggest that carry-over effects acting across different phases of the annual cycle of migratory species can have major impacts on evolutionary processes.