Tamara Emmenegger

Barrier crossing in small avian migrants: individual tracking reveals prolonged nocturnal flights into the day as a common migratory strategy

Over decades it has been unclear how individual migratory songbirds cross large ecological barriers such as seas or deserts. By deploying light-level geolocators on four songbird species weighing only about 12 g, we found that these otherwise mainly nocturnal migrants seem to regularly extend their nocturnal flights into the day when crossing the Sahara Desert and the Mediterranean Sea. The proportion of the proposed diurnally flying birds gradually declined over the day with similar landing patterns in autumn and spring. The prolonged flights were slightly more frequent in spring than in autumn, suggesting tighter migratory schedules when returning to breeding sites. Often we found several patterns for barrier crossing for the same individual in autumn compared to the spring journey. As only a small proportion of the birds flew strictly during the night and even some individuals might have flown non-stop, we suggest that prolonged endurance flights are not an exception even in small migratory species. We emphasise an individual’s ability to perform both diurnal and nocturnal migration when facing the challenge of crossing a large ecological barrier to successfully complete a migratory journey.

Variable detours in long-distance migration across ecological barriers and their relation to habitat availability at ground.

Migration detours, the spatial deviation from the shortest route, are a widespread phenomenon in migratory species, especially if barriers must be crossed. Moving longer distances causes additional efforts in energy and time, and to be adaptive, this should be counterbalanced by favorable condition en route. We compared migration patterns of nightingales that travelled along different flyways from their European breeding sites to the African nonbreeding sites. We tested for deviations from shortest routes and related the observed and expected routes to the habitat availability at ground during autumn and spring migration. All individuals flew detours of varying extent. Detours were largest and seasonally consistent in western flyway birds, whereas birds on the central and eastern flyways showed less detours during autumn migration, but large detours during spring migration (eastern flyway birds). Neither migration durations nor the time of arrival at destination were related to the lengths of detours. Arrival at the breeding site was nearly synchronous in birds flying different detours. Flying detours increased the potential availability of suitable broad-scale habitats en route only along the western flyway. Habitat availability on observed routes remained similar or even decreased for individuals flying detours on the central or the eastern flyway as compared to shortest routes. Thus, broad-scale habitat distribution may partially explain detour performance, but the weak detour-habitat association along central and eastern flyways suggests that other factors shape detour extent regionally. Prime candidate factors are the distribution of small suitable habitat patches at local scale as well as winds specific for the region and altitude.

Individual migration timing of common nightingales is tuned with vegetation and prey phenology at breeding sites

BackgroundThe timing of migration substantially influences individual fitness. To match peak requirements with peak resource availability, we hypothesized that individual migrants schedule spring migration in close relation to seasonal changes in environmental conditions along the route and particularly, at the breeding destination.To test this hypothesis, we investigated the timing of spring migration in male common nightingales Luscinia megarhynchos, a small Palearctic-African long-distance migrant, by linking spring migration timing to the phenology of local environmental conditions at non-breeding migratory stopover and breeding sites. In particular, we related individual migration decisions (i.e. departure and arrival) of nine males to site-specific vegetation phenology (based on remotely sensed vegetation index) and a proxy of food availability (based on insects’ thermal requirements).ResultsWe found weak relation of departures from non-breeding and no relation of stopover timing with local phenology. However, our results showed that individuals, which departed early from their non-breeding sites and arrived early at the breeding site closely matched spring green-up there. Early arrival at the breeding site meant also a close match with peak food availability for adults and in a time-lagged manner, for offspring.ConclusionOur findings suggest that male nightingale used cues other than local phenology for their departure decisions from non-breeding grounds and that there is some evidence for equalizing late departures during the course of migration.

Directional shifts in migration pattern of rollers (Coracias garrulus) from a western European population

Abstract Individual migrants often fly detours when travelling between breeding and non-breeding sites, resulting in specific changes in flight directions along a migratory leg. Western European populations of the European Roller (Coracias garrulus), the only member of the roller family of birds to breed in Europe, differ substantially in their predicted flight directions, leading to different hypotheses being suggested for passage areas and non-breeding destinations. In this study we have tested the hypotheses on a western or eastern detour and different crossings of the Sahara desert by tracking European Rollers breeding in southern France using light-level geolocators. After the departure from the breeding site between mid and end July, the three European Rollers which were tracked crossed the Mediterranean and the Sahara desert heading in straight southern direction. When arriving in the Sahelian zone they abruptly changed their direction eastwards to circumvent the Gulf of Guinea and reached the western Lake Chad basin, where they made a final direction shift to reach the non-breeding sites in western Angola. Our findings support the hypothesis of a straight north–south Sahara crossing with subsequent directional shifts. Whether western Africa serves as the non-breeding residence of European Rollers from the Iberian Peninsula and northern Africa remains to be elucidated.ZusammenfassungÄnderungen der Zugrichtung in einer westeuropäischen Population der Blauracke (Coracias garrulus) Auf ihrem Zug zwischen Brut- und Überwinterungsgebieten weichen Vögel oftmals von der direkten Route ab. Diese Abweichungen führen zu spezifischen Änderungen in der Flugrichtung. Für Blauracken (Coracias garrulus) der westeuropäischen Brutpopulationen existieren seit längerem verschiedene Hypothesen zur Lage der Durchzugs- und Überwinterungsgebiete, die sich deutlich in den zu erwarteten Zugrichtungen unterscheiden. Wir überprüften die Hypothesen eines östlichen, westlichen Umweges von der direkten Zugroute sowie verschiedener Formen der Saharaüberquerung mit Blauracken einer südfranzösischen Brutpopulation, die wir mit Geolokatoren ausgestattet hatten. Nach dem Verlassen des näheren Brutgebietes (etwa ab Mitte bis Ende Juli) überquerten die Blauracken das Mittelmeer und die Sahara in gerader südlicher Richtung. Nach Ankunft in der Sahelzone änderten die Vögel abrupt ihre Zugrichtung gegen Osten, umflogen so den Golf von Guinea, und erreichten den westlichen Teil des Tschadsee-Beckens. Hier änderten sie letztmalig ihre Zugrichtung um die Überwinterungsgebiete im westlichen Angola zu erreichen. Unsere Ergebnisse unterstützen die Hypothese einer geraden, in Nord-Süd-Richtung verlaufenden Querung der Sahara mit anschließenden Richtungswechseln. Ob Westafrika den Blauracken der iberischen oder nordafrikanischen Brutpopulationen als Wintergebiet dient, bleibt jedoch weiterhin offen.

Longer wings for faster springs – wing length relates to spring phenology in a long‐distance migrant across its range

Abstract In migratory birds, morphological adaptations for efficient migratory flight often oppose morphological adaptations for efficient behavior during resident periods. This includes adaptations in wing shape for either flying long distances or foraging in the vegetation and in climate‐driven variation of body size. In addition, the timing of migratory flights and particularly the timely arrival at local breeding sites is crucial because fitness prospects depend on site‐specific phenology. Thus, adaptations for efficient long‐distance flights might be also related to conditions at destination areas. For an obligatory long‐distance migrant, the common nightingale, we verified that wing length as the aerodynamically important trait, but not structural body size increased from the western to the eastern parts of the species range. In contrast with expectation from aerodynamic theory, however, wing length did not increase with increasing migration distances. Instead, wing length was associated with the phenology at breeding destinations, namely the speed of local spring green‐up. We argue that longer wings are beneficial for adjusting migration speed to local conditions for birds breeding in habitats with fast spring green‐up and thus short optimal arrival periods. We suggest that the speed of spring green‐up at breeding sites is a fundamental variable determining the timing of migration that fine tune phenotypes in migrants across their range.

Low intensity blood parasite infections do not reduce the aerobic performance of migratory birds

Blood parasites (Haemosporidia) are thought to impair the flight performance of infected animals, and therefore, infected birds are expected to differ from their non-infected counterparts in migratory capacity. Since haemosporidians invade host erythrocytes, it is commonly assumed that infected individuals will have compromised aerobic capacity, but this has not been examined in free-living birds. We tested if haemosporidian infections affect aerobic performance by examining metabolic rates and exercise endurance in migratory great reed warblers (Acrocephalus arundinaceus) experimentally treated with Plasmodium relictum pGRW04 and in naturally infected wild birds over consecutive life-history stages. We found no effect of acute or chronic infections on resting metabolic rate, maximum metabolic rate or exercise endurance in either experimentally treated or free-living birds. Oxygen consumption rates during rest and while undergoing maximum exercise as well as exercise endurance increased from breeding to migration stages in both infected and non-infected birds. Importantly, phenotypic changes associated with preparation for migration were similarly unaffected by parasitaemia. Consequently, migratory birds experiencing parasitaemia levels typical of chronic infection do not differ in migratory capacity from their uninfected counterparts. Thus, if infected hosts differ from uninfected conspecifics in migration phenology, other mechanisms besides aerobic capacity should be considered.

Intra-tropical movements as a beneficial strategy for Palearctic migratory birds

Migratory birds often move significantly within their non-breeding range before returning to breed. It remains unresolved under which circumstances individuals relocate, whether movement patterns are consistent between populations and to what degree the individuals benefit from the intra-tropical movement (ITM). We tracked adult great reed warblers Acrocephalus arundinaceus from a central and a southeastern European breeding population, which either stay at a single non-breeding site, or show ITM, i.e. move to a second site. We related ITM to the normalized difference vegetation index (NDVI) describing vegetation conditions and probably reflecting food abundance for these insectivorous birds. Three-quarters of birds showed ITM across the non-breeding range. We found no difference in range values and mean values of NDVI between the single non-breeding sites of stationary birds and the two sites of moving birds. The vegetation conditions were better at the second sites compared to the first sites during the period which moving birds spent at the first sites. Vegetation conditions further deteriorated at the first sites during the period the moving birds resided at their second sites. Our study provides evidence that birds probably benefit from improved conditions after ITM compared to the conditions at the sites from where they departed.

Host migration strategy and blood parasite infections of three sparrow species sympatrically breeding in Southeast Europe

Mobile hosts like birds occupy a wide array of habitats in which they encounter various vector and parasite faunas. If the infection probability for vector-borne parasites varies among seasons and biomes, a migratory life can critically influence the infections of a host. The growing body of literature on avian blood parasites suggests that host migrations do not only influence prevalence of infection but can also evoke higher infection intensities and increased parasite diversity in migrant compared to resident host species. We investigated the prevalence, intensity and diversity of Plasmodium and Haemoproteus infections in three closely-related and sympatrically breeding sparrow species with different migration strategies ranging from residential house sparrow and partially migratory tree sparrow to the obligate migratory Spanish sparrow. With a prevalence of 49%, the migratory Spanish sparrows were significantly less frequently infected than the resident house sparrows (82%). The partially migratory tree sparrow showed an intermediate prevalence of 60%. The parasitaemias were similar in all three host species and indicated mostly chronic but also few acute infections. While we found Plasmodium parasites in all three sparrow species, only Spanish sparrows were infected with Haemoproteus parasites in our study. With nine clearly identified parasite lineages in our study and the highest number of lineages per infected individuals (i.e. relative diversity), Spanish sparrows harboured the most diverse parasite fauna. Our results suggest that migration strategies can affect Plasmodium and Haemoproteus infections of sparrows resulting in a lower parasite prevalence and higher parasite diversity in migratory hosts—at least during our host’s breeding period. A general scope for all annual cycle periods and across various bird taxa remains to be elucidated.