A comparative study of migratory behaviour while crossing large geographical barriers in three long-distance migratory passerines

Abstract

Migratory behaviour explains how individuals or populations perform their migration. In birds this can include nocturnal or diurnal flights, long or short flights as well as low or high flight altitudes and several other behaviours. These behaviours shift throughout the migratory journey and especially when crossing barriers (large geographical areas where birds might have difficulties to find rest and refuel e.g. deserts and oceans). This master thesis examines how three long-distance songbird migrants, the Thrush nightingale (TN; n = 8) Luscinia Luscinia, the Red-backed shrike (RBS; n = 11) Lanius collurio and the Great reed warbler (GRW; n = 10) Acrocephalus arundinaceus perform their migrations and focuses especially on the common barrier: the Mediterranean Sea and Sahara Desert.

I was given data from multi sensor data loggers on the three species and analysed the pressure and activity data. I summarized the annual structure of the migratory activity of the species, and the differences between them when crossing the common barrier. All species performed clustered migratory flights which divided the migration into flight segments with major stopovers in between. The TN and RBS migrate longer distances and performed their migration in 6 and 5 segments, respectively. The GRW who migrate a shorter distance used 4 segments. The migratory intensity was higher during the spring migration in the RBS and GRW but not in the TN. A general positive effect of stopover duration on migratory intensity over the barrier was found for all species, but it did not differ between the species. The RBS was also found to have a higher proportion of inactivity in the major stopover before the barrier crossing compared to the other species. The flight altitudes were generally low over Europe and higher over the barrier, the Mediterranean Sea and Sahara Desert for all species, but the flight altitudes also varied over the migratory route. The RBS was found to have significantly lower flight altitudes when crossing the barrier than the other species. All species were found to sometimes prolonged nocturnal flights into daytime and always climbed to higher flight altitudes at these occasions. The GRW performed on average more and longer prolonged flights than the other two species and flew at high altitudes as long as throughout daytime flights. In conclusion, the species differed to some extent in their migratory behaviour during barrier crossings, and most pronounced in their propensity to perform prolonged flights. The observed differences highlight the need for continued migration research to support effective conservation under ongoing environmental change.

Popular Abstract

Migratory behaviour while crossing large geographical barriers in three long-distance migratory passerines

As the days grow shorter and temperatures drop, songbirds in northern Europe leave for their autumn journeys to greener pastures. These seasonal journeys are known as migration, and birds in this temperate region have followed them for thousands of years. Birds migrate in many different ways, and these patterns are referred to as migratory behaviour. Some species migrate steadily over a few weeks, while others travel in short bursts with long stopovers in between. Most songbirds migrate at night, but some also fly during the day. All of these strategies are part of migratory behaviour, which can vary both within and between species. Migratory behaviour often changes when birds cross large barriers such as oceans and deserts, where landing and foraging opportunities are limited or absent.

Three bird species that cross both the Mediterranean Sea and the Sahara Desert during migration are the Thrush nightingale Luscinia luscinia, the Red-backed shrike Lanius collurio, and the Great reed warbler Acrocephalus arundinaceus. All three species breed in northern Europe during summer. In autumn, the Thrush nightingale and the Red-backed shrike migrate to southern Africa, while the Great reed warbler travels to West Africa south of the Sahara. Using multisensor loggers, eight Thrush nightingales, eleven Red-backed shrikes, and ten Great reed warblers were tracked for an entire year. Data from accelerometers and pressure sensors made it possible to extract and compare different aspects of migratory behaviour between the species.

Before analysing the data, I considered what differences I expected to find. The Thrush nightingale typically inhabits dense and swampy environments, while the Great reed warbler specialises in lakes and wetlands. The Red-backed shrike, in contrast, is usually found in open grasslands and has a broad diet that includes insects, berries, and even small birds. Based on these differences, I expected the Thrush nightingale and the Great reed warbler to hurry across the Mediterranean Sea and the Sahara Desert, as suitable stopover habitat is scarce for them in these regions. I expected the Red-backed shrike to migrate more slowly and forage more along the way. I also expected the Thrush nightingale and the Great reed warbler to fly at higher altitudes, as lower altitude flight may make it easier to locate potential landing and foraging sites.

The results partly supported these expectations. There was a tendency for the Thrush nightingale to migrate more intensely than the Red-backed shrike over the barrier, although this difference was not statistically significant. The Great reed warbler did not show higher migratory intensity than the other species, which did not align with my initial expectations. I also found that longer stopovers before the barrier were associated with higher migratory intensity for all species during the crossing. One explanation may be that longer stopovers allow birds to store more fuel before attempting the crossing. Another possibility is that the birds experienced poor stopover conditions and needed more time to accumulate sufficient energy, which they then compensated for by migrating more intensely. As expected, the Red-backed shrike flew at lower altitudes over the barrier, which may reflect a greater tendency to search for potential landing sites.

All three species mainly migrate at night, but they sometimes extend their flights into daylight. These extended flights are known as prolonged flights and may represent another strategy to hurry across ecological barriers. During the crossing of the Mediterranean Sea and the Sahara Desert, all three species performed prolonged flights, but to different degrees. Just over half of the Thrush nightingales extended a flight during the autumn crossing, and none did so in spring. In contrast, almost all Great reed warblers extended at least one flight in both autumn and spring. The Red-backed shrike showed an intermediate pattern, frequently extending single flights in autumn but only rarely in spring. This result was unexpected, as I initially expected the Thrush nightingale to perform more prolonged flights than the Red-backed shrike. Perhaps the favourable habitats awaiting the Red-backed shrike further south provide a stronger incentive to keep moving rather than stopping to forage in the desert. When performing prolonged flights, Great reed warblers often continued flying throughout the day and into the following night, while the other two species typically stopped in the morning. Some Great reed warblers also performed multiple prolonged flights per individual, a pattern not observed in the other species. This suggests that Great reed warblers are particularly reluctant to stop over in this region if they can avoid it.

In conclusion, prolonged flights were the most striking behavioural difference between the species. The Great reed warbler showed the most extreme behaviour, the Red-backed shrike displayed an intermediate response, and the Thrush nightingale showed the least extreme behaviour, rarely extending flights. These differences likely reflect variation in migratory routes and the habitats encountered along the way. Great reed warblers cross the vast and arid Sahara Desert in both seasons to reach wetlands and lakes in West Africa. Red-backed shrikes migrate across the Sahara on a broad front in autumn and return along the East African coast in spring. Thrush nightingales follow the Nile during the autumn crossing and migrate along the East African coast in spring. To better understand why species extend their flights, future studies should examine where prolonged flights occur in relation to deserts and open water. When birds cross areas where landing is impossible, continuing to fly may be their only option. Such is the life of a migratory bird, adjusting its behaviour to the landscape below. Long uninterrupted flights are therefore not acts of excess, but necessary choices shaped by deserts, seas, and coastlines that leave birds with no other option than to keep going.

Master’s degree project in Biology 45 hp 2025
Supervisors: Sissel Sjöberg & Pablo Macias-Torres
Biology department, Lund University

• author: Gustafsson, Lydia
• supervisor: Sissel Sjöberg; Pablo Macías Torres
• organization: Degree Projects in Biology
• course: BION02 20251
• year: 2026
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 9222136
• date added to LUP: 2026-02-05 14:25:25
• date last changed: 2026-02-05 14:25:25