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The gateway to Africa: what determines sea crossing performance of a migratory soaring bird at the strait of Gibraltar?
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Orientador(es)
Resumo(s)
1. Large bodies of water represent major obstacles for the migration of soaring birds
because thermal updrafts are absent or weak over water. Soaring birds are known
to time their water crossings with favourable weather conditions and there are
records of birds falling into the water and drowning in large numbers. However, it
is still unclear how environmental factors, individual traits and trajectory choices
affect their water crossing performance, this being important to understand the
fitness consequences of water barriers for this group of birds.
2. We addressed this problem using the black kite Milvus migrans as model species at
a major migration bottleneck, the Strait of Gibraltar.
3. We recorded high-resolution GPS and triaxial accelerometer data for 73 birds
while crossing the Strait of Gibraltar, allowing the determination of sea crossing
duration, length, altitude, speed and tortuosity, the flapping behaviour of birds
and their failed crossing attempts. These parameters were modelled against wind
speed and direction, time of the day, solar irradiance (proxy of thermal uplift),
starting altitude and distance to Morocco, and age and sex of birds.
4. We found that sea crossing performance of black kites is driven by their age,
the wind conditions, the starting altitude and distance to Morocco. Young birds
made longer sea crossings and reached lower altitude above the sea than adults.
Crosswinds promoted longer sea crossings, with birds reaching lower altitudes
and with higher flapping effort. Birds starting at lower altitudes were more likely
to quit or made higher flapping effort to complete the crossing. The location
where birds started the sea crossings impacted crossing distance and duration.
5. We present evidence that explains why migrating soaring birds accumulate at
sea passages during adverse weather conditions. Strong crosswinds during sea
crossings force birds to extended flap-powered flight at low altitude, which may increase their chances of falling in the water. We also showed that juvenile birds
assume more risks than adults. Finally, the way in which birds start the sea crossing
is crucial for their success, particularly the starting altitude, which dictates how far
birds can reach with reduced flapping effort
Descrição
Research Article
Palavras-chave
animal tracking bird migration crosswind ecological barriers powered flight sea crossing soaring flight thermal updrafts
Contexto Educativo
Citação
J Anim Ecol. 2020;89:1317–1328
Editora
John Wiley and Sons