The gateway to Africa: What determines sea crossing performance of a migratory soaring bird at the Strait of Gibraltar?

Abstract

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 1318 |

Sea crossing as a major determinant for the evolution of migratory strategies in soaring birds

“…Due to logistic and financial constraints, such studies normally deal with low sample sizes (but see Sergio et al., 2014), and are focused on very wide geographical areas, at the expense of analyses at a finer scale. Within this scenario the research published by Santos, Silva, Muñoz, Onrubia, and Wikelski (2020) is of striking novelty: they choose a well‐known hot‐spot for migratory raptors, the Strait of Gibraltar, and analysed the relationships between topography, weather conditions, internal factors and flight behaviour during a small portion of the migratory journey, all with unprecedented detail. Taking advantage of the opportunity to capture a significant number of black kites Milvus migrans , a medium sized, facultative soaring, long‐distance migratory raptor, they tagged 73 individuals with GPS accelerometers just before their 14‐km jump towards Africa.…”

“…Santos et al. (2020) succeeded in describing each sea cross attempt in terms of duration, length and tortuosity, flight altitude and speed, flight style, and whether the crossing was successful or not. Then, they evaluated whether these parameters were influenced by external factors like weather conditions (wind and solar radiation), time of day, starting altitude, as well as by internal ones (age and sex).…”

“…The processes highlighted in Santos et al. (2020) research take place in a small geographical area, but depict one of the main selective agents among those that shaped the migratory flyways of soaring birds. Species that minimize flight distances over water have to fine tune their travels from the onset of migration, in order to reach the intermediate goal areas where the sea crossings are shorter (Vansteelant, Shamoun‐Baranes, van Manen, van Diermen, & Bouten, 2017; Vidal‐Mateo et al., 2016).…”

“…The complex geography of the Central Mediterranean region seems to have favoured the emergence of a range of sea crossing strategies, and the external influences on sea crossings demonstrated by Santos et al. (2020) may also interact with wing morphology to produce different trade‐offs among species (Figure 1).…”

“…Therefore, expanding the framework proposed by Santos et al. (2020), will be key to forecasting the far‐reaching impacts of ongoing environmental change. Since early arrival on breeding grounds can increase the chances of successful breeding (Sergio et al., 2014), future research should be carried out during the pre‐breeding migration, possibly with longer lasting transmitters.…”