A study led by Doñana Biological Station – CSIC in collaboration with European universities and research centres has demonstrated how Eleonora's falcons cope with the diverse environmental conditions they encounter on their seasonal migrations between the Canary Islands and Madagascar. In particular, the study looked at how winds determine the falcons' movements along their peculiar migration corridor. The researchers combined data from adult falcons equipped with GPS-trackers with global atmospheric models and satellite imagery to understand how falcons navigated seasonal wind fields across Africa's diverse landscapes.

Eleonora's falcons breed on small islands throughout the Mediterranean Sea and the eastern Atlantic Ocean. In the case of the Canarian falcon population, the most western of all colonies, a direct flight along the shortest possible route to Madagascar would span about 8000km, and would run straight across major barriers like the Sahara, the tropical rainforest of the Congo Basin, and the Indian Ocean. However, the study that was recently published by the scientific journal Movement Ecology shows that falcons from the Canary Islands actually take zigzagged routes spanning on average more than 9000km in autumn, and more than 11000km in spring. Nevertheless, and although the falcons spent about six more days at pit-stops in spring, the falcons were able to complete those longer spring routes in the same amount of travel hours as in autumn. This was largely due to strong tailwinds along the falcons' spring routes, which allowed falcons to reach much greater speeds during their spring return to the Canary Islands.

 "This led us to the question to what extent the zigzagging behaviour of the falcons helped them to maximize wind support", explains the team at the head of this study.  To answer this question, they compared the wind support that falcons actually experienced at each location along their chosen travel direction, with the wind support they could have received by taking the shortest possible route instead. It turned out that in both seasons the falcons zigzagged to maximize wind support across open ocean and desert, a common strategy among migrant birds.

In autumn, however, falcons also detoured to reduce the negative influence of strong headwinds over the East African savannahs, even though this landscape is not as dangerous as the desert or the sea. Moreover, falcons largely compromised wind support by travelling directly east-/westward over the Sahel-Sudan zone across several thousand kilometres in each season. The seasonal winds are relatively weak in this region, and in making these longitudinal movements the falcons compensated for previous drift, and also greatly reduced the flight distance over the rainforest in autumn, and over the Sahara in spring. 

The scientists conclude that "detours are a common feature in bird migration, and it is likely that many other species respond to winds in a similarly flexible way as Eleonora's falcons. When confronted with opposing wind fields and geographical barriers birds are better off tolerating wind drift until they can get back on track through weaker or more supportive wind fields. When winds are weak or favourable, however, birds have considerable leeway to pursue other goals, and will tolerate a small reduction in wind support by detouring around barriers, or to familiar feeding areas."

Referencia:

Vanteelant et al. 2021. Adaptive drift and barrier-avoidance by a fly-forage migrant along a climate-driven flyway. Movement Ecology DOI : 10.1186/s40462-021-00272-8.