The Eucalyptus warbler belongs to the warbler subfamily, a subfamily known for the long-distance migration of large populations, and is not related to the forest willow warbler of the Americas. Named for their preference for clearings between willow forests, they are the most common birds in most breeding areas, breeding between the July isothermal line of 10 and 22°C, north to the Arctic tundra. In the wintering regions of Africa, the Willow Warbler occupies almost all areas where trees are distributed, including acacia savannas and evergreen forests.
Migration between Europe and Africa
Of all the migratory birds that migrate from Europe and Asia to the warm climate zones of Africa for wintering, there is one willow warbler every five, and the total number of adult and juvenile birds after breeding can reach almost 1 billion. The birds fly thousands of miles south across mountains, oceans and deserts.
As male willow warblers occupy breeding areas in April and May, competing to attract mates, their soft, trembling cries flutter across the vast expanse of Europe and Asia, which has traditionally been seen as a sign of spring. A male bird singing immediately triggers neighboring birds to participate in the song, and the whole forest or wasteland becomes vibrant due to these song choruses. Willow warblers breed quickly, raising a litter of 4 to 8 young birds in an average of 26 to 28 days, so that by the end of July to August, they are ready to migrate south.
Different migration routes
Through the data obtained from the annual monitoring of cyclic individuals, we have a good understanding of the migration patterns of the European willow warbler. By 2004, British ornithologists had labeled more than a million euphorbia warblers, of which 2,500 were subsequently captured or picked up. This recovery rate is superior in cyclic studies and provides us with a reliable source of data. Data from cyclic recycling show that populations in western Europe migrate south or southwest in the autumn, crossing France and Spain and reaching West Africa for wintering. Populations in northern and eastern Scandinavia, on the other hand, migrate south-eastward, overwintering in central, eastern and southern Africa. Populations that breed in Siberia migrate the farthest. The birds fly south and then southwest through Russia, and most of them cross the Ural River all the way to southern Africa, a marathon journey of at least 14,000 kilometers. The Warbler will arrive in Africa in September and December, depending on the starting point of the migration. In the wintering areas, they form loose groups with local birds, wandering between different regions, and living a life of back and forth.
The flight strategy of the European willow warbler
The detection of the captured Willow Warbler revealed the secret of the difficult migration of this bird weighing only 8 to 12 grams. These nocturnal migratory species appear to be able to replenish energy quickly, allowing them to stay and feed during the day during migration. The scalding sand of the Sahara Desert creates a very large obstacle for the migration of the Oyster, who quickly stores enough fat to complete the migration before crossing the Great Desert: the migratory Oyster, captured in Egypt in the autumn, has enough fat in its body for them to cross the desert for three nights, while resting during the day.
Like many other migratory birds, the migration period of the Willow Warbler varies with sex and age. In autumn, sub-adults migrate south before adults, and adults catch up with or overtake them later. The reason why sub-adults start first can be explained by their slightly different body structure: sub-adults have more rounded wings, while adult birds have longer wings, which means that sub-adults are less efficient in long-distance migration. On the other hand, on the return trip in spring, males cross Europe two weeks earlier than females, indicating that they set out from Africa earlier. An early departure gives the male enough time to build territory before the female arrives, waiting for the arrival of potential mates, where they will become the most abundant species. There is growing evidence that climate change is causing changes in this inherent migration pattern. For example, the European willow warbler in the UNITED Kingdom lays its eggs a week earlier in the warmer spring and leaves the UK later than it did 40 years ago.
The biggest threat to the European willow warbler may be drought in northern Africa. Spread over the southern edge of the Sahara Desert, from eastern Senegal to Sudan, the Sahel strip has dried up what was once a reliable source of water and turned once a jungle oasis into bare sand. This situation was exacerbated by prolonged overgrazing, with the result that the Sahara Desert was expanding southward. This also reduces the chances of tired migratory birds resting during the day, lengthening the distance they cross the Sahara Desert. While this inference requires more valid evidence, environmental disasters in the Sahel may also be a major reason for the recent decline in the number of willow warblers returning to breeding grounds in the spring. In the 1980s, the population of the European willow warbler declined rapidly, and the number of some European populations even fell by more than 30%. Scientists believe that the intensification of desertification is related to the cutting down of oasis trees by humans, and the indirectness of desert expansion has led these birds to extinction.