Some birds have bi-annual migration because they move to find food resources and favorable climatic conditions for their reproduction, and this may be necessary at different times of the year depending on their life cycle.
The biannual migration of birds is often dictated by the variable availability of food resources. Indeed, as winter approaches, finding food becomes a struggle: insects are scarce, berries are absent, and plants are less generous. Therefore, to avoid starvation, many birds decide to head towards milder climates where food is abundant. Another major factor is obviously temperature. Some birds poorly tolerate the cold, and migrating twice a year allows them to stay within their thermal comfort zone. The seasonal rotation of day and night, known as photoperiod, also plays a significant role. The length of the day signals to birds when it's time to leave: no need for a schedule, nature takes care of it. Lastly, some birds closely follow changes in their habitats: if wetlands, grasslands, or forests undergo significant changes throughout the seasons, it encourages our feathered friends to migrate twice a year to ensure their survival.
Migrating twice a year allows birds to enjoy a favorable climate and abundant food resources year-round. By avoiding excessively cold or hot conditions, they limit their daily energy expenditure. This regular movement also reduces competition for food and nesting habitats. As a result, they optimize their chances of survival and reproduction. Finally, regularly changing environments also helps to decrease the risks associated with predators and region-specific diseases.
To navigate during their long migrations, birds possess an internal biological clock, precisely calibrated by the length of the day, known as photoperiod. When the days shorten or lengthen sufficiently, it acts as a clear signal: "Hey, it's time to move!". Their hormones, especially melatonin and prolactin, activate in response to this signal, prompting the bird to accumulate fat in preparation for the journey. In terms of navigation, these little travelers use a kind of internal compass, sensitive to the Earth's magnetic field, as well as their sharp senses to locate the sun, stars, or familiar landscapes along their route. Certain specific areas of their brain, highly developed, allow them to integrate all this information to migrate effectively twice a year to favorable locations for food and reproduction.
Among birds, some long-distance travelers are particularly known for their biannual journeys. For example, the Arctic Tern holds almost all migratory records: it travels about 70,000 kilometers each year between its breeding sites in the Arctic and its wintering grounds in Antarctica. Less extreme but just as regular, the Barn Swallow spends the beautiful season in Europe to nest, then heads to Africa when winter arrives. The same adventure is true for the Yellow-billed Cuckoo in North America: this clever little bird leaves the United States and Canada each year to enjoy the warmer regions of Central and South America. These species follow available food resources and take advantage of the best possible conditions throughout the year.
Climate change significantly disrupts the migratory habits of birds. With warming, cold periods arrive later or are milder, causing some birds to shift or modify their biannual migration. As a result, many species are starting to move earlier or later than usual. This shift can be problematic: their natural calendar, synchronized over generations with food availability and breeding periods, becomes completely out of sync. In some birds, it is even observed that they shorten their journey or remain sedentary for part of the year because winter is becoming less harsh in certain regions. Paradoxically, other species must undertake even longer migrations to avoid weather conditions that are now too unstable. In short, all of this seriously disturbs the natural cycles established for centuries and forces birds to adapt quickly or risk their survival.
Birds use several tools to navigate during their migration, including the Earth's magnetic field, the position of the sun, and even scent landmarks in some cases.
Small migratory birds can increase their body mass by nearly 40% due to fat reserves accumulated before their migration period to have energy for the journey.
Some migratory birds can travel up to 70,000 kilometers per year, which is almost twice the circumference of the Earth, during their biannual migrations.
The black swift can stay in flight for up to 10 months without ever landing, eating, drinking, and even sleeping while in midair during its annual migrations.
Several signs include changes in feeding behavior, the formation of significant migratory groups, increased activity at dusk and dawn, as well as hormonal and physiological variations that prepare birds for long journeys.
Yes, human activity can significantly influence biannual migrations through habitat loss, light and noise pollution, as well as climate changes that alter the crucial natural conditions and resources for these migratory journeys.
They primarily react to environmental signals such as day length (photoperiod), temperature variations, and food abundance, triggering hormonal and physiological changes to prepare for their journeys.
You can help them by preserving natural habitats, planting vegetation that provides food and shelter, limiting your use of toxic pesticides, and reducing light pollution during migration periods.
The Arctic tern holds the record for the longest known biannual migration, traveling nearly 70,000 kilometers each year, connecting its breeding grounds in the Arctic to its wintering areas in Antarctica.
No, even though many migratory birds engage in a biannual migration (round trip each year), other species have irregular, partial, or even conditional migrations depending on their environmental and food needs.
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