The second Saturday in October is observed as World Migratory Bird Day, an event dedicated to one of the most outstanding annual phenomena in the animal kingdom: bird migration. Although many animal species migrate long distances every year, the case of birds has always drawn a lot of attention, due to the mysteries and myths surrounding these journeys. Aristotle, for example, thought that swallows spent winter hibernating in the sand.
It wasn’t until the 18th century when Buffon began to understand the nature of migrations; he assumed that birds travelled to warmer regions in search of food. During the following decades, a series of questions arose from the initial research and studies: what triggers migration? How do birds manage to navigate? In this article, we’re going to answer these (and other) questions and we’ll learn about some of the most fascinating migratory birds.
Why do birds migrate?
Migrations are annual large-scale movements between a breeding (and reproduction) area and a wintering area. The ‘migratory route’ is the journey from one area to the other. Birds stay in the breeding areas during the warmer months (spring and summer) and move to wintering areas before the cold arrives – this journey is called ‘postnuptial movement’. The movement back to the breeding regions once winter is over, on the other hand, is referred to as ‘prenuptial migration’. Interestingly enough, the prenuptial migration (the way back) is done in less time than the postnuptial one.
But what triggers bird migration? Signs suggest that it’s both the variation in day length and the availability of food. This explains why the migrations occur in autumn and spring, when the daylight hours and resources available vary. There are other factors that can push forward (or delay) these journeys, such as weather conditions or competition between individuals. For example, a severe winter with little food availability can accelerate the departure of birds to their wintering areas.
The weather and the availability of resources can also play a key role in other species that are not considered migratory. Extreme environmental conditions can cause the movement of species in search of territories with more benign weather conditions. These migrations are referred to as ‘irruptions’, as they’re considered exceptional. An example is the case of the snowy owl, which can move from one region to another when there are food shortages.
Migration strategies and how birds prepare for their trips
Birds can present a variety of migration strategies, which will depend on different factors, such as the ecology of the species, its migratory route or the environmental conditions. While there are species in which all individuals migrate, there are others in which only some populations move. There are also species that are nomadic and spend short periods of time in each region, as well as accidental species (stray birds). The migrations of the different species can happen in stages (stopping in resting areas) or without stopovers.
In any case, regardless of the type of migration, there is one thing that all birds do before starting a long trip: they accumulate energy reserves in the form of fats and sugars that allow them to go without food for long periods of time. Some small birds, for example, can double their weight before starting their migration.
Orientation is a sequence of motor actions ordered in time and space in a specific manner, always under the animal’s control. During migratory journeys, birds use a number of navigation mechanisms that allow them to keep on course.
One of the main ones is visual navigation, which is based on external references: mountain ranges, coasts, rivers, swamps, etc. In this sense, the position of the sun is particularly important. Although its position changes throughout the day, birds can compensate for this movement and maintain the right course, thanks to the existence of a biological clock that allows them to perceive the circadian rhythms and know the diurnal cycle. Many birds can continue their journeys during the night, as they understand the rotation of the sky.
Birds are also able to hear low-frequency sounds below 0.06 Hz (human beings can’t perceive frequencies below 10 Hz). This skill allows birds to hear, from hundreds of kilometres away, the waves on the coast, the wind in the mountains or the desert sands moving, as well as to anticipate storms or earthquakes. Some birds have apparently altered their migratory routes in response to the low-frequency sounds produced by airplanes.
Birds find their way to their destination thanks to their ability to sense the Earth’s magnetic field. Its lines create an angle with the surface of the Earth, except at the equator. The angle and field intensity is sensed by birds, thanks to a type of protein located in their eyes called ‘cryptochromes’. Therefore, their eyes work like a magnetic compass that guides them during their journeys.
Outstanding migratory birds
The bird that travels the most kilometres during its migrations is the Arctic tern. This small bird of only 100 grams travels for several months from its Arctic breeding grounds to the Antarctic (and back) – this adds up to over 70,000 kilometres per year, the longest migration in the animal kingdom!
The bird that flies the fastest during its migratory journeys is the great snipe. In 2011, a group of scientists tagged several great snipes with geolocators and discovered that this wader can travel the more than 6,000 kilometres that separate the north of Europe from sub-Saharan Africa in just 48 hours, at an average speed of almost 100 km/h.
The bird that flies the highest during its migrations is the bar-headed goose, which has to cross the Himalayas during its journey from Mongolia to India. Some individuals have been recorded above 7,000 metres, just by flapping their two wings. In order to make such an incredible journey, bar-headed geese have extraordinary physiological adaptations.
Translated by Carlos Heras