How migratory birds find their way? Secrets of migration without GPS

On May 9, the world celebrates World Migratory Bird Day. It is a good opportunity to reflect on the phenomenon of aerial migrations. How is it possible that entire flocks travel thousands of kilometers, beginning their journey at the right time and almost always reaching their destination? Observation skills? Hormones? Or perhaps a sixth sense? What makes migratory birds set off on a journey, and a carefully planned one at that?

When, where, which way – innate mechanisms showing migratory birds the time and route

The secret behind the precision of bird migrations lies in their genes. Martin Wikelski, director of the Max Planck Institute for Ornithology, explains that migrations already appeared 50,000 years ago. Migratory birds evolved internal mechanisms that allow them to use, among other things, the Earth’s magnetic field for navigation. Thanks to these mechanisms, they can also detect subtle changes that are almost imperceptible to humans: pressure, sound, light, and smell.

Migratory species have developed a remarkable sense of observation. However, they do not choose the right direction solely on the basis of what they see in the landscape, such as mountain ranges or rivers. As early as the 1960s, two scientists, John Emlen and Stephen Emlen, conducted an interesting experiment proving that stars serve as signposts for birds. They placed selected species in boxes with stars painted in ink and in a planetarium displaying a starry night sky. It turned out that when birds could see the night sky, they were able to orient themselves toward the north.

Migratory species are highly skilled at detecting signals indicating when it is time to begin their journey. The most important of these signals is the shortening or lengthening of the day. Greater or lesser intensity of sunlight causes birds to produce hormones that change their behavior and trigger preparations for departure.

One might assume that these abilities are learned from bird parents. In reality, however, this is not the case. The cuckoo provides a perfect example, as it is often raised by unrelated, non-migratory foster parents. Despite having no contact with members of its own species, this bird instinctively knows how, when, and where to undertake its seasonal migrations.

Bird records and curiosities

There are more birds undertaking seasonal migrations than one might think. According to some sources, nearly half of the world’s 10,000 bird species migrate.

The record holders in terms of migration distance are Arctic terns (Sterna paradisaea). These small birds living in Arctic regions migrate seasonally to Antarctica. Their journey can last several months, the route averages 70,000 km, and in some cases exceeds 90,000 km.

Studies of migratory birds, such as the ICARUS Project, also make it possible to identify other avian record holders. The great snipe (Gallinago media) can reach speeds of up to 100 km/h during migration.

Small birds also migrate, such as the calliope hummingbird (Selasphorus calliope), which measures only 7 cm, and even… flightless birds. The latter group includes Adélie penguins (Pygoscelis adeliae), which travel around 12,000 km on foot every year.

Seasonal migrations involve significant weight fluctuations for birds. The garden warbler (Sylvia borin), a small Eurasian migratory bird, can increase its body mass up to threefold before departure. Later, it loses this “extra weight” at a rapid pace: 3–3.5 g for every 1,000 km traveled.

And the phenomenon of aerial migration does not end there. Birds are also capable of temporarily changing the size of their internal organs. Before departure, their hearts enlarge and their muscles grow, while the stomach, intestines, and liver shrink. The organs return to their normal size during stopovers. In this way, migratory birds smoothly switch between feeding and migration modes, and it is impossible not to admire their extraordinary achievements.