Migratory birds

Figure 1- Arctic tern

From stomping zebras in Africa to clouds of birds crossing the oceans, animal migrations are one of the most widely observed phenomena in biology. The search for food, water, and favourable breeding grounds are the most common causes of these massive movements.

Nowadays, these behaviours are considered normal to us, as we can see or hear about them in various natural documentaries, however, it wasn't always as this. In the past, we were not capable of tracking animals like nowadays. For example, when avian populations suddenly vanished from their usual habitat and appeared months later, people didn't know how that occurred, so they created many hypotheses in order to explain this phenomenon, from hibernating in hidden spots to metamorphosing into other forms. It was a white stork that helped us to put an end to the mystery. In 1882, one individual of this species appeared in Germany with a spear through his neck. The spear was made of African wood, giving zoologists critical insight into the bird's origin, and thus putting us on the right way to the answers.

Nowadays it is easier to track animals, with birds, for example, we can use unique metal and coloured rings or even attach GPS to them. These technological advances allowed us to begin to understand the amazing and notorious journeys that some birds can accomplish.

Usually, we don't acknowledge what some birds are capable of, especially when talking about migrations. Many species perform annual large-scale migrations, breeding in one location and spending the winter in another. The Western Alaskan bar-tailed godwit (Limosa lapponica baueri) for example, in order to reach its favourable breeding habitat in Alaska, flies more than 15000 km, of which 10000 are usually non-stop! Then they return to New Zealand, travelling a little under 30000 km overall on their migration, most of it travelling over oceans. These distances can be hard to understand, so to simplify, these godwits fly roughly three-quarters of the circumference of our planet.

However, despite the godwit migration being remarkable, there is a bird that annually flies the distance of approximately two circumferences of the Earth! -The Arctic tern (Sterna paradise) (Fig. 1)- is a relatively small bird (90-120 g), that can fly over 80000 km in one year throughout its migration. After breeding in Arctic Greenland and Arctic Iceland, these birds fly all the way to the Atlantic sector of the Southern Ocean to spend the winter, flying back to the north for the new breeding season. These migrations are not in a straight line, individuals tend to make stops in different feeding areas and even can perform these travels with different routes (Fig. 2).

Figure 2- Geolocation tracks of Arctic terns during their migration from North to South (Green), wintering period (red) and return to breeding colonies in the North (yellow). We can see two main southbound migration routes, (A) West African, and (B) Brazilian coast. Dotted paths indicate locations during the equinoxes. Extracted from (Egevang et al., 2010)

These are just some examples of what birds can do, they are fantastic animals and true flying machines.

References

• Battley, P. F. et al. (2012) ‘Contrasting extreme long-distance migration patterns in bar-tailed godwits Limosa lapponica’, Journal of Avian Biology, 43(1), pp. 21–32. doi: 10.1111/j.1600-048X.2011.05473.x.

• E.J. Milner-Gulland, John M. Fryxell, and A. R. E. S. (2011) Animal Migration: A Synthesis. New York: Oxford University Press. doi: 10.1093/acprof:oso/9780199568994.001.0001.

• Egevang, C. et al. (2010) ‘Tracking of Arctic terns Sterna paradisaea reveals longest animal migration’, Proceedings of the National Academy of Sciences of the United States of America, 107(5), pp. 2078–2081. doi: 10.1073/pnas.0909493107.

• Richter, S. and Bick, A. (2020) ‘Zoological Collection Chapter 49 Chapter 49 ROSTOCK : The Zoological Collection of the University of Rostock’, (January). doi: 10.1007/978-3-319-44321-8.

• Information from the All About Birds website, https://www.allaboutbirds.org/guide/Arctic_Tern/id, © Cornell Lab of Ornithology. Accessed in 27/03/2022

Eduardo Alves

Biologist