Following their noses to Lake Victoria – seagulls use smells to navigate
To travel from one continent to another, birds must navigate and, first and foremost, be able to answer the question: “Where am I?” They then have to orientate themselves: “I will now fly in a certain direction.” It is possible that they use different senses to fulfil these two tasks. To find their direction, many species use a magnetic compass which they gauge using the angle of the Sun at sunset, as the Sun always sets in the west. Whether the Earth’s magnetic field also helps them with the accurate identification of their location is still a matter of debate.
In addition, young birds of many species follow older animals on their first migration and note landmarks along the way including coasts, mountains and rivers. They then use them to find their direction on subsequent journeys. Indications that birds also use their sense of smell to determine their location have been increasing for some years. The study by an international research team shows for the first time that some birds follow their noses along their entire migratory journey to the south.
Seagulls are not necessarily known as a migratory bird species. However, many seagull species cover very long distances, as much as 7,500 kilometres in the case of lesser black-backed gulls (Larus fuscus fuscus). In late summer and autumn, lesser black-backed gulls from Russia and Finland fly along a narrow corridor over the western Black Sea and Nile Delta on the Mediterranean Sea to Lake Victoria in East Africa. They spend the winter there and then return to the north to breed.
Using GPS satellite transmitters fitted on the animals’ backs, the researchers tracked the journeys of almost 120 lesser black-backed gulls from Finland and the Solovetsky Islands, which are part of Russia. The solar-operated transmitters, which weighed 30 grams, sent position data with an accuracy of 30 metres to a research satellite up to six times per day.
The researchers wanted to find out whether lesser black-backed gulls would find their way to Lake Victoria if deprived of their sense of smell and magnetic cues. For this reason, they sectioned the olfactory nerves of one group of the birds under anaesthetic and sectioned the trigeminal nerve in another group as this nerve is thought to be necessary for the mediation of information about the Earth’s magnetic field. The nerves grow together again after a few months; thus the animals’ navigation capacity is not permanently impaired. They are always able to make their way back to their breeding grounds in the following years.
The scientists transported the birds that were caught in Finland via aeroplane 1,250 kilometres south-west to the island of Helgoland and released them there. The birds from the Solovetsky Islands started their journey from the city of Kasan on the Volga, which is 1,260 kilometres to the south-east of the islands. “Our assumption was that the two starting points were outside the gulls’ normal migratory corridor, so the birds would be forced to take an unknown route,” says Martin Wikelski, Director at the Max Planck Institute of Ornithology and head of the study. This enabled the researchers to establish which senses are used by the birds for navigation.
The migratory data show that the lesser black-backed gulls can also find their way to Lake Victoria from Helgoland. However, they need their sense of smell to do this: the birds with the sectioned olfactory nerve ended their journey too far west in Central Africa. “Without the help of their noses, they obviously did not realize that they had started further west than usual. While the intact birds were able to compensate this misalignment by directing their journey eastwards for a time, the birds with no olfactory sense were unable to do this,” explains Wikelski. In contrast, the lesser black-backed gulls did not use information about the Earth’s magnetic field for navigation: the birds with sectioned trigeminal nerves were just as successful at reaching their destination as their intact counterparts.
The analysis of the birds’ migratory route revealed that when their olfactory sense was in working order, the gulls constantly moved their route in an easterly direction and got a ‘noseful’ of the locations along their migratory route. They therefore did not generally use prominent landmarks, for example the African Mediterranean coast, to adjust their direction. The researchers do not yet know which smells the birds followed. One thing is certain: “The smells must be transmitted at least 300 kilometres in the air. Individual olfactory cues along the route, for example the Black Sea and Nile Delta, probably provide ‘smellposts’ for the general migratory direction,” explains Wikelski.
To the astonishment of the scientists, all of the birds released in Kasan found the way to their destination – even those deprived of their sense of smell. Unlike the birds that started their journey in Helgoland, the birds with sectioned olfactory nerves altered their direction. “Our data show that Kasan does not lie outside the natural migratory corridor of the lesser black-backed gulls as we thought, but within it. So the birds apparently knew the route and took direction from familiar points in the landscape without relying on their sense of smell,” says Wikelski.
Thus, lesser black-backed gulls need their noses to navigate, and magnetic cues and information from other members of their species are of little or no importance to them. “The fact that the gulls rely on their sense of smell was a huge surprise for us. Based on theoretical navigation models, it had been assumed that the navigation skills of many migratory birds and marine mammals can only be explained by smell. However, this theory was highly disputed until recently,” says Wikelski. The role played by the Earth’s magnetic field and other navigation systems like the stars in the navigation of avian migration remains largely unclear as no experiments have been done following birds during their entire natural migration.