Chameleon Eyes

Winged Globetrotters

Dr. med. vet. Martina Schybli Swiss Ornithological Institute Sempach, 2020 Translated by Meret Kaufmann

Why birds migrate

Bird migration is a unique natural phenomenon recurring year after year. Billions of birds leave their breeding grounds every autumn and fly to faraway wintering grounds, only to eventually take up the long journey again in reverse in springtime. The reason for this mass migration is not so much the cold winter temperatures, but rather the concomitant shortage of food. Insects and other invertebrates are in short supply at this time, while ground frost and snow further impede the search for other food sources such as seeds. Besides the lack of food itself, other factors influence the migration of specific bird species, such as frozen lakes and rivers, or insufficient hours of daylight to hunt for food.

Various migration strategies

For many birds, the migration between breeding and wintering grounds represents a strategy for survival. A number of different migration strategies have emerged. Some birds, known as short-distance migrants, spend the winter in Europe or the North African Mediterranean area, thereby covering a comparatively short distance of roughly up to 2000 km. Among them are many water birds and several songbird species. Long-distance migrants, on the other hand, usually cover distances of over 4000 km, crossing the Sahara desert. Many insectivores, such as the common and alpine swift, the barn swallow and the red-backed shrike, are long-distance migrants. As the latter are more restricted in their nutritional base than short-distance migrants during winter, those with a longer journey not only take off earlier but also return later in the year.

Not all bird species migrate, however. Some species remain in their breeding grounds throughout the year; these are called resident or non-migratory birds. Resident birds are less restricted in their winter eating habits than pure insectivores, and have developed other strategies to cope with the environmental conditions in winter. Most European species of wildfowl, woodpeckers and owls are non-migratory birds.

Partial migrants are a special case. As their name suggests, one part of the population migrates to wintering grounds, while the other keeps to their breeding grounds all year round. Many birds in urban areas are partial migrants; well-known examples are blackbird, robin and chaffinch. Most other birds in urban areas are short-distance migrants or resident birds.

During daytime, the heat-sensitive cameras in the Chameleon Eyes artwork in Heiligfeld Park are most likely to capture typical city birds, such as crows, tits and finches; as well as the common and alpine swift, who are typical to this area. In contrast to most birds in urban areas, swifts are long-distance migrants. But you never know: especially in spring and autumn time, a great number of other species can be seen passing through at night. The thermal imaging cameras may not able to precisely determine the bird species; yet it remains very exciting to be able to witness the sheer extent of this mass migration of birds happening right above our heads, usually without us noticing.

When and why birds migrate

Some species, including birds of prey, migrate during the day. However, the majority of migratory birds migrate by night. The position of the sun, the moon and stars as well as the Earth’s magnetic field set the approximate direction for migration. Finally, landscape structures, i.e. hills and mountain ridges, bodies of water and coasts, allow for more precise orientation. For many bird species, the migration route is genetically anchored; this is why young birds migrating for the first time will find their way on their own. But there are also some species where adult birds guide their young to show them the way. Geese, for example, are well known for this.

Migration is an exhausting time for birds and demands maximum performance. Therefore, migratory birds need to put on fat before departure and rely on places to roost during migration. Safe and quiet habitats offering food, shelter and rest are therefore essential. On their journey, birds do not want to waste any energy. Some species, including cranes and geese, fly in formation to minimise air drag, for example.

Large birds of prey and storks in turn use upwind, known as thermals, to cover great distances while gliding. If they need to cross the sea, they choose routes that involve as little flying over water as possible, since there are no thermals there. Nor does the sea offer any possibility for rest. For this reason, even smaller birds—that cannot use thermals, but only flutter—try to keep flight over water to a minimum. Most European migratory birds traverse the Straits of Gibraltar in the West or the Bosporus in the East; fewer fly over Italy, Corsica and Malta. Migration is a risky undertaking, not only due to natural phenomena like storms, extended rainy seasons or droughts but also due to human activities, such as bird hunting, which is still very common in the Mediterranean.

Bird migration and climate change

Bird migration is, above all, an adaptation to seasonal shifts in food supplies worldwide. These food supplies are currently changing rapidly due to climate change. Surveys already show that birds are trying to adapt their migration strategies to the altered circumstances caused by climate change. Many Nordic water birds, which previously flew from Northern Europe and Siberia to spend the winter in Switzerland, now find sufficient foods in ice-free lakes and rivers even in winter and therefore fewer and fewer come to Switzerland during the cold season. This is true of the tufted duck, for example.

In the ever warmer winters with less snow, short-distance migrants now also find enough food in Central Europe. They no longer fly so far and therefore depart later and return sooner, if they have not given up migrating entirely. A typical example is the red kite.

Long-distance migrants that spend the winter in tropical Africa have been particularly negatively affected by climate change. They usually only spend a relatively short part of the year in one area and depend on encountering optimal conditions in their respective habitats. In addition to the destruction of wetlands and deforestation, climate change also poses substantial problems to long-distance migrants: irregular rainfall and droughts in Africa lead to food no longer being available as regularly as in the past. In Switzerland, many long-distance migrants are consequently suffering from declining populations, while resident birds and short-distance migrants are able to confidently hold their ground.

Researching bird migration in Sempach

Bird migration is a fascinating area of research in ornithology, which still holds many questions. For example, the exact location of winter quarters or how environmental conditions impact the birds’ point of departure are still unknown for many species. The Swiss Ornithological Institute in Sempach pursues questions such as these. In recent years, research using geolocators has been particularly successful. Geolocators are small and very light measuring instruments, strapped to birds like a rucksack. The sensors measure atmospheric pressure, acceleration, temperature, the Earth’s magnetic field and light intensity. This data allows conclusions to be drawn regarding the wintering grounds, as well as the altitude and behaviour of birds during migration and in their winter quarters. For example, the Swiss Ornithological Institute was able to discover that nightingales from the Basel area tend to spend the winter on the Ivory Coast and Ghana. In the case of bee-eaters from eastern Germany equipped with geolocators, it has been shown that these birds migrate to Angola while travelling in stable social groups.Radar systems, including weather radars, also offer interesting approaches.

Radar systems capture migrating birds as biomass in airspace, thus enabling a large-scale visualisation of bird migration. This technology aims to make predictions regarding the temporal and spatial characteristics of bird migration, as well as revealing general behavioural patterns in migratory birds.Further information