So why do birds migrate?
Approximately 1 800 of the world’s 10 000 bird species are long-distance migrants. This amazing phenomenon is not fully understood and many reasons are given. A simple explanation is moving towards a place with more food and a safe place to breed. However, sometimes these journeys are not termed “true migration” because they are irregular (nomadism, invasions, irruptions) or in only one direction (dispersal, movement of young away from natal area). Migration occurs annually and in a specific season, while birds that are non-migratory are said to be resident or sedentary. So let’s try and understand why they fly….
After making the most of the food supplies in one area while raising their brood, migrants escape the cold weather and food shortages of the approaching winter months to spend the non-breeding season in another summer destination. The major drawback in undertaking these journeys are the huge distances between the destinations, and the high risks involved in undertaking such a journey twice a year.
For many species, the costs greatly outweigh the benefits and migration is not viable. For others, making the journey, albeit dangerous, significantly improves their chances of survival, and most importantly, allows them to raise more offspring than if they stayed in one place.
Whether a species is migratory or not is based largely on what it eats. Migration is usually a characteristic of aerial insect-eating birds, as the availability of insects varies greatly between seasons. The most common migrants are aerial feeders such as nightjars, swifts, bee-eaters and swallows, all of which feed in the open sky. These birds generally migrate in the autumn months, when prey numbers begin to drop. Water birds are also seasonal migrants, as their prey base is also dependent upon the wet and warm summer months. Having said this, not all insectivorous birds need to migrate.
Terrestrial insect feeders that forage in thickets or amongst leaf litter on the ground, such as babblers, thrushes and hoopoes, do not need to migrate, as the environment in which they forage is often sheltered and more protected from harsh weather conditions than the open sky, providing food all year round.
Generally speaking, frugivores (fruit-eating birds) such as mousebirds, turacos and barbets are resident in one area as their food supply is available in one form or another all year round, and they only have to move short distances to find new food sources. Granivores (seed-eating) such as finches and sandgrouse are more nomadic rather than migratory. They do not follow annual set routes from breeding to non-breeding areas and back. They tend to have more random, unpredictable movements that are not strictly classed as migration. To put it simply, nomadic birds follow food supplies closely, and will often impulsively move into areas that provide such food resources, usually in response to rainfall. Another movement pattern often followed by seed-eaters such as Red-billed Quelea is known as an irruption. This is when the species follows a ‘wandering’ lifestyle and arrives in new areas en masse, usually in response to drought or overcrowding in their area of origin, or abundant food supplies at their destination.
Another example of the impact that diet has on migration can be seen in kingfishers. The Malachite Kingfisher feeds on fish and is a resident species, whereas the Pygmy Kingfisher is an insect eater, and thus it is a migrant bird.
While some bird species are international travellers, migrating vast distances between continents every year, others are more local travellers and remain within the African continent. Based on this, we can loosely classify the Southern African migrants into three main categories, namely:
Palaearctic-African Migrants: birds that undertake long journeys, which are often between continents. Most of the time, these species arrive in Africa, not to breed but to enjoy the abundant food sources and are termed as non-breeding migrants. Examples are the Barn Swallow, European Roller, Steppe Eagle and Steppe Buzzard.
Intra-African Migrants: birds that move annually between their breeding and non-breeding grounds within the African continent. Many of these migrants breed here, such as the African Reed-Warbler and many of the cuckoo species. However, some species breed north of the equator and then migrate south during the non-breeding season, such as the Abdim’s Stork. Other species even breed in both northern and southern Africa, spending the non-breeding season in the tropics, such as the Diderick Cuckoo.
Local Migrants: birds that do not undertake long journeys, but rather seasonal trips over short distances. A prime example is the Mangrove Kingfisher, which breeds inland, and then migrates back to the coastal mangroves at the end of the breeding season. Finally, altitudinal migration can also take place. This is when a bird takes advantage of the resources and breeds at a higher altitude during the warm summer months before moving to lower altitude areas for the cooler winter months. An example of this is the African Stonechat.