Migratory birds are one of the greatest wonders of the natural world. They have inspired awe and wonder in human cultures globally for millennia. And for a long time, they were a big mystery. Some Europeans and Americans used to think that birds hibernate underwater when they disappear every winter. Others speculated that they transform into different species. Some thought that Barnacle Geese Branta leucopsis grow on trees. One Havard scientist even insisted that birds fly to the moon!
Ornithology has come a long way since then and we now know a good deal about bird migration. When it comes to Africa’s birds, we know a lot about those that move between Eurasia and Africa (the Palearctic migrants). Major routes, wintering grounds and timing of movements have been documented quite well.
We still know comparatively little about the birds that migrate within Africa; the intra-African migrants (but we definitely know they don’t go to the moon!)
Why do birds migrate?
Migration is driven by seasonal changes in the environment. Migratory birds move back and forth every year between ‘breeding grounds’ and ‘wintering grounds’. The breeding grounds are very productive, but only at a certain time of year. Wintering grounds are places where they can survive when life gets hard in their breeding grounds. These movements are quite predictable and happen around the same time annually.
Birds that breed in areas where conditions are quite stable year-round are generally sedentary as they have no need to migrate. In these areas, there are no extreme changes in food availability, temperature and other environmental factors that would otherwise cause them to migrate.
The more extreme the seasonal changes, the greater the need to migrate. This is why almost all birds that breed in the arctic are migrants while many that breed along the equator do not migrate.
The African context
Bird migrations within Africa are on a less dramatic scale than those epic inter-continental journeys that the Palearctic birds undertake. Palearctic migrants have to cross vast tracts of inhospitable land or sea, like the Sahara and Arabian deserts, the Mediterranean Sea and the Indian Ocean. In addition to this already risky task, they also have to avoid getting trapped, shot and eaten by people along the way, especially in the Mediterranean region.
Intra-African migrants do not face quite the same problems, although they have their own set of challenges to overcome.
Palearctic migration is mainly driven by by changes in temperature and day length. In Africa, these two things do not change very much throughout the year. Intra-African migrations are instead driven mainly by the seasonal switch between wet and dry periods. This is caused by the annual movements of the Inter-tropical Convergence Zone (ITCZ).
The ITCZ is a low-pressure atmospheric belt that oscillates annually between the Tropic of Cancer (northern tropic) and Tropic of Capricon (southern tropic), and is the major driver of rainfall patterns in sub-Saharan Africa. Basically, the rain follows the ITCZ. In July it is in the north and in January it’s in the south. For this reason, East Africa experiences two rainy seasons (the ITCZ passes over us twice), while most of the rest of Africa only has one rainy season. You could say that East Africa has two summers!
Different types of migration in Africa
The interesting thing about Africa’s migratory birds is that most of them are only ‘partial migrants’. Some populations migrate while others don’t. The general trend is for the populations breeding along the equator to be sedentary while those in the northern and southern tropics are migratory.
Examples of this are birds like the White-crested Helmet-shrike Prionops plumatus, African Pygmy Kingfisher Ispidina pictus, Diederik Cuckoo Chrysococcyx caprius and Black Kite Milvus migrans (this one actually has both Palearctic and African races, which some people think should be split into different species).
Several African migratory birds are trans-equatorial migrants, meaning they cross the equator on their migrations to and from breeding grounds. Wahlberg’s Eagle is a good example, breeding in the southern hemispher and wintering in the north. Abdim’s Stork Ciconia abdimii does the same thing as the eagle but in the opposite direction, breeding in the north instead. In this way they virtually live in summer throughout the year, breeding in the summer of one hemisphere and ‘wintering’ in the summer of the opposite hemisphere. This is the same thing the Palearctic migrants do but on a smaller scale.
Some migrants move much shorter distances, breeding at high altitudes and then moving lower during the non-breeding season. These are called altitudinal migrants. The Golden-winged Sunbird Drepanorhynchus reichenowi and Lesser Striped Swallow Cecropis abyssinica are some examples of this.
Apart from the several species of migrants and partial migrants in Africa, there are also birds that are simply nomadic. This means their movements are not predictable or regular. They move randomly in response to environmental conditions like food availability and rainfall. Birds in this category are generally those that depend on food that is erratic in nature, hard to predict and prone to “boom and bust”.
Examples are birds like the Black-shouldered Kite Elanus caeruleus and Secretarybird Sagittarius serpentarius (which feed on rodents and large insects like locusts). These birds may be more sedentary in some places than others, depending on the local conditions.
Bird migration within Africa is fascinating and is something that we still have a lot to learn about. With time, projects like the African Bird Atlas (Kenya Bird Map in Kenya) will help to shed more light on the movement patterns of our birds and improve our understanding of this amazing phenomenon.
The 9th of May 2020 is World Migratory Bird Day, and it also happens to be eBird’s Global Big Day. Share your sightings of migrants and other birds through eBird and see how many species you and your fellow birders can record in your country in 24 hours! This challenge is perfect for social distancing as the more spread out you can be, the better. For those in Kenya, also make sure to submit your records to the Kenya Bird Map project.
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