Today is World Migratory Bird Day and it makes you wonder why so many species of raptor migrate?
Migration is a costly business - flying hundreds or thousands of miles takes a lot of energy and many obstacles are encountered on the way. Raptors migrate because they need to in order to survive and the key factor is food availability. In the Northern hemisphere food becomes less abundant in winter. Rodents and reptiles go underground or hibernate and songbirds migrate south to secure their own food sources (insects and seeds). So, some raptors leave their breeding grounds and migrate to places where food sources are more easily found. For most Eurasian raptors, this means a trek to Africa which becomes quite a melting pot of raptor nationalities between October and March. These migrant raptors face some competition over food from resident raptors, but because the northern winter coincides with the southern summer it is still a time of plenty.
Did you know that until the 18th century people in the Northern Hemisphere believed birds hibernated too - it was the only logical explanation for the absence of birds in northern winter months. And although we now know birds migrate, there are still unanswered questions about the details of their actual migration.
The reason for returning to their breeding grounds in the Northern hemisphere is the same as for migrating south: food availability. When spring starts in the Northern hemisphere food becomes available again. Migratory raptors return north where it is less crowded and where daylight hours are longer. For diurnal raptors, longer daylight hours means there is more time each day to feed the chicks so they grow faster and stronger.
Interestingly, their diet may vary between breeding and non-breeding areas. The Endangered Steppe Eagle (Aquila nipalensis) for example feeds mainly on small mammals on its breeding grounds, with susliks (a short-tailed ground squirrel) forming the vast majority of its diet in some areas. When wintering it appears to feed mainly on mole rats in East Africa, and termites and Red-billed Quelea in Southern Africa.
There are two migration strategies among raptors based on their wing shape - flying a direct or indirect route.
Raptors with broad wings, like eagles and buzzards, rely on the lift of thermal air currents to glide and save energy. As there are no updrafts and thermals above large bodies of water during daytime, they avoid crossing seas and large lakes because they cannot maintain active flight for long distances. Consequently these “soaring” raptors follow a longer, more indirect route over land. They can cover distances up to 300 miles a day.
The Mediterranean and other seas present a major obstacle to soaring birds, which must cross at the narrowest points such as the Strait of Messina, Gibraltar, Falsterbo and the Bosphorus. These points form bottlenecks where many birds congregate. More common species, such as the European Honey Buzzard, can be counted in hundreds of thousands in autumn. This normally solitary raptor gathers in large numbers during migration at preferred crossing points and also roosts socially.
Soaring raptors lose only 10 to 20% of their body weight during migration which may explain why many of these species can fast for days while migrating.
This map is showing the migratory route of two Steppe Eagles that were GPS tagged in the Mara. They follow coastlines and cross seas at the narrowest points – such as the Suez Canal, and avoid large bodies of water like the Caspian Sea by flying around them.
Active flight route
Falcons, ospreys and harriers have more narrow-shaped wings and use more active, flapping flight while migrating. This costs a lot of energy, but has the advantage of allowing birds to take a more direct route across land and water. Ospreys (Pandion haliaetus) for example fly in a relatively straight line, crossing reasonably high mountains (e.g., the Pyrenees along France–Spain border) and also fly relatively long distances over the Mediterranean sea. To compensate for the more active flight these raptors need to forage more during migration than the soaring raptors.
All harriers readily migrate over expanses of water. They are generally solitary, but during migration they can gather in groups of hundreds at rich feeding sites. At these locations you can find Montague’s Harrier (Circus pygargus), Western Marsh Harrier (Circus aeruginosus) and the Near Threatened Pallid Harrier (Circus macrourus) roosting communally.
There are 8 major migratory flyways and two of them include Kenya; The Black Sea/Mediterranean Flyway and the East Asia/East Africa Flyway. Both include the 550km long coastline with its associated creeks, reefs and beaches, and the chain of lakes stretching along the Rift Valley from Turkana in the north to Magadi in the south.
Most raptors that migrate to (or through) Kenya use the Black Sea / Mediterranean route but not all. The East Asia/East Africa route is used by several species including:
The Vulnerable Sooty Falcon (Falco concolor) has its breeding grounds in Libya through Egypt, Israel, Jordan and Saudi Arabia to the coasts of the Red Sea and Persian Gulf, and South-west Pakistan. It winters mainly in Madagascar and migrates through coastal East Africa in small numbers.
The Amur Falcon (Falco amurensis) travels massive distances between East Asia and Southern Africa and typically crosses the Indian Ocean at high altitudes of > 1 km. They migrate in large groups of up to thousands, and initially the migration is slow-moving, presumably to build-up resources for the long trans-oceanic flight where there are no feeding opportunities.
The Osprey (Pandion haliaetus) and Peregrine Falcon (Falco peregrinus) are some of the most widely distributed raptors in the world and they use most major flyways to migrate from breeding areas to wintering areas. Research data shows that juvenile Ospreys stay longer in their wintering areas, skipping a full migration cycle, and this is why you may see them year round.
Partial or complete migration
Some species are complete migrants, like the Eurasian Hobby (Falco subbuteo). The whole population leaves Europe, mostly in August and September but some as late as October. They spend the winter in Africa south of the Sahara and return to Europe in April and May.
Within other species not all populations may be migratory; this is known as "partial migration". The Indian population of the Lesser Spotted Eagle (Clanga pomarina) for example are resident while all other populations are migratory. This resident population is often split and considered a separate species called the Indian Spotted Eagle (Clanga hastata).
Another example is the Eastern Imperial eagle (Aquila heliaca) of which many (especially) adult birds spend the winter in their breeding areas in Hungary, Slovakia and Austria. Other populations start their southward migration between September and November, preferring African wetlands and Middle Eastern rubbish dumps as wintering areas, and return North between February and May.
The Endangered Saker Falcon (Falco cherrug) can survive during snow and cold temperatures when there is enough prey, e.g. in eastern Austria. Birds in southern parts of the species range are often resident all year round or partial migrants while birds breeding in the northern part (where it is much colder) are often migratory. Like the Eastern Imperial Eagle, it's mainly the adult territorial birds that are less likely to migrate when there is enough food.
When raptors migrate
Most migratory raptors leave the Northern hemisphere from August and they start to return from February but the months vary between raptors based on diet, food availability and size. The European Honey Buzzard (Pernis apivorus) for example won’t return to Europe before mid-April or May because its main prey, larvae of wasps, isn't available earlier in the year.
For some raptors it takes weeks to arrive at their wintering grounds. Not so for the Lesser Kestrel (Falco naumanni). In both directions their migration is rapid and will typically take just four days. Kestrels use active flight and take a direct route to their destination to achieve this remarkably fast migration time.
When raptors fly
Most diurnal raptors migrate during the day because that is what they are built for. On average soaring raptors start flying later in the morning because they have to wait for thermals and updraft to develop.
An exception is the Levant Sparrowhawk (Accipiter brevipes) that frequently migrates at night using a flapping flight. Small accipiters and falcons get little advantage from thermals, and so can fly overnight, or early morning to cover more distance quickly.
Most Short-toed Snake eagle (Circaetus gallicus) migrants winter in tropical North Africa, with some eastern birds moving to the Indian Subcontinent. The Vulnerable Beaudouin's Snake-eagle (Circaetus beaudouini) is a rare but increasing vagrant to Kenya. If you find one of these intra-African migrants in Kenya consider yourself very lucky. Read more about their status in Kenya here.
The Vulnerable Greater Spotted Eagle (Clanga clanga) migrates less far south than the Lesser Spotted Eagle with birds leaving their breeding grounds in October and November to winter in southern Europe, southern Asia and north-east Africa. Passage and non-breeding wintering birds occur in small numbers over a vast area including Kenya. There is a Great Spotted Eagle named "Blinky" (as it has one eye) that has wintered in Nairobi National Park for going on 8 years now.
The Near Threatened Pallid Harrier (Circus macrourus) is another migrant to Kenya. When wintering in Africa, they sometimes make nomadic movements in search of local concentrations of insects, especially locust swarms. This species must have had a great time last year!
Migratory raptors face a lot of human-induced threats such as habitat loss and degradation, illegal shooting and poisoning, collisions with aerial structures and windmills and electrocution by power lines.
Climate change is another threat. Migratory species require suitable conditions on their breeding grounds, in their non-breeding range, and along migratory routes between the two. Rising temperatures and changing climate patterns impacts prey base and habitats. This will no doubt have an impact on raptors and maybe more so on migratory species.
Kenya Bird of Prey Trust
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