Bird migration is one of the enduring mysteries of ornithology. Although the complexities are still not completely understood a lot more is known than in the past. Migration can be defined as predictable journeys usually from one breeding area to a non-breeding area and back again as well as from extravagant summer banquets to another.

Over 130 bird species in Southern Africa are migrants either undertaking epic intercontinental journeys or moving within the African continent itself. The timing of migration seems to be controlled primarily by changes in day length and food shortage. Migrating birds navigate using celestial cues from the sun and stars, the earth’s magnetic field, and mental maps.

Southern Africa migrants are classified into 3 main categories…

1.    1. Palaearctic – African migrants: Have the longest journeys flying thousands of kilometres from Europe and Asia all the way to Africa and back again. Primarily non – breeding migrants arrive in Southern Africa at the onset of summer after having bred further north. Palaearctic species that migrate to southern Africa include numerous waders, the Steppe Eagle, Steppe Buzzard, White Stork, European Roller, Barn Swallow and Several Terns.

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      2. African Wanderers:  Intra – African migrants do not venture abroad like the Palaearctic – African Migrants but move annually between breeding and non-breeding grounds within the African continent. Examples of such are Wahlbergs Eagle, African Reed Warbler, Greater Striped Swallow and many Cuckoos. Red Chested and Diederik Cuckoos breed in both northern and southern Africa.

3.    3. Domestic travellers: Local migrants do not take long epic journeys but rather travel locally over shorter distances within one area eg the Mangrove Kingfisher.

Vagrants when battling extreme weather conditions, migratory birds may sometimes be blown off course or lose their sense of direction and arrive at completely unintended destinations. Examples of vagrant birds turning up in South Africa include the Little Blue Heron, Greater Sheathbill, Eleonora falcon, American Purple Gallinule, Hudsonian Godwit, Buff- breasted Sandpiper and White – Throated Bee-eater. Vagrants tends to be juveniles, inexperienced birds at greater risk of getting lost on migration, than the older, more experienced individuals.

Orientation and navigation.

Migratory species will plot their plans so each species follows the most efficient route possible. Just as we follow concrete highways that link destinations, so too do migratory birds follow apparently distinct aerial flyways that cross and link continents. These flyway routes are the most efficient paths for birds to follow, taking into account the distance, geographical barriers and landmarks, favourable winds, stop overs and their particular flying style – be it soaring or flapping.

Some take advantage of updrafts and other wind patterns or avoid geographical barriers such as large stretches of open water.

The routes taken on forward and return migration are often different. Birds may use specific stop over sights where they rest and refuel in places where there is reliable food before their next stage of their strenuous stage of their journey.

Many, if not most, birds migrate in flocks. For larger birds, flying in flocks reduces the energy cost. Geese in a V-formation may conserve 12–20% of the energy they would need to fly alone. (See my earlier article wild facts www.wildlandsafrica.co.za)

Seabirds fly low over water but gain altitude when crossing land and the reverse pattern is seen in land birds.

Navigation is based on a variety of senses.

Birds have a highly complex internal navigation system – both physical cues, such as land marks,  the position of the sun and the stars and an innate (inborn; natural) internal compass ability to detect the earth’s magnetic fields.

Some birds are able to detect ultra – violet light which passes through the clouds and can help to track the position of the sun on cloudy days.  Even many resident birds use the sun to orientate themselves in their daily activities.

Not only has navigation been shown to be based on a combination of other abilities including the ability to detect magnetic fields (magneto reception), but  visual landmarks as well as olfactory cues (An olfactory cue is a chemical signal received by the olfactory system that represents an incoming signal received through the nose. This allows humans and animals to smell the chemical signal given off by a physical object)

The ability of birds to navigate during migrations cannot be fully explained, however the ability to successfully perform long-distance migrations can probably only be fully explained with an accounting for the cognitive ability of the birds to recognize habitats and form mental maps. 

Migratory birds may use two electromagnetic tools to find their destinations. One that is entirely innate and another that relies on experience. A young bird on its first migration flies in the correct direction according to the Earth’s magnetic field, but does not know how far the journey will be. With experience it learns various landmarks and this “mapping” is done by magnetites in the trigeminal system, which tell the bird how strong the field is. Because birds migrate between northern and southern regions, the magnetic field strengths at different latitudes.

In polygynous species with considerable sexual dimorphism, males tend to return earlier to the breeding sites than their females. This is termed protandry.

The magnetic compass. 

The earth acts like an enormous magnet, producing a magnetic field that varies in angle over its entire surface, producing more magnetic force at the poles than the equator. Birds are highly sensitive to these fields and able to align themselves accordingly on migration. There is a neural connection between the eye and “Cluster N”, the part of the forebrain that is active during migrational orientation, suggesting that birds may actually be able to see the magnetic field of the earth.

The star compass.

Many birds that travel at night are known to rely heavily on stars to guide them on their journey. Experiments on captive migratory birds within a planetarium have shown that they make use of the star compass orientating themselves accordingly as the artificial star pattern if formed.

On cloudy nights when the stars are not visible, birds are able to orientate themselves according to the earth’s magnetic fields.

Inherited or learned?

An intriguing question is how birds migrating for the first time know where to go. Is this Learned or inherited?

Many observations and experiments have shown that both the inclination to migrate as well as the route to follow on migration, is largely inherited.  This is supported that by the fact that young birds leaving on migration either before or after their parents still manage to find their way on their first epic journey.

Migratory cuckoos whose chicks are raised in a family of non – migratory birds (brood parasites) demonstrate that migratory instincts are inherited.

Migratory birds in captivity show migratory restlessness where they hopped and fluttered on their perches facing the exact direction in which they would be flying on migration.

Diurnal migration in large birds using thermals.

Some large broad-winged birds rely on thermal columns of rising hot air to enable them to soar. These include many birds of prey such as vultures, eagles, and buzzards, but also storks. These birds migrate in the daytime. Migratory species in these groups have great difficulty crossing large bodies of water, since thermals only form over land, and these birds cannot maintain active flight for long distances. Mediterranean and other seas present a major obstacle to soaring birds, which must cross at the narrowest points.

Funnel – shaped migration results in where thin land corridors linking different continents may be characterised by funnel –shaped concentration of thousands of migratory birds all avoiding a sea crossing. Classic examples of this phenomenon occur over Israel and Gibraltar as millions of birds migrate. More common species, such as the European honey buzzard , can be counted in hundreds of thousands in autumn. Other barriers, such as mountain ranges, can also cause funnelling, particularly of large diurnal migrants.

Looping and leapfrogging

Rather than migrating to and from their breeding grounds along the same route, certain birds follow a circular migratory route known as loop migration. The Red – Backed Shrike migrates in a giant loop from Europe to central and Southern Africa and back again in anti-clockwise directions.

Leapfrog migration describes certain bird populations that breed furthest north then fly furthest south in the non-breeding season, when leapfrog over other populations of the same species which breed at intermediate latitudes. Reasons include food competition, escaping predators and climatic differences. Examples are the common ringed plover.

Record Holders

The Artic Terns are known to migrate up to 50 000 kms from their breeding grounds in the northern Arctic tundra to the Antarctic pack ice and back again each year. The fact they are known to live up to 25 years it is possible that they may fly up to one million kms plus in their lifetime also experiencing the most daylight hours than any other bird.

Most small migratory birds fly less than 2000m above sea level, Waders between 2500 and 3000m above sea level. Raptors are known to migrate at an altitude of 4000 – 6800m.

The Ruppells Vulture holds the record for the highest flying bird. One was recorded as being hit by a passenger jet at 11 300m.

Fuel for flight

Many migratory birds have remarkable endurance, regularly making extremely long non – stop flights without landing to feed or rest. These birds draw on large fat reserves to fuel fat reserves to fuel their long and punishing journeys. They will indulge in eating binges consuming enormous quantities of food before leaving. Fat is a very efficient fuel for such journeys.

Migrant birds are also adapted to coping with low levels of oxygen when flying at high altitudes.  

Wings also differ with migratory vs. non – migratory birds such as the Carmine Bee –eater wings are longer and narrower than the more rounded wings of the resident White – Fronted Bee – eater. 

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Nocturnal migratory behaviour

For many birds flying at night is an energy – saver as the air is cooler, denser, and calmer allowing for more efficient flight and less likelihood of headwinds or to be blown off course. Also reduces water loss. Birds also use stars for navigation.

While participating in nocturnal migration, many birds give ‘Nocturnal Flight Calls’, which are short, contact-type calls. These calls likely serve to maintain the composition of a migrating flock, and can sometimes encode the gender of a migrating individual.  They also likely serve to avoid collision in the air.

Nocturnal migrants land in the morning and may feed for a few days before resuming their migration. These birds are referred to as passage migrants in the regions where they occur for a short duration between the origin and destination.

Nocturnal migrants minimize depredation, avoid overheating, and can feed during the day. One cost of nocturnal migration is the loss of sleep. Migrants may be able to alter their quality of sleep to compensate for the loss.

Threats to habitat

Sadly migration carries a high costs in predation and mortality, including hunting by humans.  The distances involved in bird migration mean that they often cross political boundaries of countries where conservation measures require international cooperation. Several international treaties have been signed to protect migratory species.  The only way to conserve migrants is to protect their existing breeding and non – breeding grounds,  stop over sites and expand the existing global network of protected areas.

Education in conservation worldwide starting at a very young age would no doubt make a significant difference to our amazing animal planet. 

Shane Bennitt: Wildlands Guide

Ref: Birds – The Inside Story – Rael and Helene Loon

Wikipedia

Photos: Ian Sinclair and Peter Ryan 

For more wild facts refer to www.wildlandsafrica.co.za

Wahlberg’s Eagle : Red – Chested Cuckoo : Greater Striped Swallow : Mangrove Kingfisher