Bird migration: Where do they go?

Posted on Sun August 23, 2020.

Birds have been making thousand-kilometre journeys for millenia but scientists are still puzzling over their unique ability to navigate the globe on a scale that is simply bewildering for such small creatures. Some species will even return to the exact same nest site year after year.

Besides their propensity for annual travel, there is very little that makes a migratory species different to their stay-at-home counterparts, yet migration has clearly become a very successful strategy for survival.
So, why do they do it? Guide Elly has the answers...

Why do birds migrate?

The unfortunate cost of a holiday adventure in South Africa is often the long-haul flight to get there. Yes, they're not always the most comfortable travel experiences; perhaps the food was tasteless, the in-flight entertainment was limited or you had the misfortune of being seated across from the wailing baby.

But for many of us, the lights go down, we pop on a sleeping mask and wake up some hours later, almost at our destination. All in all, the journey to South Africa is relatively hassle-free. "Another glass of wine, madam?"  - don't mind if I do!

Self-powered and weighing less than a grapefruit, the Amur falcon is just one species that takes on the marathon journey to South Africa every single year - a round trip of about 22,000km. Beginning in their breeding grounds in eastern Asia, the falcons continue en masse across the Indian Ocean and down the east coast of Africa.

If you are like me, right about now you are probably thinking "What is the point? Wouldn't they be better off staying put?" In many instances, yes, birds will spend their entire lives in the same region and less than 10% of South Africa's bird species actually opt to make some sort of annual migration.

If you live somewhere that has an abundant source of food year-round, there is very little reason to leave - rainforests, for example. On the other hand, if you live in a habitat which changes drastically between the seasons, it may be best to go somewhere else when food is scarce in winter.

This is usually why birds migrate: to follow the fruitful, summer season and take full advantage of the months in which food is easier to come by. Most species will also take this opportunity to breed, with the abundance of insect food often allowing them to produce more young on average than their stay-at-home relatives.

While many of us would be partial to the idea of living an eternal summer, migration to do so presents an almighty challenge...

Where do they go?

Different bird species will embark on migrations that vary greatly in length, but almost all will attempt to take the most direct - and often dangerous - route. This helps them to save energy but may also expose them to some treacherous conditions along the way, like storms or predators.

There are several broad patterns of migration in African birds, the most common being Palearctic and intra-African migration. The former are those who travel to Europe, Asia, northern Africa and the northern and central parts of the Arabian Peninsula, while the latter travel shorter distances throughout the African continent.

Some examples of Palearctic migrants that we sometimes see in the Klaserie are the White stork (pictured above; who breed in Europe and Asia) and Lesser kestrels (who breed in central Asia). Examples of intra-African migrants are the Diederick cuckoo (who travels to and from the Arabian Peninsula), the Woodland kingfisher (who winters as far north as Sudan) and the Wahlberg's eagle (pictured below; who also winters in Nigeria and Sudan). 

How do they find they way?

Interestingly, migratory behaviour appears to be passed down from parent to offspring, and is ingrained into the genetics of the species. Most newly fledged youngsters will accompany flocks of adults on their first migration and thereby learn where to go.

Others, however, depart later than their parents and are genetically "pre-programmed" to know which direction to fly, slowly expanding this area as they become more familiar. Yep, that's right: something inside these birds tells them on a certain date that they need to fly in a certain direction for a certain length of time. Amazing!

One of those 'somethings' is known as a 'circannual clock' which, combined with changes in day length and climate, triggers the birds to prepare for their perilous journey. The exact moment of departure is usually based on the size of the gathering flock, wind speed and the time of day. When the conditions are right, it's time to fly!

Rather than sticking to a fixed path, young birds will often make some exploratory detours along the way and are able to do so while sticking to their programmed general direction. Birds can orientate themselves (not the same as navigation) using several different compasses, including the sun, stars, and the Earth’s magnetic field. 

In doing so, these virgin migrants will arrive in the general vicinity of their non-breeding grounds and will then scratch around for a suitable place to spend the winter. On this maiden voyage, they will also memorize various spatial aspects of the route, inlcuding topographical, celestial, meteorological and magnetic cues that they perceived along the way.

In creating this complex "mental map" of waypoints on their journey, birds are able to travel thousands of kilometres to the exact nest site that they used the year prior. At Klaserie Sands, we enjoy the annual return of a mated pair of pale-morph Wahlberg's eagles who arrive in the summer months to breed.

How is climate change affecting bird migration?

As the climate changes, species' migratory patterns are continuously evolving and the main factor driving these changes is temperature. Given that most of these birds are migrating to breed and access plentiful food supplies, timing is absolutely critical - arriving too early or too late to their destinations could cause them to miss out on vital resources.

Many birds make stop-overs on their journeys in order to refuel, and will often time these pitstops with certain prey events. For example, the aforementioned Amur falcons make a stop in Nagaland to feast on trillions of termites as they seasonally erupt from their underground colonies. As climatic conditions change, these termites may begin emerging sooner than usual and missing this event means the falcons will not have enough fuel to make it safely across the Indian Ocean.

Migratory birds are critically important for ecosystems to function properly, eating insects and distributing seeds around the landscape. Despite their migrations starting earlier, many are struggling to keep pace with the rate of climate change. In tracking these birds, scientists hope to uncover more secrets about their remarkable resilience in the face of a changing climate.

 

Article written by Elly Gearing with photos courtesy of Barth Bailey, Santiago Lacarta, Nerise Bekker, Geoff Downs, Anca Muresan and Elly Gearing.