Some birds will and some birds won’t…

The southern tip

Here in South Africa we see the end result of the mass movement of a variety of birds, large and small, aquatic and terrestrial. We are at the end of the line for virtually all these species, because beyond our shores there is only the Southern Ocean and ultimately Antarctica and only a very few species such as the Arctic tern keep heading south.

The often wild Southern Ocean

A Northern Giant Petrel cruises the Southern Ocean with ease and although not a migrant will range over vast distances and even circumnavigate the pole at these high latitudes.

We are at the end of the migration routes whether these migrants are coming from Eurasia or central Africa. As such we do not see massed flocks passing particular landmarks such as occurs at Falsterbo in southern Sweden or in Costa Rica and Panama in Central America where many migrants are funnelled into the relatively narrow strip of land between South and North America. Some fly for eleven days without pause while others will wander through a range of habitats making journeys of 12 000 km and even roundtrips like that of the Arctic Tern from pole to pole, the longest migration of any animal. Migrations can be short, simply up and down a mountain for altitudinal migrants or longer across oceans and hemispheres.

A Cape Rock Thrush sometimes seen in the upper Highway area during winter. An altitudinal migrant which will come down from the sub-alpine habitats to lower levels during winter.

Our migrants celebrated! 

This is not to say that the arrival of migrants at the beginning of our summer goes unnoticed – the sighting of the first Yellow-Billed Kite (our YBKs) from central Africa is always avidly awaited and reported. Other species are less obvious but announce themselves by their calls – the cuckoos being particularly characteristic. Early summer (November) is also a particularly good time to keep a lookout for migratory waders, those long-legged wanderers of our beaches and estuaries, the reason to look early being that one may catch a glimpse of the remaining breeding colours of these frequently confusing species when wearing their drab non-breeding plumage.

A Yellow-billed Kite – one of our much loved summer visitors

Migration is therefore something that has been observed for millennia as reflected in the observations of the presence or absence of particular species in a particular area. Without any knowledge at the time of where these species went to, a variety of legends and stories have arisen dating back to Aristotle who thought some species hibernated while others were transformed from one species to another, for instance birds into frogs. The fact that we now know that many small species migrate at night and would not have been detectible would have added to the riddle.

A Diederik Cuckoo a common summer intra-African migrant to gardens in the Upper Highway area with their distinctive call announcing their presence on their return from their wintering ground.

Why migrate?

Approximately half of the world’s 10,000 species migrate. This includes a wide range of species from songbirds to seabirds and waterfowl to raptors. A plethora of questions is generated by the phenomenon of migration, possibly the first being “Why migrate?”. The simplest answer is that this is a particular survival strategy which works for some species. Different strokes for different folks. One could argue however that all species have their own survival strategies otherwise individuals would not survive at all and the species would go extinct. It is clear that long migration involves risks from weather conditions and predators en route but at the same time long distance migrants include some of the most abundant species on the planet so the strategy obviously works for them.

Birds that migrate take on considerable effort and risk. They burn large amounts of energy and expose themselves to new predators, diseases, and hazards while passing through unfamiliar territory. In the modern world, there are also human-induced dangers that have been added to the mix that birds were not designed to cope with. One interesting perspective comes from James Currie who intimates that perhaps our perspective is wrong. He describes that birds that fly south or north for the winter and leave their breeding grounds are not going on a winter vacation to warmer climes but rather that their breeding territory is a nice place to visit and raise offspring, but not one they would necessarily like to stay in during the harsh winter. Staying in these areas means that many insect-eating species such as warblers and flycatchers would miss out on the feeding bonanza of feeding opportunities available further north or south.

Lesser Striped-swallow. An intra-African breeding migrant seen here making the most of the insect resources of the summer in the south.

One of the theories to speak to the origins of migration relate to the melting of the ice sheets during the last Ice Age. Tropical birds are thought to have ventured further north as the ice retreated following longer days and a seasonal increase of insects and other food. Over time these distances became longer until they covered hundreds or thousands of miles. Some stayed where they were benefitting from less competition for the same food source and fewer predators. The benefits for the migrants include those already mentioned, the longer days at higher latitudes and untapped food resources with less competition. Whatever gets them started, once migrants are on their way, many follow predetermined routes that their ancestors have travelled for thousands of years.  Long-distance migration involves complex routes and patterns. Immature birds of some species like the Western Osprey will migrate slightly later than the mature birds and follow different routes which may involve detours which help discover new territories. Other birds migrate in family groups so that older birds help the younger ones avoid potentially fatal mistakes.

Amur Falcon migrate in large flocks from the north-east Asia region

Long distance migration demands some remarkable physiological features. There are constraints on flying machines, in this instance birds, which do not affect mammals to the same degree. Flying machines need to be as light as possible in order to maximise the quantity and optimal use of the fuel capacity if a long flight distance is involved. The reproductive organs expand or shrink depending on the season, fat is accumulated as a fuel supply and can constitute up to one third of the body weight. There can also be enlarging of the flight muscles. In a sense the migrating bird does not get “tired”, it will keep flying until its fuel/fat deposit runs out, hopefully this is after it reaches its destination!

The distances covered and time spent during migration can be quite amazing. The record for long distance, non-stop flight is held by a Bar-tailed Godwit which flew from Alaska to New Zealand, a distance of 12,854 km in 11 days. Interested readers can work out the average speed per hour. A similar distance of about 13,000 km is covered by willow warblers flying from north-east Russia to East Africa while some individuals carry on to South Africa as well. Another thing to be aware of is that not all individuals within a population migrate. Some will stay on the summer grounds all year while others travel, migrating to other areas only to return again in summer. One of the local species that is an example of this is the African Paradise Flycatcher which is often present year round at coastal locations but will disappear from other areas as day-length cues the approaching winter.

An African Paradise-flycatcher. Some individuals migrate while some stay as residents.

When and how?

Having established that these migrants, although not all, can cover remarkable distances the next questions that come up include – what tells them it is time to move? how do they do it? How do they find their way? How do they know when they have got there? A point that emerges when studies or research are carried out to provide answers to these questions is that it is relatively easy to establish what environmental stimuli birds are responding to but much more difficult to establish just how they use these cues.

The first question is an easier one as the standard stimulus here is a change in day length, a much more reliable cue than a few unseasonal cooler or rainier days. This might not work so well around the equator where there is little change in day/night hours but changes become much more obvious at higher latitudes where more migrants occur. Migration occurs, depending on the species, by day and night. Larger, soaring species such as storks, which use thermals, will fly during the day when rising air currents are available. Their migration routes therefore tend to be over land where thermals occur.

Daytime migration also allows the use of landmarks while various experiments have shown that birds can also use the sun for navigation and direction finding. If however the sun is going to be used for navigation this requires a timing device or mechanism to allow for the change in the position of the sun during the day.

White Stork, the well known baby delivery mechanism that breeds on chimneys in Europe and then feasts in Africa for winter.

Where the situation becomes more difficult to visualise or to understand is in relation to small migrant species which generally migrate at night at altitudes which make them difficult to see although not necessarily impossible to hear. What sort of altitudes are we talking about? Bar-headed Goose is a species which is known to fly over the Himalayas which immediately gives one some idea. But what about the rest? Use of radar has shown that waders and small songbirds regularly fly at 1000 to 2000 metres and sometimes up to 6000 metres or more. Significant amounts of energy are required to reach these altitudes but once up there in the much thinner air there is the benefit of less air resistance – the same reason airliners fly so high. The thinner air which we coastal dwellers find a bit of a problem if we try energetic activities up on the Reef does not seem to present any problems to birds. Experiments using mice which had weights similar to sparrows showed that the mice could hardly crawl at air pressures equivalent to 6000 metres while the sparrows flew quite happily. One might think that low temperatures at such high altitudes would or could be a problem. In reality the flight muscles generate so much heat that the low temperatures become a benefit in terms of offloading heat.

Night migration helps to avoid diurnal raptors but obviously again poses the question – how do they know where they are going? Numerous experiments have shown that birds – along with a surprising variety of other animals – have a magnetic sense based often on minute deposits of magnetite which are affected by the earth’s magnetic field just as a compass responds. A compass can also be used as an indicator of latitude, i.e. the distance north or south of the equator. This arises because of the orientation of the earth’s magnetic field which dips towards the poles resulting in the compass needle lying horizontally at the equator but at an increasing angle as one moves north or south. So, if you, i.e. the bird, has the necessary sensory structures and is able to integrate the incoming stimuli you can find out where you are.

Sometimes it’s in the stars

Just to add to the story there have also been experiments where birds have been held in planetaria where star and constellation patterns can be manipulated. The birds’ responses indicated that star patterns were also significant in navigation. Clearly being able to fly at altitudes high enough to be above the clouds would be significant for star-based navigation on its own.

A star filled sky to guide you there and back again.

So – where does that leave us? The plethora of scientific papers and books dealing with bird migration are testimony to the interest and drive of researchers and ornithologists to resolve and understand these almost unbelievable feats that migrant birds accomplish. How close we are to a real understanding is moot. We are compelled to remember that our awareness of the environment out there is constrained by our relatively limited senses. Trying to establish what the world looks like to a different species is not easy. Our eyesight and colour vision are not bad, our sense of smell is pretty poor, and our hearing is limited to relatively low frequencies. When it comes to infra-red and ultraviolet radiation we don’t feature and trying to fathom how exactly the magnetite response to a magnetic field translates into a response that says “fly that way” is still a mind bogglingly difficult problem. Maybe we should just marvel, enjoy it all and the next time you see a tiny ringed plover or equally small barn swallow think a little more with a little more wonder about just how they got here.

A young Barn Swallow newly arrived from the northern hemisphere breeding grounds to experience his first African summer