Most bird migration is a long-distance business. The idea is to breed in long summer days at middle latitudes, and to move to the equator or other hemisphere for the winter. But in KZN we see a very special short-distance migration where birds breed at high altitudes, moving downhill for the winter.
This is altitudinal migration. In most parts of the world it would make little sense. For example the temperature on equatorial mountains is much the same year-round. So birds just stay there. In cold regions it is still too cold at sea level for insect-eaters to stay the winter. And in flat country there is nowhere to go either up or down. But in KZN we have the unique combination of high mountains only a day’s flying time from a sub-tropical coast. So in spring many bird species start shuffling uphill, looking for a territory. The higher the climb, the better the chance of finding unclaimed breeding space.
The Cavern is located in the middle of this system. This means that in winter, and only then, we see birds that breed on top of the Berg. The Fairy Flycatcher is the best example. The Rock Martin is also much more likely to be seen in winter. Other Berg-top breeders do not descent as far as The Cavern but can be seen on walks to the Sugar Loaf. Examples are the Sentinel Rock-thrush, Yellow Canary and Orange-breasted Rockjumper.
In summer dozens of migrants arrive, most from the Tropics or Eurasia, but a few from lower altitude. Most obvious is the Black Sawwing Swallow, two pairs endlessly circling the garden. Cape Wagtails become abundant, whereas in winter there are only one or two pairs.
Some apparently resident species are actually altitudinal migrants – the birds we see in summer are not the same individuals present in winter. The Chorister robin provides interesting proof of this through its great ability to mimic other birds. One bird living near May’s Falls has the Crowned Eagle call in its repertoire. But the Crowned Eagle has never been seen here, and indeed only occurs at lower altitude. The only explanation is that the robin learned this call while on its winter holiday.