South African aloes were introduced in Gibraltar in the 19th Century. They did well in the Mediterranean climate, similar to that in their home, and spread. They fared best on warm limestone slopes.
The mild, frost-free, winter climate of Gibraltar has allowed the aloes to flower in December and January, the corresponding South African summer.
the slopes take on a spectacular colour as the stands of aloe come into full bloom
These aloes are pollinated in their native South Africa by sunbirds, which have coevolved with the plants and have developed specialised bills for extracting the nectar. The red of the aloes attracts the birds and pollen is transferred by the birds who get their nectar reward. There are no sunbirds in Gibraltar but some species have learnt, in the short time since the 19th Century, that the red aloes mean food.
The main species visiting the aloes is the Chiffchaff, a winter visitor. It seems that the nectar is used as supplement to their usual diet of insects.
Chiffchaffs (above and below) drinking nectar
The Sardinian Warbler, a local resident, is another regular visitor to the aloe stands (above and below)
Sardinian Warblers (male above and female below) are larger than Chiffchaffs and will supplant them from the flowers
but the Chiffchaffs have a trick - they hover and collect nectar from the flower heads that are inaccessible to the heavier Sardinians
So the sunny slopes of Gibraltar in late December and January are a little laboratory with a natural experiment in action. Here we observe a behavioural change in the behaviour of bird populations, both migratory and resident. And, while for a long time the aloes seemed unable to cross pollinate and only spread vegetatively, there is clear evidence now that plants are getting fertilised. The Chiffchaffs are doing the trick, it seems!
Butterfly on aloe - with the warm weather (temperatures around 20C in the middle of the day) many insects are active around the flowers too, providing additional temptation to the insectivorous warblers