The knobthorns are in full bloom again, heralding in a new season of hope and expectancy. Among over seventy species of Acacia in South Africa the Acacia nigrescens, in the lowveld, probably gives as good a show as the yellow flowering species in other areas.
I have mentioned them previously but would like to draw attention to the variation in flower colour. Many of our trees have flowers that vary in colour, particularly the white ones with pink individuals and the blue or purple ones with white mutations.
The usual flower colour of the knobthorn is cream, with flowers opening from spikes of reddish buds. Certain trees have particularly 'lemon' coloured flowers and on closer inspection you will notice that these lighter coloured flowers are opening from green coloured buds.
The bud colour appears to vary from red, through a khaki colour to bright green. It is a genetic variation that makes what might seem monotonous colouration just that little bit more interesting.
Acacia blossom does not produce much nectar for birds but enough for numerous insects which conduct most of the crosspollination. Go-way birds and monkeys, among other species, feed on the buds and the attendant insects provide a good feeding ground for many insectivorous birds. When they have young, even seed-eaters and nectar feeding birds will feed their chicks on a high protein diet of insects.
I have previously mentioned some of the variety of nesting materials, including spider web, used by different species of birds. Nest building is controlled by the availability of these materials. The varieties of 'wild cotton' form an important source of nest material for many species.
The true wild cotton belongs to the genus Gossypium of which there are three species in South Africa - (G. herbaceum - which is a scrambling shrub. G. hirsutum - which produced useful cultivars, and Gossypioides kirkii with less woolly seeds). The genus is restricted to the north and east in South Africa and it was in this area and in central Africa where the first plants for cultivation were discovered.
Also restricted to the north and east of South Africa is a creeper - Ipomoea albivenia (climbing kapok) - which produces a slightly less woolly cotton. The white flowers are prominent when they climb over trees and shrubs. The cotton-covered seeds persist for many months and are probably helped in their dispersal by birds taking the cotton fluff for nest building.
Many species of birds line their nests with wild cotton - mousebirds, certain sunbirds, canaries and warblers. There are certain species that use wild cotton as the main nest construction material. The penduline tits weave an amazing little nest that looks like a little bag of white felt, while the Marico sunbirds use the cotton in a more coarse form to construct their white nests.
It is interesting that the tits and Marico sunbirds (Nectarinia mariquensis) - which are one of our rarer sunbird species - have almost the same distribution in the northern and eastern areas as the plants they use for nest building. The benefit of a prominent, white nest is not clear. Although they may reflect heat they appear to be an easy target for predators - despite a slight effort at camouflage by the sunbirds.
With such reliance on a particular nest material, we have a good example of the dependency of one species on another - and there are numerous others. We have been fortunate, up to now, not to have had many veld fires in the agricultural and game farm areas this year.
If there had been fires the wild cotton and other nest material would have been destroyed and certain bird species, like the Marico sunbirds, would not have been able to breed. Always bear in mind the likely ripple effect on other species before you carry out any action.
By Dave Rushworth