Ecological notes

Nature Pics

Habitat description

The section of the Blyde River between the Blydepoort Dam and its confluence with the Olifants River some 50km downstream supports a healthy riverine forest with over 110 tree species and about 250 bird species. Because the Blyde River is perennial, its banks close to the waterís edge are graced with a gallery of evergreen trees like matumi (Breonadia salicina), waterberry (Syzigium cordatum) and quinine tree (Rauvolfia caffra). These trees, having well developed anchor root systems, are supremely adapted to withstand high water velocities during floods. They form a dense and mostly closed canopy allowing little sunlight penetration to the ground, which in turn, is mostly covered with leaf litter but also with some creepers like river climbing thorn (Acacia schweinfurthi) and shrubs like water elder (Nuxia oppositifolia). The well-developed and largely unspoilt gallery forms an effective baffle that protects the floodplain vegetation that is set back from the waters edge from high water velocities during the regular, seasonal floods. In some places where floods have felled the odd tree at the waterís edge, sunlight penetration allows stands of reeds (Phragmites mauritanius) to flourish. On the alluvial floodplain itself, set back from the waters edge, one finds an open, decidious and semi-decidious canopy of trees, typically river bush willow (Combretum erythrophyllum) sycamore fig (Ficus sycamorus) and brack thorn (Acacia robusta), interspersed by shrubs. Where boulders and bedrock dominate the floodplain, one finds a fourth vegetation type consisting only of shrubs, typified by potato bush (Phyllanthus reticulatus). In most places along the river, the floodplain is bounded by a natural levee of deep alluvial soil supporting the fifth vegetation type that consists of large, semi-decidious trees like jackallberry (Diospyros mespiliformis) and large, evergreen trees like natal mahogany (Trichilia emetica) often with a closed canopy and an understory of shrubs and creepers. Set back even further from the river one finds the sixth and deciduous vegetation type which is the transition zone between the riverine and terrestrial habitats, typified by tamboti (Spirostachys africana). If one adds the six distinct riverine vegetation types to the adjacent two terrestrial vegetation types (depending on whether the substrate is alluvial deposits originating from the Blyde Canyon or parent material that is mostly archaic granite), one can count eight vegetation types or -communities in an area as small as the 50ha that Otters Den comprises. Small wonder then, that we find more tree species (111) at Otters Den than in the Tsitsikamma Forest, and more bird species (226) than in, say, Golden Gate National Park.

ďSpecialĒ bird sightings

1. Thick-billed Cuckoo

Though they might have been overlooked earlier, Thickbilled Cuckoos have been seen in display flights and giving their characteristic calls during the periods December to February since 2002. Their hosts, Redbilled Helmetshrikes, are sometimes seen at Otters Den so it is possible that Thick-billed Cuckoos breed here. All sightings were in deciduous woodland.

2. Pelís Fishing Owl

Once sighted during daylight in 1996, it was thought that this bird was only passing through. At the time of the sighting it was being mobbed by variety of birds including Blackeyed Bulbul and Puffback. More recently, in August 2005, however, guests at Otters Den, Christine and Johannes Maree,  reported seeing a Pelís Fishing Owl flushed from dense evergreen trees along the river. They even provided two photographs. There is some evidence that it is resident, since catfish heads and what could be Fishing Owl droppings are often seen under a small, dead tree at the edge of a pool in the river. The local fish eagles usually perch in evergreen trees or over the water so it is possible that the droppings and fish heads are from a Pelís Fishing Owl. No nest has yet been found and no subsequent sightings recorded.

3. Squaretailed Drongo

Though Forktailed Drongoes are commonly seen in the deciduous woodland, they donít venture into the evergreen forest at Otters Den. The Squaretailed Drongoes are occasionally seen in the evergreen and semi-deciduous riverine forest at Otters Den. Though their tails also appear slightly notched, they can be distinguished from the Forktailed Drongo by their more energetic calls and their habit of foraging in the mid-stratum of the forest. Perhaps overlooked earlier, Squaretailed Drongoes have been seen at Otters Den since 2002.

4. Yellow-rumped Tinkerbird

Somewhat outside of their recorded distributional range, a population of Yellow-rumped Tinkerbirds, has established in the Blyde River forest. This event seems to have occurred in 2001. It is unlikely that they had been overlooked here prior to 2001, since their call is distinctive and had never been heard here before that date. Since then they have been heard throughout all summer days and also quite often during winter. Though difficult to spot, the patient observer armed with good binoculars can locate a calling bird on top of the forest canopy.

5. Red-fronted Tinkerbird

Somewhat further removed from its recorded distributional range is the Red-fronted Tinkerbird that is distinguished by its faster series of notes that the Yellow-fronted Tinkerbird and by its golden wing patch. Red-fronted Tinkerbirds were heard and seen in the summer of 2002/2003 and again in the summer of 2003/2004.

6. Southern Banded Snake Eagle

An observation was made of an over-flying Southern Banded Snake Eagle in 1995. Since the bird was not recognized at the time of observation, reference books were immediately consulted. Its identification was based on the fine, white barring of its lower breast and the two, distinctive, white bars on its tail. An independent observer, Chris Steyn, also saw and over-flying Southern Banded Snake Eagle a little earlier the same day. The bird seen at Otters Den approached from the Northwest, the direction in which Chris Steyn had made his observation. No subsequent sightings have been recorded it remains a singular observation of a passing bird.

 

Conclusion

All six the birds reported on here were sighted at the limit of- or outside their reported distributional ranges. The habitat along the lower Blyde River is, however, congruent with the respective habitat requirements of all six species. The lower Blyde River is somewhat unique in South Africa and, perhaps, it is only lack of observational effort that has excluded it from the reported distributional ranges of many forest-dwelling bird species.

Wynand Uys

November 2005