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The Knysna Warbler Bradypterus sylvaticus is classified as regionally Vulnerable due to its small, severely fragmented range and small population (c. 2 500 mature individuals). In addition, all sub-populations contain less than 1 000 mature individuals and there is a perceived continuing decline in population size, range size, and area, extent and quality of habitat.
The Knysna Warbler is endemic to the region and has a highly restricted and fragmented distribution, being found in four zones in the littoral of Eastern and Western Cape provinces. The northernmost zone, which once extended to the vicinity of Durban (Berruti et al. 1993), now covers the stretch of coastal vegetation between Mbombazi Nature Reserve, south of Margate in KwaZulu-Natal (Wragg 2003, Smith 2005), to Dwesa-Cwebe Nature Reserve in Eastern Cape. A vagrant recently recorded in Pietermaritzburg may be attributable to this population (subspecies pondoensis).
The next sub-population occurs between Tsitsikamma and Sedgefield (Berruti 1997), with a third sub-population persisting on the southern slopes of the Langeberg Mountains, near Swellendam (Berruti 2000). A fourth sub-population occurs on the eastern slopes of Table Mountain on the Cape Peninsula (Pryke et al. 2011). It is not known if there is movement between these sub-populations and ringing records are inconclusive (n = 14, retraps = 2).
The EoO estimates, provided by SABAP2, indicate that there has been a slight increase (15.9%), which is due to a single record in southern KwaZulu-Natal. There has been a 12.9% decrease in AoO. Conclusions drawn from atlas data may be biased owing to a lack of coverage in remote areas between the KwaZulu-Natal South Coast and East London in the former Transkei, and should be treated with caution. The cryptic nature of the Knysna Warbler means that it is likely to be under-recorded and it may be more abundant than atlas data suggest.
The Knysna Warbler is extremely secretive and its presence is normally revealed only during the breeding season when it sings. Contact calls, uttered by both sexes, are diagnostic but indistinctive. It is certain that this, coupled with the inaccessibility of most of its habitat, has led to it being under-recorded in both atlas projects (DG Allan pers comm.).
The south-western sub-population, due to its proximity to Cape Town, has received the most attention from researchers. In 1977, Pringle (1977) estimated that there were less than100 pairs on the eastern slopes of the Cape Peninsula. This number may have been further reduced by the removal of thickets of alien Rubus brambles, notably in the Kirstenbosch Botanical Gardens. More recently, this sub-population has been estimated to be in the region of 25-30 pairs (PAR Hockey unpubl. data), indicating a substantial decline. Hockey et al. (1989) suggested that there were less than 200 pairs in the former south-western Cape, which would include the two southernmost sub-populations. Further east, Berruti et al. (1993) make mention that the species is rare in Eastern Cape, but do not provide an estimate. They did, however, estimate that the total population size was of the order of 10 000s, which is far lower than the 100 000s suggested by Siegfried (1992). The figure provided by Berruti et al. (1993) in 1993 was revised in 2000 to c. 2 500 individuals (Berruti 2000). BirdLife International (2014) provided an estimate of 2 500 birds. The variance in population estimates can be attributed to the cryptic nature of this species, which has made population estimates particularly problematic and does not necessarily indicate rapid fluctuations or population declines. The current population estimate is less than 2 500 mature individuals. The confidence in population estimates is low, given the lack of information on the species.
There is no current estimate of population trends. Pryke et al. (2011) made mention that, over the past 20 years, the Knysna Warbler has declined precipitously on the Cape Peninsula, but do not provide an estimate of sub-population size. The clearance of habitat for developments, agriculture and silviculture in the four zones in which the species occurs will have led to decreases in the amount of available habitat, as well as its quality, resulting in a decrease in the population. This decrease is expected to continue given the development pressure throughout the species' range. The rate of decline is unknown.
Berruti (1997) attributed the contraction in range of the northern sub-population to habitat loss brought about by clearance of coastal forests and development on the coastline of southern KwaZulu-Natal. Habitat loss would also seem to be the main cause for decline in the western sub-population, with much of the species' original habitat on the Cape Peninsula having been lost to agriculture, urbanisation and exotic tree plantations (Pryke and Samways 2008).
Interestingly Visser and Hockey (2002) found that this species fares better in transformed urban landscapes than in adjacent protected areas, but this may have been due to unsuitable management practices within the protected area.
Wildfires on the Cape Peninsula are of concern because these may burn substantial patches of habitat. Although Fynbos is fire-adapted, Knysna Warbler habitat in Western Cape is often adjacent to large stands of introduced pine trees, where fires are more intense and its effects more devastating (Berruti 2000). Given the close proximity of the key remaining sites holding this species on the eastern slopes of Table Mountain, this sub-population could be driven to extinction by a single massive fire event (Berruti 2000).
Little is known about the dispersal ability of this species, but continued fragmentation and isolation of sub-populations could lead to inbreeding depression.
Despite this species being listed as regionally as well as globally Vulnerable, there are currently no conservation measures underway. The Percy FitzPatrick Institute of African Ornithology has been responsible for research on the western sub-population.
Conservation of this species depends on the maintenance of its prime habitat within Afro-temperate forests (Berruti 2000). In much of the bird's range on the Cape Peninsula, adjacent Fynbos areas have been protected from fire since the 1970s (Forsyth and Van Wilgen 2008) despite the fact that the natural fire regime for the area is 12-15 year burns. Fynbos protected from fire becomes invaded by, and eventually replaced by forest. As such forest spreads laterally from riverine ravines into adjacent Fynbos, and lateral light is reduced, making the understorey vegetation more sparse (Pryke et al. 2011). The warbler, whose breeding requirements include a dense understorey, has been impacted upon by this fire regime. In addition, manual removal of the non-flammable forest component of invaded Fynbos vegetation may be a prerequisite (Pryke et al. 2011). The species' dependence on dense riparian vegetation indicates that clearing of undergrowth, including alien species, would be detrimental. Eggs and chicks are subject to intense depredation, much of which is probably caused by rodents (Visser and Hockey 2002). Management practices that impact negatively on populations of predators of rodents may have knock-on effects on the productivity of warblers.
* Regional population estimates of the three eastern sub-populations need to be obtained, and surveys of the seldom-visited isolated forest patches in Eastern Cape are particularly important.
* An investigation into whether the montane, riverine forest habitat preferred by the species can be returned to its natural state through controlled burning is needed.