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Detecting change in the status and habitat of Hinde's Babbler Turdoides hindei: 2000 to 2011

Ibis2013Vol. 155(2), pp. 428–429

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Phil Shaw, Peter Njoroge, Vincent O. Otieno, Edson Mlamba

Abstract

The globally Vulnerable Kenyan endemic Hinde's Babbler Turdoides hindei has its stronghold in the foothills of Mt Kenya and the Aberdares, where it is associated mainly with patches of scrub and riverine thicket (Stattersfield et al. 1998, Njoroge & Bennun 2000, BirdLife International 2012). A decline in its known range, first described in the 1970s, was attributed to scrub clearance for cultivation, perhaps compounded in some areas by human disturbance and hunting (Plumb 1979, Njoroge et al. 1998). Surveys in 1994 and 2000–2001 showed that the species was much less abundant within protected areas than in fertile, densely populated farmland, offering limited scope for conservation through further site designation (Shaw & Musina 2003). The global range of Hinde's Babbler lies predominantly within the catchments of the upper Tana River, with outlying populations in Meru District, Machakos, Kitui and Nziu (Shaw et al. 2003). By 2001 the species was known from just seven main areas, and was thought to have a global population of c. 1500–5600 birds. Although scrub clearance has impacted on the species’ range and population, its effect may have been partly mitigated by the spread of the exotic invasive Lantana camara, which provides scrub cover in areas previously cleared of native shrubs, enabling the Babbler to persist in, and possibly to re-colonize, intensively farmed land (Njoroge & Bennun 2000). Like most members of its genus, Hinde's Babbler breeds cooperatively, groups typically comprising three to four adults (range 1–8), often accompanied by one or two juveniles (Njoroge & Mutinda 1996, Shaw & Musina 2003). Some groups include individuals with extensive light feathering on the chest, head and back, sometimes distributed asymmetrically, a trait interpreted by Plumb (1979) as possible evidence of inbreeding. In 2011 we reassessed the global range of Hinde's Babbler and resurveyed three sites previously surveyed in 2000–2001, to determine whether changes had occurred in its known range, abundance, demography or habitat over the intervening decade. To determine the relationship between plumage type, population density and breeding success we assigned a plumage score to all individuals seen sufficiently clearly, noting their age class and group composition. Changes in Hinde's Babbler's global range were estimated from the number of occupied 10 × 10-km squares (its Area of Occupancy) and from the number of squares bounded by these known sites (its Extent of Occurrence). Three time periods were compared: 1900–1970, 1971–1990 and 1991–2011. Since its discovery in 1900, Hinde's Babbler has been recorded in 53 squares, indicating an Area of Occupancy of c. 5300 km2, its Extent of Occurrence spanning c. 22 700 km2. While the species’ Area of Occupancy in each period had increased progressively, its Extent of Occurrence contracted between 1900–1970 and 1971–1990, and then expanded during 1991–2011, as a result of new discoveries on the periphery of its range. Notwithstanding these apparent extensions, several aspects of the species’ status give cause for concern. During 1991–2011, Hinde's Babblers were recorded from 31 squares (59% of its historical range), but was unrecorded in 14 squares from which it had been reported in 1971–1990, mainly within intensively cultivated areas near to the core of its range. In July 2011 we re-surveyed 74 km of watercourse transects in Machakos, Kianyaga and Mukurwe-ini Valleys Important Bird Areas (IBAs), using field methods identical to those used in 2000–2001. Recordings of Hinde's Babbler calls were played for 1–2 min at intervals of 50 m, ensuring that each transect was sampled independently of variation in habitat quality. Each group was observed for at least 10–15 min. Individuals were assigned to one of three age categories, based on eye colour, gape and plumage. In 2000–2001, habitat details were recorded within sets of four contiguous 50 × 50-m quadrats, each set located 250 m apart, using GPS. Habitat points were relocated in 2011, to within a median of 11 m. At each point, the altitude, topography, slope, land use and width of watercourse were recorded, and the percentage of coffee, tree and scrub cover was estimated within each quadrat (see Shaw & Musina 2003). We recorded 308 Hinde's Babblers in 75 groups, indicating little overall change in the number of individuals (+1.3%) or groups (−3.8%) since 2000–2001. There was, however, a marked change in the species’ demography, with a 12% increase in the number of adults present (from 232 in 2000–2001) and a 30% drop in the number of juveniles (from 54). The three IBAs showed contrasting trends, ranging from a 68% decline at Machakos (from 47 birds in 2000) to a 40% increase at Kianyaga (from 83 birds in 2000–2001). Based on data pooled from all sites surveyed in 2000–2011, there was a positive relationship between group size category (one to three adults vs. four to eight adults) and the ratio of offspring to adults (Mann–Whitney U = 4062.50; P = 0.027). The presence of pale-plumaged adults was unrelated to breeding success at the group level but varied significantly between sites and in relation to age class, pale plumage types being significantly more common among adults than juveniles (n = 443 birds assessed). The number of Hinde's Babbler groups encountered and the number of offspring present within 1-km transect sections were positively correlated with the mean percentage of scrub cover, estimated from habitat quadrats. Similarly, there was a positive relationship between change in Hinde's Babbler abundance during 2000–2011 and change in the mean percentage scrub cover. By comparing the changing distribution of groups and scrub cover at this spatial scale, we were able to quantify the degree of change in scrub cover associated with the gain or loss of one or more Hinde's Babbler groups, thus providing a firm basis for developing management guidelines for the sites surveyed. Full results of the study are presented in Shaw et al. (2012) and will be presented in a paper to be submitted for publication in 2012. This project was awarded a research grant of £845 from the BOU in 2011, and was also supported by the African Bird Club. We are extremely grateful for the BOU's generous contribution, which met the cost of accommodation and subsistence for the survey team.

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