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SEPG 1909 - Date awarded 2001

The relationship between group size, habitat quality and breeding success in Hinde's Babbler Turdoides hindei and Northern Pied Babbler T. hypoleucus

Philip Shaw,
Scottish Natural Heritage
2 Anderson Place
Edinburgh
EH6 5NP

Abstract

Hinde's Babbler Turdoides hindei and Northern Pied Babbler T. hypoleucus are group-territorial cooperative breeders whose geographic ranges overlap in central Kenya. The former is classed as globally Vulnerable and, during 1990-2000, was recorded from just seven sites. Earlier studies had shown that its breeding performance at one site was much poorer than that of Northern Pied Babbler, suggesting that it may be more susceptible to human disturbance. This study examined factors influencing the distribution and breeding success of each species, and the abundance of Hinde's Babblers at six of the seven known sites.

Although dependent on thickets for breeding and cover, over 70% of Hinde's Babblers were found at two intensively cultivated sites, where little natural cover remains. Most of the remaining groups occurred in semi-arid areas, where the percentage of thicket cover was much higher, but the babbler population was relatively sparse. The breeding performance of Hinde's Babbler was slightly higher than that of its more widespread congener, and was positively correlated with thicket cover and adult group size class. Group density also showed a positive, though non-linear, correlation with cover. Results suggest that within intensively cultivated areas, a modest increase in thicket cover may significantly raise the density and productivity of Hinde's Babbler groups.

Background

Hinde's Babbler Turdoides hindei is a group-living, territorial, cooperative breeder, endemic to Kenya, and classed as globally Vulnerable (BirdLife International 2000). During the mid-1900s Hinde's Babbler had a global range of about 17,500 km², and was recorded mainly from the southern slopes of Mt Kenya (Plumb 1979). By 1979 its range appeared to have contracted substantially and, by the 1990s, it was known from just seven sites. 

Hinde's Babbler is associated with thickets, normally in the vicinity of watercourses (Njoroge et al. 1998), and occurs in two contrasting situations. Several sites receive moderate or low rainfall, have limited agricultural potential and, consequently, a low human population density. At two sites, however, rainfall levels are substantially higher and much of the land is intensively cultivated, supporting a human population density in excess of 400 people km^-2. Although the amount of thicket available is small, and much of it transitory in nature, these highly modified sites appear to support higher babbler densities than the drier, less fertile and less disturbed sites (Njoroge & Bennun 2000). They also support populations of Northern Pied Babbler T. hypoleucus, a more widespread and seemingly more adaptable species, whose breeding performance in one year was higher than that of its threatened congener (Njoroge & Mutinda 1996).

In many cooperatively breeding birds, reproductive output has been shown to increase with the number of non-breeding helpers present (Brown 1987). Although this relationship has been confirmed in one Turdoides babbler (T. squamiceps; Wright 1998, but see Zahavi 1990), group size and breeding success appear to be unrelated in several other species (Shaw & Shewry 2000). Relatively little attention has been paid to the relationship between breeding success and territory quality in babblers. Theoretically, large group size may be a consequence, rather than a cause, of high breeding output, perhaps enabling successful pairs to secure better quality territories. In this study, the relationship between habitat, group size and breeding outcome was examined in Hinde's and Northern Pied Babbler, to determine whether successful breeding is independently correlated both with group size and territory quality. The population size of Hinde's Babbler at six of its seven known sites was also examined, providing a baseline for future surveys. This work was carried out as part of a joint study, involving J. Musina and P. Gichuki of the Ornithology Department, National Museums of Kenya.

Methods

Surveys were conducted during 27 June to 11 July 2000 and 19 June to 7 July 2001, at a total of six sites: Kianyaga (37°21'E, 0°30'S), Mukurweini (37°03'E, 0°33'S), Machakos (37°15'E, 1°30'S), Mwea National Reserve (37°37'E, 0°50'S), Kitui (38°02'E, 1°25'S) and the NW border of Meru National Park (38°04'E, 0°12'N), referred to here as 'Meru'.

Hinde's Babblers respond vigorously to tape-recordings of their calls. Taped calls were therefore played at fixed (50 m) intervals along transects, ensuring that each transect was sampled independently of any variation in habitat 'quality'. This approach maximised the amount of time spent searching the species' preferred habitat and the number of individuals located, thus providing an indication of breeding output as well as density. Based mainly on eye-colour, the majority of individuals encountered were assigned to one of three age classes: adult, immature or fledgling.

Habitat parameters, which included measures of topography, land use, stream width, and the percentage of coffee, tree and thicket cover, were recorded within four 50x50 m quadrats, at 250 m intervals. These data were recorded at all sites other than Meru N.P., where habitat assessments were limited by time and access constraints.

Results and discussion

A total of 502 Hinde's Babblers were encountered during the two surveys. Each group's position was recorded, providing a baseline for future surveys (Figure 1). A further 163 birds had been recorded in additional areas surveyed in 1994 (Njoroge & Bennun 2000), yielding a composite total of 665 birds. Over 70% of these were found at two intensively cultivated sites, and less than 10% occurred within the two protected areas surveyed (Shaw et al. in prep.). Populations encountered during 2000-01 spanned a geographic range of about 200 km (N-S) by 100 km (E-W), within what appear to be 3-4 discrete blocks. The most isolated of these, at Meru, is 120 km from the nearest known population (Shaw et al. in prep.).

Group size in both species was low in comparison with that of several other Turdoides species, typically comprising 2'5 adults (hindei, mean = 3.8; n = 109 groups; hypoleucus, mean = 3.6; n = 44 groups). In Hinde's Babbler, breeding success, in terms of the proportion of offspring present, was slightly higher than in Northern Pied Babbler, and was positively correlated with adult group size class in 2000, but not in 2001 (Shaw & Musina, in prep.). A similar relationship was evident between group size category and breeding outcome in Northern Pied Babbler, although only when pairs of birds were compared with larger groups of adults.

Factors likely to influence the abundance and breeding outcome of Hinde's Babbler varied markedly between the five sites at which habitat parameters were recorded (Table 1). Separate analyses were therefore performed for the two high rainfall sites (Mukurweini and Kianyaga) and two low rainfall sites (Machakos and Mwea). Hinde's Babbler abundance was much lower at the latter, where mean thicket cover (48%) was six times that of the two high rainfall sites (8%). Despite these differences there was a significant, positive correlation between Hinde's Babbler distribution and thicket cover at each site type. In contrast, the distribution of Northern Pied Babbler was negatively correlated with thicket cover.

At the two high rainfall sites, Hinde's Babbler group density showed a positive, non-linear correlation with adjacent thicket cover (i.e. within 250 m of each group), levelling out at about 15-20% cover. Breeding output was also positively correlated with adjacent cover, indicating that productivity was influenced by territory 'quality' as well as by group size (Shaw & Musina, in prep.). Within intensively cultivated areas, variation in thicket cover was slight, suggesting that a relatively modest increase in cover may be sufficient to raise the species' population density and breeding performance.

For a more detailed account of this study, see Shaw et al. (2001, 2002).

Figure 1.  

The distribution of babbler groups along streams in the southern half of Mukurweini Valleys Important Bird Area. The location of each habitat survey point and babbler group was recorded by GPS, and is shown against a 1 km UTM grid.

Table 1. 

Environmental parameters and Hinde's Babbler densities at five survey sites

Acknowledgements

This study was part-funded by Small Ecological Project Grant No. 1909. The Royal Society for the Protection of Birds (the BirdLife International Partner in the UK) kindly contributed to the Kenyan team members' field expenses in 2000. I am grateful to Leon Bennun for his invaluable support and advice, and to the Kenya Wildlife Service for allowing the survey to proceed in Mwea National Reserve and Meru National Park.

References

Brown, J.L. 1987. Helping and communal breeding in birds. Princetown: Princetown University Press.

BirdLife International (2000). Threatened birds of the world. Barcelona and Cambridge, UK: Lynx Edicions and BirdLife International.

Njoroge, P. & Bennun L. (2000). Status and conservation of Hinde's Babbler Turdoides hindei, a threatened species in an agricultural landscape. Ostrich 71(1&2): 69-72.

Njoroge, P. & Mutinda, D. (1996). Notes on nesting of Hinde's Babbler Turdoides hindei and Northern Pied Babbler T. hypoleucus in Central Kenya. Ostrich 67: 170-172.

Njoroge, P., Bennun, L.A. & Lens, L. (1998). Habitat use by the globally endangered Hinde's Babbler Turdoides hindei and its sympatric relative, the Northern Pied Babbler T. hypoleucus. Bird Conservation International 8: 59-65.

Plumb, W.J. (1979). Observations on Hinde's Babbler Turdoides hindei. Scopus 3(3): 61-67.

Shaw, P. & Musina, J. (in prep.). Correlates of abundance and breeding success in the globally threatened Hinde's Babbler Turdoides hindei and its congener, Northern Pied Babbler T. hypoleucus.

Shaw, P. & Shewry, M. (2000). Abundance, group size and breeding success of Bare-cheeked Babblers Turdoides gymnogenys. Ibis 142: 58-64.

Shaw, P., Gichuki, P. & Musina, J. (2002). A survey of Hinde's Babbler Turdoides hindei at five sites, June-July 2001. Research Reports of the Centre for Biodiversity, National Museums of Kenya: Ornithology 42.

Shaw, P., Musina, J. & Gichuki, P. (2001). Surveys of Hinde's Babbler Turdoides hindei and Northern Pied Babbler T. hypoleucus in four Important Bird Areas. Research Reports of the Centre for Biodiversity, National Museums of Kenya: Ornithology 40.

Shaw, P., Musina, J. & Gichuki, P. (in prep.). Estimating change in the geographic range and population size of Hinde's Babbler Turdoides hindei. Submitted.

Wright, J. (1998). Helping-at-the-nest and group size in the Arabian Babbler Turdoides squamiceps. Journal of Avian Biology 29: 105-112.

Zahavi, A. (1990). Arabian Babblers: the quest for social status in a cooperative breeder. In: Stacey, P.B. & Koenig, W.D. (Eds.). Cooperative breeding in birds: long-term studies of ecology and behavior. Cambridge: Cambridge University Press.

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