by Grzegorz Kopij
Quantitative studies on bird assemblages are conducted in areas ranging in size from few hundred m2 to several thousand km2. In very small areas, like gardens around private residences, usually comprising less than one ha in surface size, studies on species diversity and their temporal changes are exemplified by Brown 1997, Kaestner 1997, and Thomson 2019. Relative abundance may also be estimated by means of ringing in such small places (e.g. de Swardt et al 2004, Thomson 2019). However, under bushveld/savanna conditions, population densities and dominance of particular species may be reliably estimated only in areas larger than 10 ha. For less common passerines and most non-passerines such studies should be conducted in areas larger than 100 ha and for species nesting in very low densities (e.g. raptors) the study area should be at least 1000 ha in surface size (e.g. Kopij 2016, 2019, 2020).
A male of the Namaqua Dove.
In Windhoek, interesting studies on avian diversity were conducted in three small suburban gardens, each one less than 1 ha in surface size, i.e. in Ausspannplatz (Kaestner 1996), Windhoek West (Brown 1997), and Klein Windhoek (Thomson 2019). Kaestner (1997) during one year (August 1996-August 1997) recorded 84 species in his garden and around it. Brown (1997), on the other hand, conducted very intense observations on birds in his garden over a period of 12 years (1987-97), while Thomson (2019) had ringed in his garden a total of 12 494 birds, recapturing 7 589 of them over a period of 10 years (2008-2018) (The total includes multiple recaptures of individuals – Ed.). Although these studies constitute valuable contributions to our knowledge of birds in the Namibian capital, so far no quantitative studies on birds in larger areas have been conducted in this city.
The purpose of this study was to determine the status of birds occurring in an area larger than 10 ha. An attempt was also made to estimate abundance of species breeding in this area. The area was selected to facilitate further studies aimed at monitoring changes in species composition and abundance of particular species.
Study area
The National Botanical Garden of Namibia (NBGN), 12 ha in surface area, is situated in the centre of Windhoek. It was established in 1993 and presently it is run by the National Botanical Research Institute. It is the only botanical garden in Namibia and focuses exclusively on indigenous flora. The larger part of the garden is not landscaped and vegetation is only slightly modified. It constitutes a bushy rocky hill, with numerous outcrops occupied by Rock Hyrax. It There are 52 tree and shrub species, 139 herbs, 32 grasses and 31 other vascular plants in the NBGN. The vegetation can be classified as highland savannah. It is, in fact, a fairly dense bush composed mainly of acacias (A. erubescens, erioloba, hereroensis, karroo, hebeclada reficiens), butter tree Cyphostemma spp., kudu bush Combretum apiculatum, Diospyros licioides, Grewia flava, Searsia spp., buffalo thorn Ziziphus mucronata, Sheperd’s tree Boscia albitrunca, candelabra euphorbia Euphorbia virosa, puzzle bush Ehretia alba and others. The garden is a stronghold for dense stands of Windhoek aloe A. littoralis, and quiver tree Aloe dichotoma. Among grasses are mainly species of Aristida and Eragrostis.
Quite common among reptiles are the Namibian Rock Agama Agama planiceps, having a few social groups in the garden. There are a dozen or so Rock Hyraxes Procavia capensis in the rocky outcrops, a few Porcupines Hystrix africaeaustralis and Yellow Mongoose Cynictis penicillata.
Methods
The mapping method (Bibby et al. 2012) has been employed to determine the status and numbers of bird species occurring in the NBGN. The total of four complete surveys of the entire NBGN were conducted; two of them in 2013 (3 July and 13 August) 2013; and also two in 2020 (20 and 28 August). All surveys were conducted in the mornings by walking slowly along all paths in the garden. All birds seen and heard were identified, sexed and aged whenever possible, and plotted on a map. Special care was taken not to count the same birds twice. Therefore records of simultaneously calling/cooing birds (at the same time) were important in this regard. Movements of birds were also recorded.
As recommended in the mapping method, a pair, not an individual, was a census unit. It has been assumed that one isolated or two adult birds together represented one (potentially breeding) pair; 3 or 4 adult birds – 2 pairs, and so on. However, a flock composed of 1-2 adult birds and a few juveniles were regarded as representing one pair. In polygamous species, e.g. Southern Masked Weaver, the number of potentially breeding pairs was equal to the number of females recorded at a nesting site. The number of breeding pairs of Little Swift was assessed based on the number of occupied nests, and those of the Southern Masked Weaver and White- browed Sparrow-Weaver – based on the number of nesting sites.
Results
A total of 44 resident breeding species were recorded in 2013 and 2020. At least 23 other species nest in the garden irregularly (in some years only) or they nest in the neighbouring areas and visit the garden occasionally (the botanical garden probably comprises only a small part of their breeding territories).
The most common (dominant) species among regularly breeding residents are: the Laughing Dove, White- browed Sparrow-Weaver, Blue Waxbill, Chestnut-vented Warbler, Southern Masked Weaver, African Red-eyed Bulbul and Helmeted Guineafowl.The following less common species were recorded as residents in the garden: Rockrunner, Pririt Batis, Swallow-tailed Bee-eater, Marico Flycatcher, African Paradise Flycatcher, Ashy Tit, Barred Wren- Warbler and Karoo Thrush. The NBGN appears to be a main refuge for the Helmeted Guineafowl in Windhoek. The species is vulnerable to predation by people, dogs, cats, etc. The NBGN is a good resting and foraging place for these birds, with low predation risk, posed probably only by the Yellow Mongoose.
The Little Swift breeding colony is situated under the eaves of the NBRI building. In 2020, it was composed of ten occupied nests. Four hornbill species nested in the areas neighbouring the NBGN: African Grey, Southern Yellow-billed, Damara Red-billed, and Monteiro’s. In some years, the NBGN may constitute a small part of their fairly extensive territories. The same is applicable to three piciform species and Gabar Goshawk. Other species nested in the NBGN in some years only; at least once between 1991 and 2020.
The list of non-breeding visitors and vagrants includes 27 species. Among them, 15 are African visitors (recorded annually or every few years), six are African vagrants (recorded occasionally, 1-3 times per decade), and six are Palearctic visitors.
Discussion
The total number of 84 bird species recorded in the NBGN over a period of 30 years (1991-2020) is relatively low. Over a period of one year only, Kaestner (1997) recorded in and around his small garden in Ausspannplatz also 84 bird species. Over a period of 12 years, Brown (1997) recorded 112 bird species in and around his small garden in Windhoek West. However, most of the species recorded in both places were non-breeding visitors or vagrants, while the number of breeding residents was even lower than in the NBGN. The more intense observations and more years devoted for such observations in a given place, the higher number of vagrant and visitor species were recorded there. However, the number of resident species may increase only slightly due to this effect.
It should be pointed out that the numbers of potential breeding pairs of some species were only roughly assessed. At least eight surveys are required for precise estimation of numbers of all species. The numbers of more vocal and conspicuous species, e.g. Laughing Dove, Helmeted Guineafowl, Acacia Pied Barbet, Grey Go-away-bird, were assessed more precisely than less vocal and elusive species, such as Ashy Tit, Black-faced Waxbill, Long- billed Crombec or Barred Wren- Warbler.
Due to the good natural conditions, presence of water, lack of human disturbance, and low predation pressure by dogs and cats, many bird species find favourable feeding and nesting conditions in the NBGN. It may play a role in the protection of species such as the Helmeted Guineafowl, Red-billed Spurfowl, Pearl-spotted Owlet, Rockrunner, Barred Wren-Warbler, Violet-eared Waxbill and Black-faced Waxbill.
Both species diversity and population densities of some species may still increase in the NBGN with human help. Some species, such as hornbills, barbets and starlings can be attracted to the garden by artificial nesting boxes specially constructed for them. Other species (e.g. Helmeted Guineafowl, Red-billed Spurfowl, waxbills) may increase their numbers if additional food is provided by man, either in the form of seeds, boiled rice or bread crumbs or sliced apple exposed in feeders (Kiekebusch-Steinitz 2003).
Although the Yellow Mongoose were recorded, their dens were not found in the NBGN. They penetrate the garden only occasionally and seem to have no impact on the local avian assemblage. Also, the porcupine does not affect it. It is a plant feeder, and as such may pose a problem for some plants, especially lilies and related species, but not for birds. The presence of a sizable Rock Hyrax population in the garden may also pose a problem in the future, if the population level exceeds its limits, as was recorded for example in some places in Bloemfontein (Wiid, Butler 2014). They may deplete some plant food resources causing a decline of those bird species which rely on them, e.g. Helmeted Guineafowl, Red- billed Spurfowl or White-browed Sparrow-Weaver.
Since the NBGN plays a very positive role in nature protection, water retention and environmental education, it is postulated to enlarge it by inclusion of the neighbouring Aloe Trail with a large stand of the Windhoek Aloe. At present, this trail is not properly protected and is threatened by devastation.
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Kopij G. 2016. Birds of Katima Mulilo town, Zambezi Region, Namibia. International Science & Technology Journal of Namibia (Windhoek), 7: 85-102.
Kopij G. 2019. Structure of avian communities in a mosaic of built-up and semi-natural urbanised habitats in Katima Mulilo town, Namibia. Welwitschia International Journal of Agricultural Sciences, 1: 68-75.