A study of the ecology of the Namaqua Sandgrouse and other arid-zone birds

This field study set out to identify the key ecological factors influencing the population dynamics of the Namaqua Sandgrouse Pterocles namaqua, through an investigation of diet and the nutritional demands during different stages of the annual cycle, the timing of breeding seasons and movements in relation to patterns of rainfall-dependent food availability, breeding success and the factors limiting productivity. In addition, the nesting habits and success of 11 coexisting arid-zone bird species were examined to test a variety of hypotheses regarding the relationship between nest-predation rate and nest site, nest density, predator-avoidance behaviour, stage of the nesting cycle and season, and degree of residency. Furthennore, the importance of rainfall as a breeding stimulus and its effects on clutch size were investigated for several species. The Namaqua Sandgrouse is an obligate granivore at all times, feeding on the seeds of annual plants, primarily of the family Fabaceae. Even while breeding, energy is the first-limiting nutrient in the foods of adults. Growing chicks have a proportionally greater protein demand, and are more dependent than adults on protein-rich legume seeds to satisfy first-limiting amino acid requirements. The chick growth phase was identified as the most nutritionally demanding stage in the annual cycle. The breeding season was found to be unexpectedly variable, and not consistently correlated with periods of peak food availability. The peak nesting period ranges from January-May in northwestern Namibia; June. ()ctober in the Kalahari; September-December in the Nama Karoo all late-summer (January-April) rainfall regions. The possible complicating influences of biannual breeding, moulting, adult nutritional reserves and seasonal variation in nest-predation pressure are discussed. The Namaqua Sandgrouse is largely nomadic within its range, but also undertakes an annual "east-west" partial migration within South Africa. This movement is largely between two late-summer rainfall regions, so is not directly linked to gradients in food availability. Nesting success ranged from 5.7% to 13.5% over four consecutive breeding seasons (n = 278 nests). Predation, primarily by small mammals, was responsible for 96% of nest losses. Estimates of annual recruitment in a core breeding area ranged from minima of3-10% to maxima of 6-20% over three seasons. Possible reasons for this sustained low level of breeding success are explored. The implications of these findings to the management of the Namaqua Sandgrouse as a resource for sustainable wing-shooting are discussed. Nesting success among other arid-zone species ranged from 3.5% to 75.4%, predation accounting for 94% of nest losses (n = 588). Predator-avoidance behaviour and nest site accounted for much of the variation in nesting success. Nest predation was inversely related to nest density, and decreased as the breeding season progressed from spring to midsummer. Daily egg-predation rates were higher than daily nestling-predation rates for seven of the nine altricial species studied. Among ground-nesters, territorial residents incurred significantly lower (U4.6 = 24.0, P < 0.02) daily nest-predation rates (3.41%, n = 4) than nomads (6.91%, n = 6), suggesting that residency enhances nest survival. Breeding activity was strongly correlated with rainfall in most species, but resident insectivores responded immediately to rainfall events as small as 6 mm while nomadic granivores exhibited a delayed response only to larger rainfall events (> 40 mm). Following large or cumulative rainfall events, over 50% of species studied showed evidence of an increase in clutch size, sometimes within a week. For example, average clutch size in the Spikeheeled Lark Chersomanes albofasciata ranged from 2.00 (± 0, n = 13) during three relatively dry seasons, to 2.83 (± 0.39, n = 23) following a 54 mm rain shower, to 3.30 (± 0.66, n = 10) and a maximum clutch size of five following 78 mm of follow-up rainfall.