Adaptations of desert birds: Sandgrouse (Pteroclididae) as highly successful inhabitants of Afro-Asian arid lands

Sandgrouse show a multiplicity of adaptations to the low biological productivity, water shortage, large heat fluxes and the associated temperature extremes which characterize conditions in deserts. Their adaptations are discussed under the various headings: Thermoregulation; Water and salt balance; Population densities and movements; Foods; Ecological isolation; Adaptations for breeding; and Conclusions. Thermoregulation. Unlike most birds, some sandgrouse regularly experience ambient temperatures (Ta) well above body temperatures (Tb) and can thermoregulate effectively under these conditions. Heat balance is effected by behavioural and physiological means (choice of microclimate, regulated periods of activity, increased insulation by feather erection and huddling behaviour in hot as well as cold conditions, metabolic heat production and evaporative heat dispersal). Mechanisms which conserve body energy and water reserves are generally favoured. When Ta > Tb, some species defend typical avian Tb values (39–41 °C) but others may tolerate hyperthermia. Water and salt balance. Sandgrouse have a parsimonious water economy, drinking regularly but probably at intervals of up to 5 days. This saves energy and water on flights between feeding areas and watering places, and probably allows more extensive foraging excursions. Salt intake is potentially low, compensated by selective intake of salty items, and the renal-intestinal system seems adapted for conservation of both salt and water. Population densities and movements. Even in harsh deserts, population densities may be high (>50 birds/km2), enabled by a vegetarian diet and nomadic movements which allow sandgrouse to exploit temporary pockets of improvement and to quit deteriorating areas. Some species in environments with predictable precipitation show regular migrations. Foods. Sandgrouse are essentially selective seed eaters, particularly on leguminous species. Members of certain plant orders, families and genera recur in the diets of different sandgrouse species over a wide geographical range, possibly because in practice utilization is restricted to the comparatively few species with relatively nutritious seeds, low toxicity and suitable physical characteristics. This may limit dietary differentiation and hence the ecological isolation possible between sympatric sandgrouse species. Ecological isolation. There are rarely more than two sympatric species common in any locality. They may differ in size, habitat preference and/or feeding techniques; similar species with similar diets may coexist where a food source is temporarily superabundant. Adaptations for breeding. Eggs are relatively small allowing clutch-completion with reduced resource investment, and they have low water vapour conductances conferring high resistance to desiccation. Females incubate through the heat of the day, but this may allow them protracted feeding during cooler times to restore body reserves used in egg production. Semi-synchronous hatching may occur despite laying spread over 5 days and incubation from the first egg. Young drink water carried to them on modified belly-feathers of males, whose own body water reserves are therefore not depleted; gross morphological modifications of feathers are associated with physico-chemical changes allowing rapid water imbibition by keratin, and hence speedy adoption of the feather's water carrying conformation. Conclusions. Sandgrouse are successful in deserts as a result of a large number of individually apparently minor adaptations in ecology, behaviour and physiology, whose cumulative effect is substantial. It is likely that this is the normal pattern of adaptation to deserts and other major habitats (as distinct from major changes in a few key functions).