Effects of season, water and predation risk on patch use by birds on the African savannah

Abstract

Birds from semi-arid regions may suffer dehydration during hot, dry seasons with low food availability. During this period, both energetic costs and water requirements for thermoregulation increase, limiting the scope of activity. For granivorous birds feeding on dry seeds, this is a major challenge and availability of water may affect the value of food. Water availability could (1) increase the value of a food patch when the surrounding environment is poor, due to an increase in the marginal value of energy, and (2) increase the value of the entire environment to the forager when environmental quality increases, due to an increase in the marginal value of time. We aimed to test this by measuring giving-up densities (GUDs, remaining food densities after foraging) of granivorous birds in the presence or absence of filled water pots, at different seasons differing in background food and water availability. We predicted that GUDs will increase with water provision during the dry season with moderate food, but in the early wet season with low food and water availability, GUDs will decrease with water provision. Later in the wet season, our experimental addition of water should have no effect. During seasons with low water availability but differing in food availability, results confirmed our predictions. However, when water became more abundant as the wet season progressed, birds still foraged more intensely during days with added water. In all seasons, birds fed more intensely in cover than in exposed areas, suggesting that predation risk rather than heat influenced microhabitat selection.

Body Temperature Regulation in Hot Environments

“…Furthermore, as increasing body temperature in hot environments may be viewed as a trade-off between the use of hyperthermia and access to energy and water, the need for hyperthermia may be predicted to be less in captive birds with usually unrestricted access to food and water compared to wild birds. Previous studies of wild birds in the same study area has shown measurable costs of foraging exposed to the heat from direct sunlight, and the value of access to water [25]. It should, however, be noted that this study was conducted during the dry season and results would probably have been different if conducted during the wet season.…”

Breeding limits foraging time: evidence of interrupted foraging response from body mass variation in a tropical environment

“…The difference in food availability between the wet and the dry season is a fundamental assumption in this study. We believe this assumption to be reasonable because it is based on empirical observations from previous studies in the same study area (Brandt and Cresswell , Molokwu et al , ). Furthermore, insect abundance is generally lower in the dry season as in other seasonal tropical environments (Frith and Frith , Arun and Vijayan , Silva et al ) so that parents may work harder to raise young when they breed during the dry season.…”

“…We expect that body reserves will increase with breeding stages that constrain foraging such as incubation but with an interaction with availability of resources. For the common bulbuls, the dry season in a tropical savannah is expected to be less favourable for breeding than the wet season because insect abundance is low, and food and water are less widely distributed (Brandt and Cresswell , Molokwu et al , ). The occurrence of moult may confound the effect of breeding on body mass variation (Gosler ) and, therefore, we also consider the possible confounding effects of moult here.…”

Foraging Patch Selection in Winter: A Balance between Predation Risk and Thermoregulation Benefit

“…The hierarchical prioritization of predator avoidance over reduction of thermoregulation cost brings a sub-optimal exploitation of the thermal environment, as the time spent in patches with the lowest thermoregulation expenditure is not maximized [2],[7],[35],[36],[37]. An optimal exploitation of the thermal environment may gain importance in colder environments, and thus the relative importance of predation and thermoregulation is prone to change with the environment.…”