Adaptive thermoregulation during summer in two populations of an arid-zone passerine.

Heterothermy plays an important role in lowering the costs of thermoregulation in endotherms by reducing water and energy requirements. We tested predictions that birds in arid habitats should express fine-scale variation in their thermoregulatory patterns as a function of prevailing climatic conditions. We assessed effects of air temperature (Tair) and water vapor pressure deficit (D) on body temperature (Tb) in free-living White-browed Sparrow-Weavers (Plocepasser mahali) during summer in two arid habitats in the Kalahari Desert, South Africa, using data from a dry period at a hot, desert site (n=7 birds), and during a dry period (n=4 birds) and a wet period (n=5 birds) at a milder, semi-desert site. The desert birds maintained a significantly higher set-point Tb (41.5 degrees+/-0.2 degrees C, mean-SD) than semi-desert birds (40.2 degrees+/-0.2 degrees C). During the warmest part of day (12:00-18:00 hours), Tb increased significantly during periods of high Tair and/or high humidity, and mean and maximum Tb were up to 1.40 and 2.3 degrees C, respectively, above normal levels. However, as Tair increased, birds at the desert site maintained Tb at or below set-point levels for a greater proportion of the time than birds at the semi-desert site. Birds at the desert site also expressed a greater magnitude of daily heterothermy (heterothermy index, HI=2.4 degrees+/-0.3 degrees C, mean+/-SD) than birds at the semi-desert site: the latter population showed a greater magnitude of heterothermy during a dry period (HI=2.1 degrees+/-0.3 degrees C) than during a wet period (HI=1.6 degrees+/-0.2 degrees C). Birds continued foraging throughout the warmest part of the day, despite the fact that heat dissipation (percentage of time spent panting and wing-spreading) increased significantly with increasing Tair. Our findings reveal that populations can vary in their thermoregulatory responses in both space and time and suggest that small changes in Tair can have significant effects on thermoregulation in free-ranging desert birds, even when Tair

The influence of fledgling location on adult provisioning: a test of the blackmail hypothesis.

One theory to explain the existence of conspicuous solicitation is that it is a way for young to 'blackmail' carers into provisioning them, by threatening their own destruction. Fledgling birds offer a unique opportunity to investigate the 'blackmail theory', as their mobility enables them to influence the predation risk they face. We investigated a novel solicitation behaviour in fledgling pied babblers (Turdoides bicolor), where fledglings use their location to influence provisioning rates. We show that fledglings face a trade-off: the ground is a much more profitable location in terms of provisioning rate from adult carers, but they are at greater risk from predators owing to their limited flying ability and slow response to alarm calls. Young babbler fledglings move to the ground when hungry, signalling their state, and this stimulates adults to increase their provisioning rates. Once satiated, fledglings return to the safety of cover. By experimentally increasing terrestrial predation risk, we found that adults increased their provisioning rate to terrestrial but not arboreal fledglings. Thus, by moving to a riskier location, fledglings revealed their need and were able to manipulate adults to achieve higher provisioning rates. These results provide support for the 'blackmail theory'.

The riddle of the sands: why are shorebird densities so high in Southern estuaries?

Population data for migratory waders (Aves: Charadrii) during the middle of the nonbreeding season at 31 coastal wetlands in the western Palearctic and Ethiopian regions were analyzed to examine the relationships between bird density and energy intake, intertidal foraging area, and latitude. Wader density is closely linked to population energy intake at individual sites during the nonbreeding season and increases from northern to southern latitudes independently of wetland size. A simple mathematical model linking the variations in the seasonalities of invertebrate production and bird predation is used to provide a mechanistic explanation of the observed variation in bird density with latitude. Results are discussed in the light of current competition- and survival-based models of wader distribution, and, contrary to predictions of some models, we conclude that wader populations track the carrying capacities of coastal wetlands across a wide latitudinal range.

Kelp gulls Larus dominicanus as predators in kelp Macrocystis pyrifera beds.

Minimum daily consumption of bivalves Gaimardia sp by kelp gulls in a Macrocystis kelp bed at Porvenir, Tierra del Fuego, was calculated to be ca. 525 000 individuals, corresponding to an energy removal of ca. 21 kJ·m-2·d-1. This is far in excess of energy removal by any other higher-trophic-level predator in southern Chilean Macrocystis beds, but previous work has ignored kelp gulls as a component of the kelp bed community.

Life-history patterns of populations of the limpet Patella granularis: the dominant roles of food supply and mortality rate.

Rates of algal production were measured at 6 rocky intertidal sites on the southwestern Cape coast, South Africa, where populations of the limpet Patella granularis occur. Rates of algal production and limpet mortality were recorded at each site and both factors affected limpet reproductive fitness. Limpet growth rates and biomass at different sites were significantly correlated with rates of algal production, and limpet mortality was related to the density of African Black Oystercatchers, important predators of P. granularis. Life-time gametic output of a hypothetical cohort of limpets at each site was modelled using the measurements of growth, reproduction and mortality made at each site. Limpet cohorts at sites with rapid rates of algal production were predicted to have a larger life-time production of gametic material than cohorts at sites with slow algal production rates, except in instances of acute predatory pressure. In the light of the overriding influence of food supply on the expression of limpet life-history parameters, it is imperative that researchers consider food availability before assuming that local, population-specific differences in life-history patterns are due to genetic differences.

The influence of coastal upwelling on the functional structure of rocky intertidal communities.

Relationships between organisms at all trophic levels are influenced by the primary productivity of the ecosystem, and factors which enhance rates of primary production may modify trophic relationships and community structure. Nutrient enrichment of intertidal and nearshore waters leads to enhanced production by intertidal algae, and it was hypothesized that where rocky shores are washed by nutrient-rich upwelled waters, the intertidal communities should show a characteristic functional structure, based on the effects of enhanced primary production. Study sites were chosen on rocky shores in southern Africa, central Chile and the Canary Islands, in areas with and without coastal upwelling, and mid-shore community structure at these sites was analysed in terms of the abundance of certain functional guilds of organisms.It was found that algal cover and the biomass of herbivorous limpets supported per unit area on rocky shores were significantly greater in regions of coastal upwelling than in regions where upwelling did not occur. Ground cover by sessile filter-feeding organisms was significantly greater on shores in non-upwelled areas. However, correspondence analysis showed no functional aspect of intertidal community structure that was characteristic of coasts washed by upwelled waters. Primary reasons for this are probably the large variations in the nature of nutrient enrichment that accompanies upwelling, and in the nutrient status of non-upwelled areas. Other factors are man's exploitation of intertidal organisms and differences in the genetic origins of the intertidal species involved.