From daily movements to population distributions: weather affects competitive ability in a guild of soaring birds

Abstract

The ability of many animals to access and exploit food is dependent on the ability to move. In the case of scavenging birds, which use soaring flight to locate and exploit ephemeral resources, the cost and speed of movement vary with meteorological factors. These factors are likely to modify the nature of interspecific interactions, as well as individual movement capacity, although the former are less well understood. We used aeronautical models to examine how soaring performance varies with weather within a guild of scavenging birds and the consequences this has for access to a common resource. Birds could be divided broadly into those with low wing loading that are more competitive in conditions with weak updraughts and low winds (black vultures and caracaras), and those with high wing loading that are well adapted for soaring in strong updraughts and moderate to high winds (Andean condors). Spatial trends in meteorological factors seem to confine scavengers with high wing loading to the mountains where they out-compete other birds; a trend that is borne out in worldwide distributions of the largest species. However, model predictions and carcass observations suggest that the competitive ability of these and other birds varies with meteorological conditions in areas where distributions overlap. This challenges the view that scavenging guilds are structured by fixed patterns of dominance and suggests that competitive ability varies across spatial and temporal scales, which may ultimately be a mechanism promoting diversity among aerial scavengers.

Can accelerometry be used to distinguish between flight types in soaring birds?

“…Both species are obligate soaring birds although they differ in body mass, with the Andean condor being the heaviest terrestrial soaring bird (ranging from some 9-16 kg; wingspan ca. 3 m [34,35] …”

Wing size but not wing shape is related to migratory behavior in a soaring bird

“…In addition, for terrestrial soaring birds, the wing shape may relate to the selective pressure of the foraging strategy and habitat. Soaring flight is highly dependent on environmental conditions (Spaar and Bruderer , Shepard and Lambertucci ), and low aspect ratio and wing loading in general give birds the ability to exploit weak thermals (Hedenstrom , Ákos et al , Shepard and Lambertucci ). This may be particularly significant for scavenging birds, as the ephemeral nature of their food resources often requires them to search large areas, and because the possibility of using weak thermals early in the day increases their chances of getting food (Kendall ).…”

Meteorological and environmental variables affect flight behaviour and decision‐making of an obligate soaring bird, the California Condor Gymnogyps californianus

“…). Finally, estimates of thermal updrafts and orographic updrafts provide direct measurements of the velocity of air movements likely to be used by soaring birds (Shepard & Lambertucci ). The spatial resolution of these weather variables was coarse (32 to ~80 km; Table ) but was the best available from these sources.…”

“…). The properties of fluid media, such as air and water, are highly variable, and that variation can have significant impacts on the costs of movement (Shepard & Lambertucci , Shepard et al . ).…”