A comparison of movement ecology and search efficiency between vulture species in Etosha National Park, Namibia

Background/Question/Methods

The search phase is a critical component of foraging behavior, and search efficiency can strongly affect individual fitness and the outcome of interspecific competition. Thus differences in search efficiency among species can have important implications for community dynamics and conservation. Yet we still understand poorly which factors determine interspecific variation in search efficiency, mostly due to methodological limitations.

In many ecosystems vultures of various species feed exclusively on vertebrate carcasses. The spatiotemporal distribution of the carcasses is relatively unpredictable, making them a challenging resource to find. Vultures find carcasses by visually scanning vast areas for carcasses or for aggregations of vultures and other scavengers. We aim at quantifying and understanding the difference in search efficiency between the Lappet-faced vulture (Torgos tracheliotus; LFV) and the white-backed vulture (Gyps africanus; WBV) foraging in Etosha National Park, Namibia. We quantified search efficiency by food supply experiments and high-resolution GPS tracking of free-ranging vultures, and combined the GPS tracks with a biased correlated random walk simulation model to elucidate the factors underlying the difference in search efficiency. 

Results/Conclusions

Experimental carcass deployment shows that LFVs find carcasses first, at rates exceeding random expectations based on relative abundance. Identifying potential carcass-eating events from GPS tracks shows that LFVs are also superior to WBVs in terms of per-individual carcass finding rate. Analyzing movement tracks reveals differences in flight behavior between these species, and allometry of eye size implies that LFV vision is better.

Modifying species-specific traits in the empirically-parameterized simulation model allows us to assess the relative role of each factor. We found that the differences in visual ability, time allocation, and roosting behavior between the species correspond to the empirically observed advantage of LFVs over WBVs, while other differences in other aspects of the movement patterns have minor importance.

Our results provide quantitative and statistical support for previous qualitative and descriptive evidence on the important role of LFVs as efficient carcass finders in the vulture community, and provide mechanistic explanations for their higher performance compared to WBVs. Our research highlights the need to study both i) the foraging behavior of free-ranging animals in their natural context and ii) the merits of merging these data with a model-based analysis of movement paths for comparing search efficiency among species.