Using Vigilance Behavior to Test Whether Predation Promotes Habitat Partitioning

Predation could be responsible for habitat partitioning by species if different species are safest from predation in different habitats. Alternatively, predation cannot be responsible for habitat partitioning if all species rank the safety of habitats similarly. I developed predictions to test these two hypotheses, using data on the rates at which animals scan the environment for predators while they forage. Preliminary results suggest that predation is unlikely to be responsible for the habitat preferences of sparrow species wintering along an elevational gradient in the Sonoran Desert of southern California, USA. Two predictions can be tested using vigilance behavior. (1) If species differ in which habitats they are safest from predation, some species will experience increased risk when moved from one habitat to another, whereas other species will experience decreased risk; changes in vigilance levels between habitats will be inconsistent among species. (2) If species are safest in the same habitat, they will experience similar changes in risk between habitats, and should exhibit similar changes in vigilance levels between habitats. Sage Sparrows (Amphispiza belli) and Black—throated Sparrows (A. bilineata) spend the winter in different habitats. I recorded vigilance levels in each habitat while birds foraged on naturally occurring seeds in a portable aviary. Sage Sparrows and Black—throated Sparrows exhibited similar changes in vigilance between habitats, suggesting that these two species are safest in the same habitat, and that predation is unlikely to explain their habitat preferences. Also, I tested and found support for the prediction from vigilance theory that food abundance affects vigilance level. Vigilance declined in response to increased foraging patch profitability, counter to the intuitive prediction that increased food abundance relaxes the threat of starvation, allowing more time for vigilance. This relationship between abundance and vigilance is explained by a simple model of starvation.