MOVEMENT DECISIONS BY INTERMEDIATE EGRETS

Abstract

An Intermediate Egret (Egretta intermedia) catching a tadpole in a flooded rice field in Japan. Animal movements in heterogeneous environments play a crucial role in a variety of ecological processes. But few studies have revealed what causes animals to show multispatial scale movements within the framework of optimal theories. Using detailed information on movement paths and prey captures by Intermediate Egrets in rice fields, we explored the effects of individual experience at multitemporal scales on subsequent movement parameters. A mobile GIS system (left), which consists of a portable GPS receiver, a laser rangefinder, and an electronic compass, allowed information about a target location to be recorded from a distant observation point (right). Simultaneously, we recorded the timing, duration, and type of behavior of the focal egret. This study showed that the framework of optimal foraging theories is useful for explaining comprehensively the different movement modes in animals. We also concluded that considering both area-avoided and restricted searches at multitemporal scales is important for understanding movement decisions, particularly in organisms searching for hierarchically distributed resources. The results of the study suggested that egrets make different adaptive decisions at different spatial scales. Prey attacks in the previous 1 minute caused egrets to walk away (area-avoided search, left) at a small spatial scale, whereas those in the preceding 8 minutes caused egrets to stay around (area-restricted search, right) at a large spatial scale. Also, the timing of patch departure by flying seemed to follow predictions by the marginal value theorem. These photographs illustrate the article “Hierarchical movement decisions in predators: the influence of foraging experience at more than one spatial and temporal scale” by Tatsuya Amano and Naoki Katayama, tentatively scheduled to appear in Ecology in 2009.