Search strategies of tyrant flycatchers

Abstract

Aspects of searching behaviour among free-living South American flycatchers (Aves: Tyrannidae) are compared quantitatively. Flycatchers forage with stationary searching periods, followed either by an attempted prey capture (sally) or a ‘give-up’ flight to a new perch. Search times are proportional to body size within each of three categories of foraging behaviour: aerial hawking, sally-gleaning, and perch-gleaning. Over the family as a whole, search times are directly proportional to the size of the visual field scanned during the search. Intraspecific variations in search times are caused by local variations in prey density or visual complexity of the habitat. Between foraging modes, differences in searching and movement patterns are related to prey dispersion characteristics. Aerial hawkers regularly return to favoured perches, but foliage gleaners, which reduce the resources surrounding a perch by sallying only once, rarely return to a perch. In contrast to aerial hawkers, foliage gleaners appear to follow an organized scanning procedure on each perch, by searching nearby surfaces before they examine more distant prey substrates. Throughout the family, the median flight distance after a perch is abandoned is approximately twice the median search radius. Comparisons of search time distributions preceding sallies with those preceding give-up flights suggest that there is no single, optimal give-up time in a given habitat. Foliage-gleaning species appear to assess the amount of search time each perch warrants, presumably based on the degree of complexity of the search area. They either sally at prey before that time, or give-up when the allotted time has elapsed.