Attendants of Neotropical mixed-species flocks are hypothesized to benefit primarily by gaining protection from predators, while potential costs have received little attention. The Wedge-billed Woodcreeper (Glyphorynchus spirurus) frequently joins mixed-species understory flocks but also often forages alone. We hypothesized that, because of the significant vertical component of its foraging behavior, the primary cost to flock-following might be a decrease in foraging efficiency associated with conforming to flock movement patterns. We compared the foraging behavior and microhabitat use of Wedge-billed Woodcreepers in and out of flocks at Tiputini Biodiversity Station, eastern Ecuador, during January–March, 2010–2012. We measured foraging height range, time spent per trunk, vertical movement rate, distance flown between trunks, extent of foliage cover around the focal bird, hitch rate, and peck rate. We tracked birds in 2012 using radiotelemetry to measure horizontal movement rates and to record the locations of flock-joining and -leaving. Wedge-billed Woodcreepers in flocks foraged in more exposed microhabitats, moved vertically at a faster rate despite a decrease in foraging height range, and exhibited greater hitch rates than individuals foraging alone. Peck rates, peck:hitch ratio, and distance flown between trunks did not differ between situations. Horizontal movement rate did not differ in and out of flocks, but the length of flock-following sessions was negatively correlated with movement rate above a threshold of 4 m min-1. These observations are consistent with an antipredator benefit of flocking for this species and suggest an energetic or foraging efficiency cost associated with conforming to flock movement patterns.
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Vol. 130 • No. 4