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Recent theoretical analyses have shown that anti-predator benefits in social groups depend on the attack distance of the predator relative to prey spacing within groups. Both attack distance and prey spacing depend on the ability of predator and prey to detect each other. Previous work on forest predators suggest that many depend on surprise to ambush their prey, thus we test the hypothesis that detection distances by eagles of monkeys are greater than vice versa, despite the supposed advantages of sociality in facilitating detection of predators by prey. We used field experiments in the wild to assess detection distances of both raptor predators and their natural monkey prey. Live hawk-eagles (Spizaetus), under rehabilitation from injury, were placed tethered to perches in the home ranges of two habituated wild study groups of tufted capuchin monkeys (Cebus (apella) nigritus) in Iguazu National Park, Argentina. Analysis of video footage of the eagles during the approach of capuchin monkey groups allowed us to define the first moment of behaviours indicating detection by the eagle; detection behaviours of the monkeys near the eagle were recorded observationally by field assistants. The hawk-eagles always detected the monkeys (average distance 31.9 m) before the monkeys detected the predators (average distance 9.4 m). Predators always initially detected one or two spatially-peripheral individuals of the prey group. Distance of detection by the predators (and thus maximum possible attack distances) was significantly less than the prey group spread of 42-57 m. The short detection (and consequent short attack) distances by eagles of monkey prey in this habitat suggests that early warning of attacking eagles may not be a primary benefit of grouping in this case.