Spitefulness, Altruism, and the Cost of Aggression: Evidence against Superterritoriality in Tree Swallows’

used in mobbing probably arose as a flight intention movement (Daanje 1950, Hinde 1954). A component of the evidence for its origin as a flight intention movement is the change in posture that accompanies the wing and tail movements; the movement is similar to the postural change that often precedes flight. This component of the flit (the change in posture) is missing from trogon mobbing tail flits. This suggests that in mobbing the important factor is movement, which enhances visibility to other birds and perhaps confusion of the predator (but see Curio 1978). Trogon mobbing may illustrate the value of mobbing among small birds or other passively mobbing species. Although these species are not likely to drive large predators from their territories, their calls often attract other, more aggressive birds such as jays. Often when I placed a captive owl on a perch to observe the responses of other birds, the first species to mob were smaller species such as Plain Titmice (Parus inornatus) or Black-throated Sparrows (Amphispiza bilineata). When jays arrived they mobbed aggressively; on several occasions they startled the tethered owl sufficiently to cause her to fall off her perch. This suggests that mobbing may benefit the smaller species by atttracting larger birds who can force owls, particulary small species, to flee.