Aggressive tendencies and levels of a graded display: Factor analysis of response to song playback in the redwinged blackbird (Agelaius phoeniceus)

The aggressive tendencies of territorial male redwinged blackbirds were tested using playback of conspecific songs and presentation of a male redwing mount. Factor analysis of overt aggressive responses generated three orthogonal indices of aggressive tendencies. These indices were named (1) Distant Approach (rate of flights over, and proportion of time spent within 5 m of, the mount), (2) Proximity (proportions of time spent within radii of 5, 3, and 1 m of the mount), and (3) Attack Latency (latency to perch beside and attack the mount), and were tested for correlation with the threat displays and other behavior of territorial males to assess their aggressive motivation. In addition, the behavior and time budget variables were used in a linear discriminant function analysis of males that attacked the mount during the testing periods and males that did not attack. The Distant Approach factor was negatively correlated with the proportion of time foraging. The Proximity factor was positively correlated with mean Song Spread display intensity and the proportion of time singing and defending, and negatively correlated with proportion of time foraging. Attack Latency was positively correlated with low Song Spread rate and negatively correlated with medium Song Spread rate given by the territorial males when more than 5 m from the mount. Attacking the nonattacking males could be discriminated on the basis of their (1) proportions of time spent foraging and singing and defending the territory; (2) mean Song Spread (intensity) and incipient Song Spread rate when less than 5 m from the mount; and (3) low Song Spread rate when more than 5 m from the mount. These results suggest that the extent or intensity of the Song Spread indicates the level of aggressive motivation in situations where attack is inhibited, and that threat display and other behavior have signal value and indicate to an intruder the probability of attack.