Raptors and "campo-cerrado" bird mixed flock led by Cypsnagra Hirundinacea (Emberizidae:Thraupinae)

bird mixed flocks including cypsnagra hirundinacea and neothraupis fasciata as species with sentinels were studied in "campo-cerrado" in order to investigate the possible relationship between alertness and the mixed flock leadership. this study was conducted from march to september 1996 and mixed flocks were observed on average for 2:30h. the time with sentinels were recorded for c. hirundinacea and n. fasciata. the sentinels of cypsnagra hirundinacea performed most of the vigilance (time with sentinel was on average 42 ± 17%, 41 ± 17% of which by c. hirundinacea, whereas only 1.2 % by n. fasciata) and gave all the alarm calls recorded (54% of the encounters with raptors stimulated alarm calls). a relationship was verified between time with sentinel and the rate of encounters with raptor (anova, f = 3.0, p < 0.05). the results of this study are an evidence for the anti-predatory function of mixed flocks, in campo-cerrado, and the alertness as a major feature of a leader species, since c. hirundinacea always led those flocks.