Large flocks are expected on risky environments, as it reduces predation risk. Resource availability brings frugivorous species flock size (FS) to an optimum, as small trees will not support large flocks and resource detection can be improved by larger ones. Here we describe Alipiopsitta xanthops vocal repertoire, relating it to FS. We recorded vocalizations and registered FS of Alipiopsitta xanthops in Brasília (Brazil) during 2006. Vocal communication signals can be divided in long range (LR) and short range (SR) sounds. Five call types were found: flight call (LR), short range alarm call (SR), sentinel call (SR), and two agonistic calls (LR & SR). Flight call is equivalent to the functional song of specific recognition, it is also uttered when perched, or in quick series as a long range alarm, or still by overlap as a congregation signal. As congregation signal is attained by flight call overlap, it can get 10dB louder than single calls. The average FS during feeding was 7.7 individuals (n = 116, SD = 8.2), and while congregating to sleep was 79.1 individuals (n = 7, SD = 10.5). Larger flocks were found in tall trees (9-11 individuals) than in the smallest tree (4 individuals; Kruskal-Wallis p < 0.01). Two opposing forces seem to act on FS: food consumption efficiency, and predation risk. These forces also act on the communication. Short range calls (sentinel call, SR alarm call) maintain communication channels while minimizing predator detection. High intensity vocalizations allow for long range communication, therefore frequent separation and reunification of the flocks, improving consumption efficiency. Moreover, as congregation is achieved through flight call overlap, the louder the sound is, larger the flock, hence lower the predation risk.
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