SEXUAL, SEASONAL AND GROUP-SIZE DIFFERENCES IN THE ALLOCATION OF TIMEBETWEEN VIGILANCE AND FEEDING IN THE GREATER RHEA, RHEA-AMERICANA

We studied the effect of sea and group size on the proportion of time a greater rhea, Rhea americana, allocates to vigilance and feeding dur ing thc breeding and the non-breeding seasons. We analysed 175 records of focal animals that were feeding alone or in groups of 2 to 26 bird s. In both seasons, males spent more time in vigilance and less time i n feeding than females. Both sexes spent more time in vigilance and le ss time in feeding during the breeding season. Sexual and seasonal dif ferences in vigilance were the result of different mechanisms. Males h ad shorter feeding bouts than females but there mere no sexual differe nces in the length of the vigilance bouts. On the contrary, seasonal d ifferences were the result of males and females having longer vigilanc e bouts during the breeding season but there were no seasonal differen ces in the length of the feeding bouts. During the non-breeding season , individual vigilance was higher in rheas foraging alone than in grou ps. In this case, solitary birds had longer vigilance and shorter feed ing bouts than birds foraging, in groups. We discuss the possible effe ct of intragroup competition and food availability on the allocation o r time between feeding and vigilance in this species.