Jc. Reboreda et Gj. Fernandez, SEXUAL, SEASONAL AND GROUP-SIZE DIFFERENCES IN THE ALLOCATION OF TIMEBETWEEN VIGILANCE AND FEEDING IN THE GREATER RHEA, RHEA-AMERICANA, Ethology, 103(3), 1997, pp. 198-207
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.