OBSERVATIONS OF GROUP BEHAVIOUR AND BREEDING BIOLOGY OF THE YELLOW-BILLED SHRIKE CORVINELLA CORVINA

Yellow-billed Shrikes were found to live in groups throughout the year. Within the group, each member helped to defend the group's territory, warn against predators and feed the breeding female, nestlings and fledglings.
During the study there was little change in the location of the boundaries and in the areas of the territories occupied by the majority of the groups. The densities of the larger groups were in general two to three times that of smaller groups. Numbers within one group varied by ±24% of the average (12) during a period of three years.
Progeny remained in a group for some years before dispersing, sometimes in parties of the same sex. Both sexes exchanged groups, the females moving on average further than males. During successive periods in the history of a group the representation of the sexes varied from a surplus of females to a surplus of males. In the population as a whole the sex ratio was probably parity.
Only one female bred in a group at a particular time and she alone incubated. Eggs were laid on consecutive days. Breeding started at the height of the dry season; the first peak in egg laying occurred at the beginning of the rains; laying continued through the wet season and ceased usually in August. The most frequent clutch size was four, and varied little within a breeding season or between seasons. The incubation period ranged from 15 to 18 days, the most frequently recorded being 17 days. The nestling period was 19 days. The percentage of total eggs laid that produced fledglings was 25% and yearlings 11%.
Young shrikes were independent in the seventh week, participated in group displays in their tenth week and fed fledglings in their fourteenth week.
The age of first breeding was not discovered. Two females in their sixth year were still helpers in a group at the end of the study.