Masahiro Fujioka

Food delivery and sibling competition in experimentally even-aged broods of the cattle egret

SummaryThe age of nestlings was made even in 12 nests of cattle egrets (Bubulcus ibis) by exchanging young chicks or eggs in a breeding colory in Japan. The growth of chicks in such synchronously hatching broods (SHBs) grew almost as fast as first-hatched chicks in 38 control (asynchronously hatching) broods (AHBs). Four last-hatched chicks in AHBs but no chicks in SHBs died of starvation. The frequency of parental nest-visits with food, that of begging food by chicks, and food mass eaten by chicks were greater in SHBs than in AHBs during the first half of the nestling period and similar thereafter. Dominance rank within each SHB was formed through ritualistic fights among siblings. It was correlated with neither growth rates nor winning ratios in food contests, but in some SHBs the subordinate chicks were attacked more frequently by dominant siblings during food contests than in AHBs. The most subordinate chick in one SHB died as a direct result of such attacks. Sibling aggressions were more frequent in SHBs than in AHBs.

Sibling competition and siblicide in asynchronously-hatching broods of the cattle egret Bubulcus ibis

Abstract Feeding behaviour and sibling competition were observed in nine families of the cattle egret ( Bubulcus ibis ) from blinds during 1359 nest-h throughout the nestling period. During days 0–19, size differences among siblings were clear; begging behaviour of chicks changed with time. At least one parent always attended the nest. Food boluses regurgitated early within a feeding period were received by senior chicks more often than by juniors. When any two siblings begged for food at the same time, the elder and younger received the first bolus on 65% and 35% of occasions respectively. Between days 20 and 39, the frequency of begging reached a peak. Begging behaviour became intense and stereotyped. The number of boluses received per begging declined rapidly, especially for junior chicks. In large broods, the success rate of begging was lower and fights occurred among siblings, especially among juniors. Out of 256 dyadic fights, the elder sibling won 85, lost one, and tied 171. The youngest chick died in two broods, apparently as the result of these fights (siblicide). No parents interfered in fights among their offspring. After day 40, the frequency of begging decreased gradually and ceased by day 80, No chicks died in the last period, although the frequency of fights in all large broods remained high.

Cooperative breeding in Azure-winged Magpies, Cyanopica cyana, living in a region of heavy snowfall

The frequency of occurrence of helpers, their age and sex, and certain behavior at nests were investigated in Azure-winged Magpies, Cyanopica cyana, living in central Japan, a region of heavy snowfall. One group of 16 birds was a summer visitor and the other two groups of about 20 birds were resident. Out of 14 nests we observed in 1983, six had one to two helpers, one had no helper, and at the other seven we could not confirm whether helpers attended or not. Therefore, 43% to 93% of nests had helpers. One helper was known to attend at least four nests, and seven of 14 marked individuals (50%) acted as helpers. These results mean that cooperative breeding occurs regularly in this population.