A FIELD-TEST OF IDEAL FREE DISTRIBUTION IN FLOCK-FEEDING COMMON CRANES

1. The densities of common cranes Grus grus in 10 zones of a wintering area were approximately proportional to the amounts of food resources , but some overuse of the zones with highest food densities was observ ed, i.e. greater numbers of birds than expected used these zones. The distribution resembled ideal free distribution only after numbers of c ranes had exceeded carrying capacity. The seasonal pattern of settleme nt deviated from ideal free distribution. During the early and late pa rt of the season, when there were fewer birds at the study site, crane s preferred to forage as close to the roost as possible provided that there was enough food, instead of selecting the zones further away wit h highest food densities. 2. However, 12 individually marked cranes di ffered in their competitive ability and foraging area selection. Large r adult birds were dominant in aggressive encounters, displacing subdo minant cranes from good feeding positions. Food intake rate of dominan t cranes tended to be higher than the hock average, the difference inc reasing with rank. Dominant cranes preferred to forage in the zones wi th highest food densities and had higher absolute daily food intakes. The relative pay-offs of different phenotypes changed across zones wit h different food densities: subordinate birds could not increase their intake rate at the richest zones as much as dominants. 3. The average daily food intake of a crane was thus positively correlated with both the quality of the foraging zone and the dominance rank of the bird. These results fulfil most assumptions and predictions of the interfere nce phenotype-limited distribution model, although truncation of pheno types between zones was imperfect.