【生態学】ハワイガラスの採餌は枝を使う

Hawaiian crows (Corvus hawaiiensis) use tools when foraging for food - a rare behaviour in birds that had hitherto been confined to New Caledonian crows within the crow family - finds a study published in this week’s Nature.

Crows living on the South Pacific island of New Caledonia (Corvus moneduloides) have long baffled scientists with their tool-use behaviour. The evolutionary origin of this remarkable behaviour has remained elusive mainly because comparisons with species of the same genus were not possible.

Christian Rutz and colleagues show that another tropical corvid, the Hawaiian crow (‘Alala'), is a highly dexterous tool user. Hawaiian crows became extinct in the wild in the early 2000s, and currently only survive in captivity. The authors tested 104 of the 109 surviving Hawaiian crows and found that 78% of them spontaneously used stick tools to probe for food that was out of reach. They also found that tool use varied with age, with 93% of all adult crows using tools compared to 47% of younger birds. Moreover, at least two lines of evidence suggested that tool use is part of the species’ natural behavioural repertoire: juvenile crows developed functional tool use without training or social input; and proficient tool use was seen to be a species-wide capacity.

As Hawaiian crows are only distantly related to New Caledonian crows but evolved in similar remote tropical-island environments, the authors suggest that convergent evolution of tool-related adaptations may have taken place. This in turn supports the idea that, in birds, tool use for foraging is facilitated by ecological conditions typical of islands, such as reduced competition for embedded prey and low predation risk.