Freeloading crows start to contribute to group efforts when hardworking birds become handicapped, a study shows. Carrion crows (Corvus corone) form stable groups that share the responsibilities of breeding and caring for the young. Dominant breeders rely on helpers to feed chicks, but they also tolerate individuals that don't seem to help at all. Puzzled about the reasons for this leniency, scientists have suggested that dominants may indirectly benefit from the survival and future reproduction of lazy relatives, and that larger groups — even those filled with dallying birds — may have a lower risk of predation or be more efficient at foraging. Evolutionary biologist Vittorio Baglione at the University of Valladolid in Palencia, Spain, and colleagues now reveal an unexpected role for the laziest members of the group. They report their findings today in the journal Proceedings of the Royal Society B: Biological Sciences.
The research team used camouflaged video cameras to collect data on how often 61 wild crows from 17 social groups in northern Spain fed chicks. They recorded for 12 hours across three days, then trapped and clipped the wings of one breeding bird from each group and repeated the data collection. When clipped crows reduced their chick feeding by about 30%, only non-breeders intensified their care-giving efforts. What's more, the laziest birds increased their helping behaviour the most. Five out of eight crows that had previously refused to visit the nest suddenly began feeding the chicks.