A study has found that when hardworking birds in a group become handicapped, scrounging crows start to contribute instead.
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.
Now, evolutionary biologist Vittorio Baglione at the University of Valladolid in Palencia, Spain, and colleagues have revealed an unexpected role for the laziest members of the group.
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 30pct, 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.
"It's really important to investigate individual variability in helping behaviour, because it could help us understand the evolution of cooperation. That's the central dilemma of behavioural ecology that we've been grappling with for a long time," Nature quoted Walt Koenig, an ornithologist at Cornell University in Ithaca, New York, as saying.
Loitering crows may help the whole group by ensuring that provisions for offspring remain constant during tough times- the increased effort of non-breeders compensated fully for the diminished offerings of the disabled crows.
What's not clear is whether the slackers offer the help because they hear chicks begging or because dominant birds force them to contribute.
Dawdling animals may be more likely to chip in voluntarily if they are strongly related to other members of the group, because they may derive indirect gains from the group's overall reproductive success.
The factors that influence helping behaviour are difficult to examine using skittish crows in the wild, so Baglione next plans to use tame birds in aviaries.
There's still a lot more to learn about how different individuals adjust their cooperative behaviour depending on the actions of others in the group, he said.
"I'd like to believe that this kind of study might shed some light on cooperation in humans and other species that cooperate in subtle ways," he added.
The findings of the study are reported in the journal Proceedings of the Royal Society B: Biological Sciences1.