Cuckoo chicks living in crows' nests may actually benefit their hosts.

Birds are celebrated for their parental care, arduously working to make a nest and provide food to feed their brood.

Using data from crow families monitored over a 16 year period, the team found that nests containing cuckoo chicks were over 20% more likely to successfully fledge a baby crow than those without.

To find out why the cuckoos might be having this effect, Dr Daniela Canestrari from the University of Oviedeo in Spain, who also led the study which is published in Science, removed one or two cuckoo hatchlings from an already-parasitised nest and introduced them into another crow's nest that was previously free from any cuckoos.

Removing cuckoos almost halved the rate of successful fledgling compared to parasitised nests, whereas adding cuckoos nearly doubled the success rate.

But how? When they become alarmed, cuckoo chicks produce a strong-smelling acid and sulphur-containing repellent secretion to deter predators.

In tests, a range of mammal and bird species actively avoided meat brushed with samples of the secretions. This, the team speculate, can help to protect both the cuckoo and its nest mates from being eaten.

There is, nonetheless a down-side. Although nests with a cuckoo increased the likelihood of fledging at least one crow, they fledged fewer crows overall compared to successful nests without a cuckoo.

So cuckoos appear to provide protection against predation but at the cost of reducing the number of crows fledging. And the extent of the benefit depends on the predator population during the breeding season. As such, this relationship varies from parasitism to mutualism from one year to the next.

"If we were to assume a scenario where predation pressure is constantly low over many, many thousands of years, we might expect defences to show up ... but the predators will always be around ... for example in our study area predation is quite high, sometimes we have 70% of nests predated," Canestari explains.

Averaged across the 16 years, there was no significant difference in breeding success due to having a cuckoo. And this lack of an overall negative effect of cuckoos on the crow nests explains why crows have not developed defensive strategies against the imposters.