Brood parasites are definitely the bullies of the avian world. They lay their eggs in the nests of other birds, sometimes destroying the host’s own eggs or just waiting for their nestlings to do the dirty work after they hatch. They then outcompete any surviving host nestlings for food, while the poor host parents are worked to the bone to feed the monstrous nest invader.
In spite of the steep costs of nest parasitism, most avian host species do not have effective mechanisms for detecting and removing brood parasites from their nests. So, why don’t mama birds notice they have a GIANT intruder in their nest and carry out some infanticide of their own? One hypothesis is that the cost of a mother bird making a mistake and pushing the wrong baby out (i.e. her own) outweighs the benefit of developing such a behavior.
Canestrari et al. (2014) focused on the great spotted cuckoo (Clamator glandarius) and their host, carrion crows (Corvus corone corone). They studied the success rate of nests with and without brood parasitism and found that carrion crow nests that contained parasitic cuckoo nestlings were actually more likely to be successful (i.e. fledge at least one crow nestling). How could this be?
As is alluded to in the species name, great spotted cuckoo nestlings produce secretions full of nasty compounds from their cloacas when threatened. Crow nestlings, on the other hand, do not. The authors hypothesized that the presence of a cuckoo nestling, and their gross secretions, may deter predators from chowing down on crow nests. Consistent with this hypothesis, the authors observed that quasi-feral cats, crows, and raptors all avoided eating raw chicken that had been contaminated with cuckoo secretions (but happily fed on the uncontaminated chicken).
However, the picture wasn’t all rosy. The authors also looked at long-term data to identify whether the same association between nest parasitism and successful fledging existed across years. Interestingly, they found that on average, parasitized nests had the same success rate as nests that contained only crow nestlings. The authors posited that this occurs due to differences in predation pressure from year to year. In years where predators are abundant, the great spotted cuckoos benefit the crows by protecting their nests. On the other hand, in years when nest predations is lower, the carrion crows suffer costs in fecundity due to their association with the great spotted cuckoo.
Thus, the relationship between the giant spotted cuckoo and the carrion crow is complicated, sometimes mutualistic and sometimes parasitic. Overall, the fluctuations in predator abundance, and the accompanying change in the relationship between the two species, prevents consistent selection against brood parasitism in the carrion crow.
Canastrari, D., D. Bolopo, T.C.J. Turlings, G. Röder, J.M. Marcos, and V. Baglione (2014) From Parasitism to Mutualism: Unexpected Interactions Between a Cuckoo and Its Host. Science. 343: 1350-1352. DOI: 10.1126/science.1249008