NPR All Things Considered “This Freeloading Bird Brings Help — And The Help Smells Gross”
It is hard to believe that feeding an entire extra non-offspring would be in the self-interest of a bird, but as this short points out, costs and benefits are always environment-specific. In this case, the “parasitic” effect of having to raise an additional chick that is not your own might be offset by increased chances of your offspring surviving under high predation pressure. Is it possible that this benefit might be great enough under some circumstances to be labeled “mutualistic” (as in thank you very much for burdening me with your smelly offspring)?
My suspicion is that for intervals the benefits associated with raising a cuckoo chick might actually outweigh the costs, and these intervals might help explain the persistence of nest parasitism. Like all parasites, nest parasites face a dilemma: to be successful, I must extract resources from my host, but if I exact too high a toll and cause my host to die I may lose my means of surviving and/or reproducing. One way to deal with this problem is to be very costly to your host in good times while being less costly (or even beneficial) when times are tough. The cuckoo parasitism seems to fit this pattern: one could imagine that a cuckoo might be costly to a crow whose offspring do not face much predation risk, but might actually tip the balance towards benefit when predation risk is high. After all, it is better to have an extra mouth to feed than to have no mouths to feed at all!
This discovery of cuckoo chick benefits might also help answer an important question: Why aren’t crows better equipped to recognize and eject cuckoos? If having a cuckoo in your nest is beneficial under some circumstances, the ability to detect the intruder offspring and the instinct to eject that intruder might be selected against. Given that crows are so cognitively advanced, one might suspect that they would be capable of assessing when having a cuckoo is valuable: it would be interesting to see if crows are more likely to tolerate a cuckoo when predation risk is high.A Minor Post, Behavior, Birds, Coevolution, Mutualism, Parasitism, Predation, Quantifying Costs and Benefits, Radio & Podcasts