I've written before about the evolutionary arms race between brood parasites (who lay their eggs in the nests of other birds, leaving them to raise their chicks) and their hosts. In these systems, host birds benefit from recognizing and removing parasite eggs or chicks from their nests. Meanwhile, the brood parasites keep trying to trick the hosts into accepting and caring for their young.

Hosts must walk a precarious line in defending themselves against brood parasites. Too lax, and they end up spending valuable time and energy raising another bird's chicks. Too strict, and they run the risk of rejecting one of their own eggs by mistake. For the best results, hosts should modify how defensive they are against parasites in relation to the risk they pose.

A few years ago, Diane Colombelli-Négrel, Sonia Kleindorfer, and colleagues from Flinders University in Australia discovered a remarkable way one bird fights back against brood parasites. Female superb fairy-wrens teach their embryos a "password" while they're still in their eggs. Each female's incubation call contains a unique acoustic element. After they hatch, fairy-wren chicks incorporate this unique element into their begging calls to ask for food. Colombelli-Négrel, Kleindorfer, and colleagues showed that chicks whose begging calls most resembled their mothers' incubation calls received more food. But the brood parasites of the fairy-wren, Horsfield's bronze-cuckoos, produced begging calls that did not so closely resemble the parental password.

In a new study, Colombelli-Négrel, Kleindorfer, and colleagues again looked at the relationship between superb fairy-wrens and Horsfield's bronze-cuckoos to see if a greater threat of brood parasitism would cause the fairy-wren to up its teaching efforts.

Cuckoo nestling in fairy-wren nest. Photo: Diane Colombelli-Negrel.

First, the researchers recorded calls from 17 fairy-wren nests in South Australia. They found the similarity between the mother's password and the chick's begging call was predicted by the number of incubation calls produced by the mother: If females made many incubation calls, their chicks ended up producing more similar begging calls.

Next, the researchers conducted a playback experiment at 29 nests. They broadcast either the song of Horsfield's bronze-cuckoo or a neutral bird. After the cuckoo calls, but not after the neutral bird calls, female fairy-wrens made more incubation calls to their embryos. In other words, female fairy-wrens that heard a cuckoo near their nest increased their efforts to teach their password to their embryos.

Colombelli-Négrel and Kleindorfer say their results provide a mechanism for how fairy-wrens could get better at decision-making and lower the probability of committing an acceptance error for a cuckoo chick or a rejection error for one of their own chicks.

"When there are cuckoos in the area, you should call more to your eggs so that they have a higher call similarity after hatching and you can decide if the offspring is yours," Colombelli-Négrel and Kleindorfer wrote in an email. "We show a mechanism that starts in the nest and involves active teaching and sensorimotor learning in embryos."

Colombelli-Négrel, Kleindorfer, and their colleagues are continuing to study how fairy-wrens teach their passwords to their chicks. They're currently looking at how the fairy-wren embryos learn using heart rate and magnetic resonance imaging (MRI) scans, and whether parents do anything special to help their offspring learn, such as investing in egg nutrients that promote learning.

To beat a tricky brood parasite, superb fairy-wrens have to start teaching their offspring early and often. And when they detect a threat, these dedicated parents double-down on their teaching efforts to make sure their chicks get the message.

References:

Kleindorfer, S., Evans, C. and Colombelli-Négrel, D. (2014). Females that experience threat are better teachers. Biology Letters 10: 20140046. doi: 10.1098/rsbl.2014.0046.

Colombelli-Négrel, D., Hauber, M. E., Robertson, J., Sulloway, F. J., Hoi, H., Griggio, M. and Kleindorfer, S. (2012). Embryonic learning of vocal passwords in superb fairy-wrens reveals intruder cuckoo nestlings. Current Biology 22: 2155-2160. doi: 10.1016/j.cub.2012.09.025.