How to protect your kids from brood parasites: Talk to them!
(Originally published December 16th, 2012 on http://scienceofbirds.blogspot.com)
Let’s do some science, shall we?
Lots of birds have problems with brood parasites, such as the cowbirds of the new world and cuckoos of the old world. Most brood parasites have adapted a faster incubation period and a faster growth period, so that they can hatch before the parasitized bird’s actual eggs and either push them out of the nest, or just take all the food so the other chicks starve. However they do it, there are various solutions that birds have adapted to avoid this problem. Some kill the parasite’s egg, some abandon their nest, but by far one of the most unique solutions are that of the Fairy-wrens, a fascinating group of songbirds native to Australia that could be described as chickadees plus hummingbird-level color.
A recent study has revealed that fairy-wren mothers begin communicating with their offspring before they have even hatched. During incubation, female Fairy-wrens give a sort of incubation call that, when the chicks hatch, will become a vocal password for food. Without this vocal password, the chicks will not be fed. She gives this call repetitively late in her offspring’s incubation period, and is embedded in their brains when they hatch. This vocal password, then, is their begging call and isunique to each nest, so mothers always know which chicks are actually theirs, or at least which chicks they incubated. Mothers use these
Let’s go through this:
The female lays and and begins incubating her eggs. The eggs typically take about 15 days to hatch, so the female waits until day 10 to start giving her incubation calls, both so that her embryonic offspring can actually hear her, and so that she doesn’t spend the entire incubation period giving her nest away to predators. Timing is where the tables are turned for the early hatching bronze-cuckoos’ (see left) chicks that frequently parasitize fairy-wrens’ broods: bronze-cuckoos hatch after 12 days rather than 15, so they don’t get nearly as much time within the egg to learn the vocal password. Without the correct vocal password, the fairy-wren parents will not feed the chick, so the brood parasite never gets fed, while the actual fairy-wren chicks, hatching three days later with the correct begging calls, are given all the food. Case closed, end of story. Suddenly, the early hatching habits of the bronze-cuckoo that were once an adaptation become a HUGE disadvantage. Cool stuff, right?
This discovery leads to a few questions, too. First off, if Fairy-wrens learn their begging calls during incubation, do they have any begging calls genetically programmed? The answer, it turns out, is no; Fairy-wren chicks develop with a blank slate, and learn everything in the egg. The significant implication here is well put by one of the researchers:
“These findings demonstrate that traits that appear to be innate, such as nestling begging calls, may actually be learned. Knowing this adds to our understanding of evolution and also has many practical implications, particularly for captive breeding and conservation biology.”
This also means that if a mother’s actual offspring are replaced by unrelated offspring during incubation, the mother’s actual offspring will not know the correct call to be fed by their actual mother. It all depends on learning the right call. Another obvious question: how do males learn the calls? They don’t participate at all in the incubation, but they still know the vocal password, so the female must communicate it in some way. The researchers in Australia had similar questions, and they found that females share the vocal password with their mates through a so-called “solicitation song”. This way, there are no loop-holes in the vocal password system. Finally, we’ll end with a prediction in the next step in what has now become an “acoustic arms race” between brood parasite and Fairy-wren: in order to survive, Bronze-cuckoos will have to adapt to their smaller window for learning the vocal password, by, well, learning it faster.
Whatever the cuckoo’s “counter-solution”, it seems the relationship between parasite and parasitized always produces fascinating discoveries. The big question now is if any species here in North America have adapted a similar system. The encroachment of farmland onto what were once eastern woodlands safe from brood parasitism has brought birds like Red-eyed Vireos, a species completely without adaptation against brood parasitism, face to face with Brown-headed Cowbirds, who like open areas more than forests. It will be interesting to see where evolution takes this conflict, and this discovery on these diminutive, color-blasted Australian songbirds gives us new insight into this process.
Have a great day, everyone.
