On Friday we introduced the concept of “brood parasitism”, where one organism will leave its offspring to be raised by another. It’s a tactic meant to conserve the resources of the parent parasite, and in extreme cases where the invading brood kicks its foster siblings out of the nest, serves to lower the amount of individuals of the “host” species that are in direct competition with the parasite.

Upon seeing the photo of the tiny warbler feeding a giant cuckoo hatchling—overflowing from its tiny nest—many wonder, “How can the parent not notice that its “child” is a giant impostor?” Enter supernormal stimuli. When the warbler sees the larger cuckoo egg in its nest, its first instinct is not “This is not my egg” but rather “This is a really healthy egg” and pays extra attention to it. Similarly, the giant cuckoo in the photograph is likely perceived by its proud “mother” as a “healthy growing boy”. In experimental settings, scientists have been able to replicate this tendency by introducing fake eggs into nests. In an unfortunately comical fashion, the mother bird would favor the larger eggs, even if they were larger than the bird itself.

It’s easy to laugh at the poor bird sliding off a giant plaster egg, but humans aren’t exactly immune to supernormal stimuli either. Harvard psychologist Deirdre Barrett argues that cute toys, junk food, and porn are all examples of supernormal stimuli in our own lives. James Randi Educational Foundation president D.J. Grothe has a great interview with Barrett on his podcast, For Good Reason. Check it out.


Now then, back to the comic. All that buildup on Friday was around this paper published 2 weeks ago in Current Biology. To combat brood parasitism, the superb fairywren has taken to identifying its children by song. It’s not rote memorization by any means, the hatchlings pick up on distinct notes from the mother’s song and include it in their begging cries. The mother wren recognizes the notes, and favors those hatchlings. The song is parent specific, which means that wren hatchlings that were switched into different nests, did not fare well either. It’s a fascinating development in the arms race between parasite and host, and it serves to illustrate the importance of recognizing offspring.


Seth Shostak joined the cast earlier this year during the 2012 Super Moon series.