The “rim-laying” Black Robins of the Chatham Islands


The islands of New Zealand are, like those throughout much of the Pacific, a hotbed of ornithological endemism.  Thousands – and millions – of years in isolation from mammalian predators led to the evolution of vast numbers of species poorly adapted to deal with nest predation from introduced cats, rats, and others.  As a result, one of the greatest adaptive radiations in vertebrate history is to us now a shadow of its former self, and many species are hanging by the proverbial thread.

One such critically endangered bird is the Black Robin (Petroica traversi) of the Chatham Islands.  This is a species saved from the very brink of extinction by Herculean conservation efforts that included an aggressive captive breeding program.  As part of that program, every egg from every fertile female was precious, even those laid by females on the rim of their nests as opposed to properly, in the nest cup.

Black Robin photo by Frances Schmechel

Black Robin photo by Frances Schmechel

In nature, there is variation in everything.  For example, I just cracked open the nearest field guide at hand (O’Brien, Crossley, and Karlson’s “The Shorebird Guide“), opened up to a random page (Red-necked Phalarope), and learned that the wingspan of this species ranges from 32 to 41 cm but length only ranges 18–19 cm.  This means that among individuals that seem to be of uniform size, some have relatively much longer wings than others. That variability in a population is the foundation on which natural selection acts, and evolution occurs.

Physical features aren’t the only things that vary of course, behaviors vary too.  Some of us are naturally outgoing while others among us are shy, for example.  In Black Robins, one key aspect of reproductive behavior concerns egg placement in nests:  some mothers lay their eggs consistently in the cup whilst others are more haphazard, and frequently lay their eggs on the rim.

By collecting every viable egg in the captive breeding program, however, conservationists were inadvertently increasing the reproductive success of females with a highly undesirable trait.  The result was an increase in the population of females who laid their eggs on the rims of their nest, because that trait was tied to an allele that was passed on to subsequent generations.

With more individuals in the recovering population, conservationists can now be more appropriately selective in determining which individuals are allowed to breed and which are not, but this was a close call. From as many as 50% of the females in the population laying eggs on the rims, that trait has now fallen to about 10%.

This sobering story for conservation is another excellent example of how natural selection acts on individuals leading to evolution of populations.  Thanks go to Ed Yong and his article “In Saving a Species, You Might Accidentally Doom It” published this week at nationalgeographic.com.  Yong’s article links to the original paper outlining this issue, Massaro et al. 2013.

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This entry was posted in animal behavior, bird banding, bird evolution, birds/nature, Endangered Species Act, environment, evolution, history, IUCN, life, Links, skepticism and science, tuatara, wildlife and tagged , , , , , , , , , , , , . Bookmark the permalink.

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