WRENS CAN BE SUPERB EXAMPLES OF EVOLUTION

by Whit Gibbons

September 30, 2007

Last week I noted that evolution can be an easily understandable concept in its simplest form but that some biological mysteries can be difficult to unravel even by evolutionary biologists. A true life example with an Australian bird, the superb fairy-wren or blue wren, shows how complex some evolutionary processes can be for scientists to decipher.

I once saw a flock of these pixie-like birds flitting around in Royal National Park near Sydney. They reminded me of blue-colored Tinkerbells. But aside from being pretty little birds, these wrens have an interesting trait in common with many other birds, including red-cockaded woodpeckers. That is, the parents have helpers during the period when young are in the nest. A helper for a pair of birds is typically a young, non-breeding male that brings food to nestlings.

The parents get a break from the chore of finding food for themselves as well as for their young. From an ecological viewpoint, the benefit to the parents and young seems obvious: the presence of helpers assures that the offspring are well fed, which would mean they grow faster and get larger than babies in a comparable situation in which only the parents are providing food. The parents benefit by producing bigger babies that have a better chance for survival.

From an evolutionary perspective, a benefit to a helper male participating in this cooperative breeding system is through what is known as kin selection, whereby the individual is assisting close relatives. Hence, the helper is assuring its own reproductive success indirectly by increasing the survival probabilities of individuals that carry many of the same genes as it does. Another way in which such cooperative behavior might benefit the helpers themselves is by increasing their own efficiency at gathering resources. By being part of a group engaged in gathering food they learn from being in the company of others.

A study by scientists at the University of Sheffield in England addressed a previously unexplained situation in which the offspring of fairy-wrens did not grow faster or get bigger when helper males were present. Instead, they usually grew at the same rate and reached the same size before leaving the nest as nestlings that were being fed solely by the parents. The mystery: why wouldn't baby birds that are having their diets supplemented by helper males get bigger faster?

The British biologists discovered a subtle and simple but very important distinction between helper-fed baby wrens, which got almost 20 percent more food, compared to those being fed only by their parents. The scientists conducted their study on two types of fairy-wren groups, one with helpers present and the other with only breeding pairs of the birds. They observed that when helper males were absent in a breeding group at egg-laying time, female fairy-wrens laid larger eggs with a higher nutritional content. If helpers were present, they laid smaller eggs. Both the larger and smaller eggs of fairy-wrens took the same amount of time to incubate, and the babies stayed in the nest for the same amount of time before fledging. So why would females lay smaller eggs with less nutrition when they clearly are capable of laying larger, more nutritional eggs?

The scientists did experiments in which, unbeknownst to the birds, they exchanged eggs between nests with and without helpers. When smaller eggs were substituted for the larger eggs of parents without helpers, the size and ultimately the survival of the young birds were reduced. The experiments confirmed that the mother wrens were anticipating whether they could expect supplemental feedings from helpers and adjusting the quality of the eggs accordingly. If helpers are present, the female lays small eggs, because helper birds provide the extra food needed for babies to reach the preferred size.

Charles Darwin presented his ideas more than a century ago, but his principles of natural selection and survival of the fittest continue to explain how many intricacies of life work. And what is the advantage to the female to lay small eggs when opportunity arises? Females that invest in smaller eggs with lower nutritional value are more likely to survive longer and produce more eggs in the future.



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