How 'Bad Parenting' Among Birds Could be Evolution at Work

Many will argue that there is no worse parent than one who abandons their child - especially for selfish reasons. Unfortunately, evolution doesn't select for what we think is "right" but for what works best. Now, new research has found that not only is abandoning a chick to be raised by others beneficial for zebra finches, but it may even be a trait that is selected for - improving the species as a whole.

What we're talking about here is called "brood parasitism" and it's all that different from when an unfit parent hands his or her newborn child off to a stranger before dashing away.

The zebra finch (Taeniopygia guttata), for instance will sometimes lay it eggs in another harmless species' nest, rather than preparing a nest itself. The unwitting victim birds will incubate the eggs, along with their own egg clutch, none-the-wiser... at least until a newborn finch hatches.

Past research has shown that nest owners will try to prevent this from happening by taking proactive action. Primarily, they try to stop zebra finches from reproducing in the first place, violently harassing coupling finches or driving off would-be egg-layers from their nest.

However, once the deed is done the 'good parent' birds often still stick to the course, raising even a finch chick like one of their own until it flies away to go and shove more foreign chicks onto another unsuspecting parent.

The fact that zebra finches will do this again and again, despite no cues, implies that this is a behavior etched into the very nature of the finch. But how did it get there? What makes being a dead-beat parent a trait that the natural order of things would favor?

For one, without having to put time into incubating eggs and caring for young, adult zebra finches can pump out more fertilized eggs than most other birds. They essentially spend all their time making babies, and let other species worry about raising them. And if this behavior is hereditary, it would then make sense how zebra finch populations quickly became flooded with practitioners of bad parenting. (Scroll to read on...)

Still, unlike with the 'chicken or the egg,' birds being good parents clearly came first. Something must of taken place to push the finches' onto this strange evolutionary path.

"Consider this example," researcher William Feeney, recently bade readers of The Conversation. "If a bird lays its eggs over several days, but its nest gets destroyed before it completes its clutch, it could salvage the remaining eggs by laying them in the nests of birds from other species. If the offspring was successfully raised by the unsuspecting host, this unexpected brood parasitism event may provide a stepping stone this behavior to evolve further."

That's where a new study, recently published in the journal Ecology and Evolution, comes in, detailing how Feeney and his colleagues conducted a revealing experiment on 17 pairs of expecting zebra finches.

In an enclosed aviary, the researchers set out three identical nests, as well as some nesting material, and let a finch pair choose a nest and lay an egg. Then, before another egg could be laid, their nest was destroyed. Additionally zebra finch egg was added to one of the remaining nests, and a Bengalese finch egg was added to the other.

"Bengalese finches are a closely related to zebra finches, and lay eggs that are larger and sometimes different in color to zebra finch eggs," the Feeney added.

The expecting finches now had a choice: start from scratch, or lay the egg in one of the two nests. Without fail, they always chose the latter option, but it did not seem to matter to them next which species - as indicated by the egg - might be raising their young.

This, Feeney and his colleagues argue, suggests that a similar scenario could have gotten the ball that is brood parasitism rolling.

"These are the kinds of biological leaps that Darwin discovered when he first studied finches," he said. And, even today, we are still learning."

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