Why these parrots sometimes kill each other’s chicks

Green-rumped parrotlets appear to have high instances of both adoption and infanticide, showcasing two very different sides of life in the animal kingdom. A study published May 6 in the journal Proceedings of the National Academy of Sciences (PNAS) details what drives them to go to two extremes when it comes to handling another bird’s chicks.

Green-rumped parrotlets (Forpus passerinus) are a very small and gregarious bird found in parts of South America and the Caribbean. They feed on fruits, seeds, and various flowers and are known for their bright green hues. Unlike other parrots that prefer to live higher up in the forest canopy, Green-rumped parrotlets tend to nest in hollowed-out trees or fence posts in grasslands.

These parrotlets are not the only animals who have been observed adopting or killing one another’s babies. Numerous species of amphibians, fish, rodents, felines, and primates have all been observed killing babies of their own kind and–in some cases–their own offspring. Both males and females may kill the offspring of their rivals in order to secure a social or sexual advantage for themselves. A male animal may kill the offspring of a female whose mate has died so that he can reproduce with her faster. Others have been observed caring for the babies of dead or missing animals.

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For Green-rumped parrotlets, both infanticide and adoption have to do with finding two necessities for survival:a nest and a mate.

“Most of the infanticide attacks happened when a breeding pair was attacked by another pair that was trying to take over a coveted nest site,” study co-author and University of California, Berkeley conservation biologist Steven Beissinger said in a statement.

In the study, a team created artificial nesting sites built out of PVC pipes and installed them on a cattle ranch in Guárico, Venezuela. Colorful bands were also placed on the parrotlets to track the individual birds and their relationships. They found dead baby birds in the nest without a clear indication of what had killed them.

“We couldn’t tell if something had attacked them, or if they had died from disease, or something else,” said Beissinger. “But when we were watching some of the nests, all of a sudden in went a male who didn’t belong—who wasn’t a parent at that nest—and out he came with a little bit of blood on his beak.”

The team began to track instances of the behavior and also referenced observations of more than 2,700 nests taken between 1988 and 2015 in the study. Among parrotlets, it appears that competition over nesting sites is the primary motivation for the attacks, and not reproduction.

In the nests that the team monitored, parrotlets killed or wounded nestlings and eggs at 256 sites. Most of the attacks appeared to be carried out by a single parrotlet or a breeding pair that came in and claimed the nesting site for themselves afterwards. The attacks also occurred more often when the parrotlet population was high and there was more competition for good nesting sites.

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“It’s not that everybody’s born a killer, but the urge to breed is very strong,” study co-author and University of Texas Rio Grande Valley avian ecologist Karl Berg said in a statement. “When the resources provided by the environment aren’t enough for all individuals to breed, they seek out alternative strategies. Unfortunately, that involves killing innocent little offspring.”

Infanticide was also observed in nests where one parent had died and the living parent had found a new mate. The new mates were just as likely to either adopt unrelated offspring or kill them. Choosing to become a stepparent did not ultimately hurt their changes at reproductive success. The males who adopted chicks that were unrelated to them and nested with females also started to breed at a younger age.

According to the team, adoption is more difficult to understand since it challenges older ideas about natural selection. An evolutionary goal for most animals is to get as many copies of your genes into the next generation as possible. Adopting a niece or nephew may make biological sense since they share roughly 25 percent of your genetic material. However, the advantage of adopting the offspring of an unrelated animal does not make quite as much sense, but still occurs.

“It was very interesting to see that the reproductive fitness outcomes were about even between adoption and infanticide and suggests that they have an alternative strategy,” said Berg. “Adoption may be a non-violent means of getting genes into the next generation.”