Scarlet Macaw parents 'play favorites,' purposefully neglecting younger chicks

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Scarlet macaws are a symbol of fidelity and virtue to many people because they are thought to mate for life—but it turns out that they also "play favorites" when feeding their young, making them excellent mates, but neglectful parents.

Fortunately, Texas A&M scientists have developed a way to ensure the birds' bad parenting results in fewer chick deaths.

Researchers at the College of Veterinary Medicine and Biomedical Sciences have discovered that scarlet macaws purposefully neglect feeding the youngest chicks in most broods, even when resources are plentiful. This results in only one or two chicks being able to fledge—the process in which parents teach their young to fly and survive on their own—even though broods may contain up to four chicks.

"Scientists have known for years that scarlet macaws hatch more chicks than they fledge," said Dr. Donald Brightsmith, a professor in the VMBS' Department of Veterinary Pathobiology. "We found that 26% of second chicks in scarlet macaw broods and nearly all third and fourth chicks die before fledging.

"We tested several theories as to why these younger chicks don't survive, and we found that it's not sibling rivalry or a lack of food. The parents just stop feeding certain chicks, so they starve to death," he said.

The deciding factor appears to be the difference in chick ages.

"Scarlet macaws lay eggs over a period of several days instead of all at once, which means the chicks don't hatch on the same day," said Dr. Gabriela Vigo-Trauco, a post-doctoral researcher with the Schubot Center for Avian Health, who led the project.

"If the second chick hatches only a couple of days after the first, there is a good chance that the parents will feed it. However, if it hatches four, five, or more days after the first chick, the parents will probably neglect it and let it die."

The researchers, who recently published their study in the journal Diversity, suspect that when chicks are young and hatch four or more days apart, they begin to need different types of parental care. For example, some chicks need feeding while others are still incubating, which contributes to the high rates of neglect and starvation.

Armed with this knowledge, the team has developed a method to save neglected chicks by giving them macaw "foster parents."

"Thankfully, scarlet macaws are not endangered or threatened, but there are many parrot species that are," Brightsmith said. "We hope that this foster program will be used to help save the populations of endangered parrot species."

Understanding brood reduction

Brood reduction, or eliminating chicks after they hatch, is a common practice among birds.

"Some species of seabirds, like boobies, gulls, and pelicans, have a high degree of sibling rivalry that can lead to death," Vigo-Trauco said. "Eagles, falcons and other species are known to attack and kill their own chicks. Often, parents target the smaller chicks, which are usually younger."

For scarlet macaws, starvation is the cause of 45% of all chick deaths; chicks that hatch third or fourth in a brood always die.

"We wanted to understand what was causing 26% of second chicks to die of starvation," Brightsmith said. "Scientists often point the finger at a lack of resources in the environment, but if it's about conserving resources, then why do scarlet macaws lay so many eggs?"

To rule out lack of resources, Gustavo Martinez, a member of the research team, marked and monitored trees to estimate the amount of fruit—scarlet macaws' main source of food—in the forests of the Tambopata National Reserve in Peru where the study was conducted.

"Once a month for several years, he would go out and check about 1,300 trees for fruit and flowers," Brightsmith said. "Looking at the data, we can tell that there were times when food scarcity forced macaws to forgo breeding for a season, but we couldn't find any association with chick starvation."

Catching macaws in the act

To see what was happening in scarlet macaw nests, the researchers installed cameras in nest boxes at the Tambopata National Reserve in Peru. For 10 years, they captured video segments showing what was happening to the chicks.

They also accessed the nests and manually checked which chicks had received food, which was how they determined that some chicks were intentionally being starved.

"Scarlet macaws have a food sac on their necks called a crop, and in chicks it's very easy to see when it's full of food," Brightsmith said. "We caught video of female macaws trying to over-feed their oldest chicks while the third chick would be running slowly around the base of the nest with an empty crop, begging for food.

"What's more, the chicks at that age can't regulate their own body temperature, so they need to be in the nest. We saw that the mother won't even share her body heat with her dying offspring," he said. "As scientists, we try not to do what's called anthropomorphizing—attaching human ideas about morality to animals. But it's hard to watch that and not think of it as parental abuse."

The "abusive behavior" goes even further—but it seems that macaw parents aren't always on the same page.

"Sometimes the female macaw will start to bury a chick that she's decided not to feed by kicking nest substrate on top of it," Brightsmith said. "But then the father will come home and unbury the chick and feed it. So, they're not always in agreement, which makes the whole process even more complex."

Saving neglected chicks

While scarlet macaws have some questionable parenting techniques for their own hatchlings, the good news is they also make excellent foster families for neglected chicks.

As part of her doctoral research, Vigo-Trauco developed a program for saving neglected chicks. The chicks are raised in captivity for a few weeks before being placed in the nests of macaws with chicks at a similar developmental stage or that have lost all their chicks to predation.

The program effectively eliminates the need for different types of parental care and allows the foster parents to raise chicks that would have starved.

"The key to success is making sure that the chicks all look about the same size," she said. "This encourages the new parents to take care of the foster chick as if it were their own."

While the macaws seem to notice that something is different, that doesn't stop them from adopting the foster chick.

"We see them on camera as they land on the nest box, look in, and then look around like they're thinking, "Did I walk into the wrong house?" It's kind of hilarious," Brightsmith said. "They turn their heads sideways to get a good look at the new chick, think about it for a moment, and then start to feed them."

The foster chick program, published in Diversity in 2021, successfully re-homed 28 chicks over the course of three breeding seasons.

"Parrots are one of the most endangered groups of birds in the world," Brightsmith said. "We hope that this program, and the understanding of brood reduction behind it, can assist with the conservation of a broad array of parrot species across the tropics."

More information: Gabriela Vigo-Trauco et al, Age Difference, Not Food Scarcity or Sibling Interactions, May Drive Brood Reduction in Wild Scarlet Macaws in Southeastern Peru, Diversity (2024). DOI: 10.3390/d16110657

Provided by Texas A&M University