Odd ‘Nestfellows’

Written by
Julie Schwietert Collazo
Dec. 14, 2022

Searching for why some South American birds form parenting co-ops

Barro Colorado, a six-square-mile island in the middle of the Panama Canal, has been a nature reserve and a site of scientific research for nearly a century. Hosting more than 400 researchers and scholars annually, Princeton University professors and students among them, Barro Colorado is “the most intensively studied tropical forest in the world,” according to the Smithsonian Tropical Research Institute (STRI) website.

For decades, Princeton researchers have flocked to STRI to examine forest biology and biodiversity, responses of invertebrates to climate change and observations of howler monkeys’ behavior, to name a few. Add to that list the cooperative breeding system of the greater ani, which has been the focus of a 15 year study led by Christina Riehl, associate professor of ecology and evolutionary biology (EEB) at Princeton.

Two anis are perched near their nest; one is banded

Two anis are perched near their nest; one is banded. Photo by Luke Carabbia

Riehl says that while the ani is one of the island’s most common birds, it had been largely overlooked by researchers. Upon landing on the island in 2006 to study alongside her doctoral adviser, Martin Wikelski, now director of the Department of Migration of the Max Planck Institute of Animal Behavior at the University of Konstanz, Germany, who was working on several projects in Panama, Riehl realized that “very little was known about the greater ani, except that it was a communal nester like the groove-billed ani,” a species common in Costa Rica that had been studied extensively by Sandra Vehrencamp, a behavioral ecologist at Cornell. Riehl had read Vehrencamp’s papers and was fascinated by the species. “An understudied tropical bird with a bizarre cooperative breeding system?” she says of the greater ani. “I immediately started planning my dissertation.”

In the decade and a half she has spent in Panama — mostly during the greater ani’s summer breeding season — Riehl and her colleagues have been working to uncover why anis have such a unique breeding system, in which two to five pairs of birds nest, lay eggs, incubate and feed offspring communally, sometimes with unrelated group members. “This is unusual in animals,” Riehl notes, as “cooperative groups are usually composed of family members, and communal nesting isn’t easy.”

The greater ani’s abundance on Barro Colorado doesn’t mean that researching them is straightforward. Fieldwork is conducted entirely by boat because anis nest in marshy shoreline  vegetation. “Any experiment requires knowledge of the nesting biology of the group — how many birds are in the group, whether they’re laying eggs or raising chicks, where their nest is — and so a huge amount of our fieldwork is finding nests, marking eggs and nestlings, and taking blood samples for later analysis,” Riehl says.

Researchers also rely upon technology — and installing cameras or playing back alarm calls from a speaker can be a tricky task while floating in a boat. “In some ways, it’s a wonderful way to spend the day,” says Riehl. “But other days, when you get alternately baked by the sun and frozen by the rain — or worse, when the boat breaks down — it’s not so wonderful.”

Three greater ani nestlings are individually marked with non-toxic paint to aid in identification on video for a study on provisioning and begging behavior.

Three greater ani nestlings are individually marked with non-toxic paint to aid in identification on video for a study on provisioning and begging behavior. Photo by Amanda Savagian

Unsurprisingly, the COVID pandemic presented all kinds of challenges for researchers like Riehl, her students and her colleagues. “It was extremely hard not to be able to interact with the scientists, not to mention the anis, during the pandemic,” she says.

Riehl and her team have recently made some significant findings. This summer, doctoral candidate Amanda Savagian, working with postdoctoral research associate Josh Lapergola, both in EEB, designed a set of experiments to confirm her hypothesis that greater anis have a “raptor alarm,” a call reserved only for when they observe a bird of prey nearby. “It’s one of the few examples of referential communication in birds,” says Riehl. Lapergola spent the summer playing back raptor calls to anis and recording their behaviors, thus confirming the hypothesis.

But not all of the team’s questions about the greater ani have been answered, and the species still offers exciting avenues for further research. “How do so many birds ‘agree’ on anything?” Riehl asks. “How do they decide when to start laying eggs or where to build the nest? Why does a bird decide to stay in the same group or try to leave and join a different group? How do groups form to begin with? We still have only a hazy understanding of most of these questions.”