In the rainforest of Central America, where predators abound, a cuckoo species found safety in numbers by building community nests guarded by two or three breeding pairs.
Why then do these pleasant aviaries sometimes abandon the collaborative lifestyle and lay eggs in nests outside the communal group, acting as social parasites, in the hope that other females will raise the pups as if they were theirs?
In an article published online in the journal NatureResearchers from Princeton show that cuckoos, known as Greater Anise (Crotophaga major), act collectively for the most part, but can become social parasites after their communal nest is destroyed. They begin the breeding season by putting all the eggs in the same basket, but if the predators intervene, the birds change to a strategy of spreading the eggs around other nests.
"When different females of a population seek different reproductive tactics, such as cooperative and parasitic nesting, this represents an evolutionary enigma. We wonder why some females lay their eggs in the nests of other groups, while other females never do, "said Christina Riehl, assistant professor of ecology and evolutionary biology. She and co-author Meghan Strong, a research specialist in the Department of Ecology and Evolutionary Biology, conducted the study with the help of undergraduate and graduate students and trainees, including several from the Princeton Environmental Institute (PEI).
"We found that parasitic strategy is more of a second-line option," Riehl said. "It's as if the birds said," If our attempt at cooperation fails, let's go to Option B. "
The team concluded that the two reproductive strategies can coexist in the population because the mixed strategy of cooperating first and then parasitizing produces approximately the same number of offspring as the pure cooperative strategy. If either was clearly better, all birds would probably adopt the most successful strategy.
The Greater Ani is one of the few bird species in which unrelated females come together to raise their offspring communally. In contrast, social parasitism is quite common, found in more than 250 species of birds.
To find out why the normally collaborative anis resort to parasitism, scientists have observed birds and their nesting behaviors over an 11-year period at the Barro Colorado Natural Monument in Panama. Birds build nests on branches that hang from the Panama Canal, requiring researchers to reach the nests by boat.
Under constant threat from predators such as snakes and monkeys, almost all anis begin their breeding season in community groups, the researchers found.
However, social parasitism is also widespread, researchers found, with about 25% of the nests being parasitized by a female who was not a member of the group.
The team found that females whose nests were destroyed by predators at the beginning of the nesting cycle were more likely to be parasitic, usually in nests near their own failed nest.
Researchers also asked whether moving to the middle station of the collective settlement for social parasitism can actually lead to more offspring.
Although putting as many eggs as possible seems like a good strategy, social parasitism has disadvantages. Eggs tend to be slightly smaller and therefore less likely to thrive. And because of the risk of being discovered, these eggs have a lower chance of survival until the puppies leave the nest.
In contrast, co-operative females lay fewer eggs, but work harder to care for them, so more chicks survive to leave the nest.
The take-away: Both reproductive strategies – community creation only versus the mixed approach of starting as communal breeders and switching to parasitism – are viable reproductive strategies.
Mark Hauber is an ornithologist and behavioral ecologist at the University of Illinois at Urbana-Champaign who was not involved in the study. "The study explains why cooperative reproduction is kept evolutionarily in this species – solitary breeding or parasitism alone are simply not productive enough," Hauber said. "But either cooperative breeding with large claws or failure of cooperative breeding followed by parasitism are equivalent strategies of adequacy. Thus, parasitism is still present in the population and so is co-operative creation. "
The team also found that the same females have turned to parasitism after the failure of their nests year after year.
"Previously, in my own work on zebra finches in captivity, I discovered that some individuals are more likely to become parasites than others," Hauber said. "This is now demonstrated and firmly confirmed in a wild population of tropical free-living birds."
The ongoing study monitors about 40 to 60 nests per year. Overall, for the current finding, the team studied genetic data of 1,776 eggs laid by 210 females in 240 nests from 2007-17.
Each year, the team begins the breeding season in June, searching for nests in the vegetation along the coast. Researchers check the nests daily, clearing each new egg to collect cells and blood left in the egg, with the goal of using genetics to identify the mother of each egg. They weigh and measure each egg and collect blood from the puppies to confirm their fatherhood.
"Getting into the nest is actually the hardest part of collecting measurements," said Luke Carabbia, of the class of 2019, who conducted field work as a PEI trainee. "While some groups of birds cooperate very well with us and nestle at accessible breast height, others build their nests to the top of their trees or in fragile reeds, and birds usually have a way of choosing places that are incredibly thorny or very close to a nest of angry wasps. "