Cuckoos and Crows Teach Us How Parasites Can Be Good

Crow sitting in a nest in a tree

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In a parasitic relationship, one species benefits in some way while another is harmed. Cuckoo birds have long been seen as parasites, because they lay their eggs in the nests of other birds. The cuckoo chicks then compete for food with the host's own babies.

However, new research is challenging our understanding of this relationship. Daniela Canestrari and her team at the University of Oviedo in Spain studied the nests of crows, both with and without cuckoos. They found that nests with both species actually fared better, because baby cuckoo birds actually defended the nests from predators, thereby boosting the crow population.

"In ecology, there are many many different kinds of interactions between different species," Canestrari told me. "What we concluded from this study is that classifying these interactions as parasitic or mutualistic maybe is not so correct, because sometimes these interactions can be quite complex."

Studying Cuckoo Birds

The data for this study was collected over the course of 16 years. Canestrari's team made the discovery while studying the social behaviors of crows. "We realized that this population was parasitized by the great spotted cuckoo," she said. As they monitored the crows' nests, they counted the number of eggs, the number that hatched and the number of chicks that fledged and left the nest.

"We realized by chance that the nests that were parasitized were more likely to be successful," said Canestrari. "So we decided to analyze the data." The analysis confirmed their findings.

Secretion Deters Predators

Newly hatched cuckoos release a noxious secretion when threatened. Researchers think this benefits all the hatchlings that share a nest, little crows and cuckoos alike, by deterring predators. Canestrari said they know there are predators in the area like cats, and birds of prey like raptors, but they aren't sure which are the biggest threat to crows. "Even crows can predate each other's nests," she said. This could be a direction for further research, with the help of continuous recording technology.

The secretion itself also merits further study. Canestrari said only cuckoo chicks produce this substance, which is a mixture of acids, indoles, phenols and sulfur-containing compounds. "Which are all compounds that are responsible for the odor. It's really bad."

Are All Parasites Bad?

Of course, the findings aren't saying there are no bad parasites. For example, it's hard to see any benefits for the host if the interaction with another species kills it. "It's a message of complexity," said Canestrari. In the case of crows and great spotted cuckoos, the benefit might be lost if the nests weren't threatened by predators. "The outcome of the interaction may change over time."

It will be interesting to see if this finding will inspire other ecologists to reevaluate other parasitic interactions. "We are curious to see if other researchers reach the same conclusions for other systems," said Canestrari.

The full findings are published in the March 21, 2014 issue of Science.