An investigator went to Central America to study oropendulas, which are communal nesting birds. Another species of bird, the cowbird, sometimes lay its eggs in the nests of oropendulas. Some of the populations of oropendulas throw the cowbird eggs out of the nest, and some dont. The investigator was interested in finding out why some birds would raise other species as their own but others would toss them out. By watching the nests closely, he found that blowflies lay their eggs in the nests of oropendulas, and that the young larvae, maggots, feed on the young birds. If young cowbirds are in the nest, the precocious cowbirds eat the blowfly larvae, protecting the young oropendulas. In colonies of oropendulas that discriminate against cowbirds, throwing them from the nest, the blowflies are not eaten by cowbirds. These colonies of oropendulas build their nests close to a particular wasp colony, and the wasps eat the blowflies.
The relationship between the oropendulas that discriminate against cowbirds and the cowbirds is one of