Read passage and answer following question.

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