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Pterolichoidea and Freyaniidae) are among the commonest ec-tosymbionts of birds. Most researchers have assumed they are parasites, having negative effects on hosts. Here we present evidence that suggests that feather mites may not be parasites. We develop a framework for considering conditional outcomes in these interspecific associations, dealing with different kinds of relationships between symbionts. The non-parasitic status of feather mites is supported by a literature review as well as by preliminary data on mites ’ food. We illustrate symbiotic rela-tionships with a graphical model showing different scenarios in which hosts ’ cost-benefit relations are determined by the interac-tions among their symbionts. Birds are hosts to numerous symbionts, some of which can be parasitic, i.e decreasing the fitness of their hosts. Failure to detect costs of parasitism might be due t