Understanding species’ abundance is a main topic in ecology. Much has been advanced to understand the abundance of free-living organisms, but less has been done to understand species abundance in host-symbiont systems. In this PhD project we study the interaction between birds and feather mites, using a double (host and symbiont) approach to understand the reasons why different host species harbor different symbiont abundance, and why different symbiont species have different abundances in different host species.