This specimen is a male Analges spiniger Giebel, 1871. The species was first described from an icterine warbler, though this one was taken from a blackcap from Sherburn in Elmet.
The enlarged legs are allegedly used to grip the female when mating and "males of this genus cause damage to the female carapace ....". However, I have never seen any signs of this having looked at many specimens over the past twenty years.
The mites spend their entire life cycle on the host and feed on feather debris, pollen and fungal spores. They also take lipids (preening oil). (Barry Nattress)
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