Showiness, non-parasitic symbionts, and nutritional condition in a passerine bird

The study of host-symbiont interactions has mainly focused on how parasites influence the fitness and behaviour of their host. Two common assumptions of such studies are that all symbiontic organisms are parasites, and that t hey are directly or indirectly related to host traits which act as signals of quality in mate choice. We assessed the prevalence and abundance of feat her mites (Proctophyllodes pinnatus; Astigmata; Protocphyllodidae) on Linne ts (Carduelis cannabina; Aves; Carduelinae) and examined how the abundance of mites may be related to the condition and plumage colour of Linnets. Mit e prevalence was so high (near 100%) that no variation could be found betwe en age classes or sexes. However, mite abundance was significantly higher i n males than in females. No significant relationship was found between mite abundance and host nutritional condition, except for yearling females. Nut ritional condition and showiness of secondary sexual traits (carotenoid-bas ed plumage patches) were not correlated in male linnets. Size and brightnes s of red patches in the plumage of males varied with age, but were unrelate d to mite abundance when age was taken into account. The absence of effects of mites is in agreement with the long held, neglected view that feather m ites form non-parasitic associations with birds. Higher mite abundance on m ales may be attributed to more secretions of the uropygial gland, on which mites feed. Gender variation in plumage colour and abundance of feather mit es suggests a common hormonal-mediated mechanism regulating both traits.