Feather mites, pectoral muscle condition, wing length and plumage coloration of passerines

I compared the feather mite (Acari, Proctophyllodidae) loads of moulting bi rds with features of the new plumage that they were growing. I examined 21 samples, each sample containing individuals of the same species, sex and ag e class (juvenile, yearling or adult). I used nine species: wren, Troglodyt es troglodytes; dunnock, Prunella modularis; robin, Erithacus rubecula; blu e tit, Parus caeruleus; great tit, P. major; chaffinch, Fringilla coelebs; greenfinch, Carduelis chloris; linnet, C. cannabina; and yellowhammer, Embe riza citrinella. As previously reported for the house finch, Carpodacus mex icanus, birds with more feather mites grew duller plumage and relatively sh orter wings than less infested individuals of the same sex and age class. T hey also had lower protein reserves judged by the shape of their pectoral m uscle. Mite load was usually a better predictor of coloration than pectoral muscle score, but the opposite was true for changes in wing length. In con trast to house finches, birds whose plumage was dull before moult had highe r mite loads while moulting. Even if feather mites do not cause dull. pluma ge or short wings, they appear to be a good indicator of birds in poor phys iological condition. (C) 1999 The Association for the Study of Animal Behav iour.