Sexual selection for white tail spots in the barn swallow in relation to habitat choice by feather lice

Many bird species have white spots in their tails or wing feathers, and suc h characters have been hypothesized to be either reliable signals (handicap s) or amplifiers that facilitate the message of a signal. In barn swallows, Hirundo rustica, the size of the white spots in the tail feathers is sexua lly dimorphic and positively correlated with feather length. We tested whet her such spots act as handicaps or amplifiers. These white spots affect sex ual selection in barn swallows, as shown by an experiment in which we rando mly subjected males to (1) a considerable reduction of the size of all the spots by the use of a black permanent marker pen, (2) a small reduction of the size of the spots, or (3) no reduction. There was a positive associatio n between spot size and the number of offspring produced per season. The wh ite tail spots were preferred by feather-eating Mallophaga as a feeding sit e: holes made by Mallophaga were more abundant in the white spots than expe cted by chance. A habitat choice experiment with Mallophaga on barn swallow tail feathers revealed that they preferred white spots over black parts of the tail feathers. We therefore expected long-tailed male barn swallows to have more Mallophaga than short-tailed males. However, the opposite relati onship was observed, indicating that long-tailed males may reliably signal their quality by the presence of large white tail spots without parasite da mage. Thus white tail spots in barn swallows appear to be a reliable signal of phenotypic quality. (C) 1999 The Association for the Study of Animal Be haviour.