Latitude, elevation and body mass variation in Andean passerine birds

1 Relationships between body mass and latitude, and body mass and elevation are examined in the assemblage of Andean passerine birds. 2 Across species, body mass is positively correlated with the mid-point of the species elevational distribution, but there is no significant relationship between body mass and latitudinal range mid-point. 3 When the assemblage is separated into Andean endemic and non-endemic species, the former group shows a significant positive relationship between body mass and elevation, and the latter a significant positive relationship between body mass and latitude ('Bergmann's rule'). Andean endemic species exhibit Bergmann's rule once elevation is controlled for using multiple regression. 4 These relationships are not a consequence of the phylogenetic non-independence of species. All the effects shown are very weak, with latitude and elevation explaining only a few per cent of the variation in body mass. Relationships are strong est when phylogenetically controlled analyses are performed just within genera. 5 The implications of these results for the mechanistic understanding of Bergmann's rule are discussed.