SEXUAL SELECTION IN THE BARN SWALLOW (HIRUNDO-RUSTICA) .5. GEOGRAPHIC-VARIATION IN ORNAMENT SIZE

Models of sexual selection in a dine predict the patterns of clinal va riation in female mate preference and male secondary sexual characters . These predictions were tested for the nominate subspecies of the bar n swallow Hirundo rustica which demonstrates clinal variation in morph ology, with several characters in both sexes showing increasing size a t higher latitudes. Sexual size dimorphism in the length of the tail o rnament and the short, central tail feathers increase with increasing latitude while size dimorphism in other morphological characters is in dependent of latitude. The main reason for the two divergent patterns of sexual size dimorphism appears to be the higher foraging cost of ha ving a long tail ornamental at low latitudes. The control of developme nt decreases with increasing latitude as demonstrated by an increasing latitudinal dine in fluctuating asymmetry of tail length. Phenotypic variance in tail length increases with latitude in males, but not in f emales, as shown by the coefficients of variation. Clinal variation in morphology is not due to natural selection associated with a latitudi nal increase in the distance between breeding and wintering areas. The geographic patterns of morphological variation suggest that the tail character has diverged geographically as a result of a sexual process of reliable signalling.