GENETIC-VARIATION AND BILL SIZE DIMORPHISM IN A PASSERINE BIRD, THE REED BUNTING EMBERIZA-SCHOENICLUS

In passerine birds morphological differentiation in bill size within s pecies is not commonly observed. Bill size is usually associated with a trophic niche, and strong differences in it may reflect the process of genetic differentiation and, possibly, speciation. We used both mit ochondrial DNA (mtDNA) and nuclear microsatellites to study genetic va riation between two subspecies of reed bunting, Emberiza schoeniclus s choeniclus and E.s. intermedia, along their distributional boundary in western Europe. These two subspecies are characterized by a high dimo rphism in bill size and, although breeding populations of the two subs pecies are found very close to each other in northern Italy, apparentl y no interbreeding occurs. The observed morphological pattern between the two subspecies may be maintained by geographically varying selecti ve forces or, alteratively, may be the result of a long geographical s eparation followed by a secondary contact. MtDNA sequences of cytochro me b and ND5 (515 bp) showed little variation and did not discriminate between the two subspecies, indicating a divergence time of less than 500 000 years. The analysis of four microsatellite loci suggested a c lear, although weak, degree of genetic differentiation in the large- a nd small-billed populations, as indicated by F-ST and R-ST values and genetic distances. The correlation between bill size and genetic dista nce between populations remained significant after accounting for the geographical distances between sampling localities. Altogether, these results indicate a very recent genetic differentiation between the two bill morphs and suggest that a strong selection for large bills in th e southern part of the breeding range is probably involved in maintain ing the geographical differentiation of this species.