Morphology and morphometry of two hybridizing buntings at their hybrid zone in northern Iran reveal intermediate and transgressive morphotypes

The closely related Black-headed Bunting (Emberiza melanocephala, a western Palearctic lineage) and Red-headed Bunting (Emberiza bruniceps, an eastern Palearctic lineage) hybridize and replace each other south of the Caspian Sea. The parental species have distinct phenotypes and therefore morphology is useful for assessing hybridization in the contact zone. In the years of 1940 and 1977, quite a few hybrids were collected and studied morphologically. Since then, the hybrid zone appears to have expanded westwards, but there has been a time gap in the collection of morphological data. Here we reanalyze bunting specimens morphologically and compare the historical data with recent data. Morphometric and phenotypic traits from three time periods (1940, 1977 and recent) were studied to assess phenotypic variation of hybrids, pattern of hybridization, and transgressive traits in the hybrid zone. Our results show that most of the birds in the hybrid zone exhibit intermediate phenotypes (both colors and morphometric characters), ranging from the pure phenotype of either of the parental species. However, hybridization has also produced novel phenotypes not seen in any of the parents. Using a canonical discriminant function analysis, the morphometric characters separated each parental species and the hybrids quite well. Our results showed morphometric intermediacy of hybrids in accordance with phenotypes. We observe a time trend in which recent hybrids are more similar to Red-headed Buntings phenotypically compared to historical samples. This pattern is likely a signature of a westward expansion of the Red-headed Bunting into the breeding range of the Black-headed Bunting.