INTRODUCTION One of the main issues in the study of evolution of organisms is to determine how the environment promotes adaptation. Divergence of populations as a consequence of adaptation to resource diversity is of particular relevance to the process of specia- tion (Futuyma 1998, Schluter 2001). Speciation among Crossbills seems linked to morphological diversification promoted by adaptation to foraging on cones of a variety of conifers (Benkman 1987, 1993, 2003, Summers & Piertney 2003). Alternatively, processes of speciation in Crossbills have also been associated with either geographical or breeding isolation (Knox 1976, Groth 1993). Due to the high heterogeneity of populations, the systematics of Common Crossbills Loxia curvi- rostra has been the aim of several studies within the Palearctic (Knox 1976, 1990, 1992, Eck 1981, Massa 1987, Clouet & Joachim 1996) and the Nearctic (Groth 1993). For the Common Crossbill in N America, Groth (1993) described seven differ- ent types proposedly belonging to distinct sibling species. Taxa of Common Crossbills, such as sub- Morphological diversification of Common Crossbill Loxia curvirostra populations within Iberia and the Balearics Alonso D., Arizaga J., Miranda R. & Hernández M.A. 2006. Morphological diversification of Common Crossbill Loxia curvirostra populations within Iberia and the Balearics. Ardea 94(1): 99–107. Two subspecies of Common Crossbill Loxia curvirostra are currently accepted to be present in Iberia and the Balearics: L.c. curvirostra and L.c. balearica. Nonetheless, some populations in S Iberia are yet to be known, especially regarding their taxonomy. Biometrics of Crossbills from Navarra in N Iberia, Alicante and Málaga in S Iberia, and Mallorca in the Balearics were analysed by means of Discriminant Function Analysis, to describe their morphological diversification, and to quantify their degree of differentiation. Crossbills from Navarra and Mallorca strikingly differed from each other, supporting their status of nominate and balearica subspecies, respectively. By contrast, individuals from S Iberia showed intermediate traits, suggesting that Crossbills from S Iberia belong to an intermediate morphotype that differ significantly from Crossbills in N Iberia and the Balearics. Thus, data support that either adaptation to different resources or isolation, or both, cause mor- phological differentiation of populations of Crossbills. Key words: Common Crossbill, Loxia curvirostra, biometrics, Iberia, Balearics, discriminant function analysis 1 Department of Vertebrates, Aranzadi Sciences Society, Zorroagagaina 11, E-20014 Donostia, Spain; 2 Department of Zoology and Ecology, Faculty of Sciences, University of Navarra, Irunlarrea 1, E-31080 Pamplona, Spain; * corresponding author (jarizaga@alumni.unav.es) Daniel Alonso 1 , Juan Arizaga 1,2, *, Rafael Miranda 2 & M. Ángeles Hernández 2