Morphological differentiation of breeding and migratory Linnets Linaria cannabina in eastern Spain

Abstract

This paper aims to assess whether breeding and migratory populations of Linnets (Linaria cannabina) in eastern Spain show morphological differentiation and whether migration timing correlates with morphometric traits. For that, we collected biometric data from over 4,000 Linnets captured in breeding sites and at coastal migratory stopovers in Castellón, Spain. We used Principal Component Analyses and General Linear Models to test for differences across age, sex, breeding status and season. Morphological traits varied significantly between first- and second-calendar-year birds, especially feather and skeletal measurements. Migratory individuals showed a larger body size (compatible with a more northern origin according to Bergmann’s Rule), proportionally higher and wider bills (which might reflect geographical trophic preferences), and a higher wing-tail ratio (compatible with a longer migration distance like morphology). Within each migration season, birds passing later tended to have traits associated with a more northern origin, suggesting population-level differences in migration phenology. Morphometric variation in Linnets reflected both migratory strategy and passage timing, highlighting the utility of biometric analyses for understanding migration dynamics and population structure in passerines.