Morphometric evidence for two subspecies of Belding’s Yellowthroat ( Geothlypis beldingi ) and implications for the species’ conservation status

Belding’s Yellowthroat (Geothlypis beldingi) is a passerine endemic to Baja California Sur (BCS), Mexico, where it breeds exclusively in a limited number of isolated and widely scattered perennial oases. Its recent downlisting from Endangered to Vulnerable was based largely on assumed effective population connectivity inferred from sporadic dispersal records, which confronts the long-standing hypothesis that posits its subdivision into two subspecies: G. beldingi goldmani in the north and G. beldingi beldingi in the south. Here, we evaluate the delimitation of both subspecies and assess the conservation implications of the species’ subdivision under the IUCN Red List framework. Using morphometric data from 45 adult individuals sampled from the largest, northern population (San Ignacio, BCS) and the second largest, southern population (Santiago, BCS), we applied univariate analyses, principal component analysis, discriminant function analysis, and 75% rule tests to quantify their morphometric divergence. Multivariate analyses revealed strong and concordant morphometric differentiation between populations, with an overall discriminant classification accuracy of 95.6% and complete diagnosability of females. No evidence of morphological intergradation was detected, and divergence patterns were consistent with long-term isolation across c. 250 km of unsuitable habitat, suggesting a restricted gene flow. Our results provide robust morphometric evidence that G. beldingi is subdivided into two allopatric population segments, meeting the IUCN definition of severe fragmentation. Given the species’ small total population size (estimated at 650–1,670 mature individuals), this fragmentation implies critically low effective population sizes, increasing vulnerability to demographic and genetic threats. Our findings challenge the assumption of a single, well-connected population underlying the recent downlisting, and support a reassessment of G. beldingi under IUCN’s criteria. While genetic data are needed to explicitly quantify reproductive isolation, in their absence, indicia of population subdivision of the kind presented here are nevertheless crucial for accurately assessing extinction risk and prioritizing the protection of the species’ critical habitats.