Geographical variation in Tachycineta Swallows (Aves: Passeriformes: Hirundinidae), with description of a new subspecies from Colombia
Authors:   Unaffiliated, Donegan Thomas M. 
Published: December 2024

Dark-winged Tachycineta swallows occurring west of the Andes in Colombia have confounded observers for decades. With a view to assessing their taxonomic rank, geographical variations in voice, plumage and biometrics were studied as between western Colombian birds ("Magdalena Swallows") and all members of subgenus Leucochelidon, i.e. Mangrove Swallow T. albilinea of Central America, White-winged Swallow T. albiventer of East of the Andes, Chilean Swallow T. meyeni and White-rumped Swallow T. leucorrhoa of southern South America and Tumbes Swallow T. stolzmanni of western Ecuador and Peru. A 'yardstick' methodology was applied, measuring and comparing differentiation between species using up to 18 vocal variables across 4 vocalization types as well as 9 biometric characters. Chilean versus White-rumped Swallow are a closely-related pair of valid, sympatric species, whose measured differentiation was compared to that between Magdalena and other swallows to assess their taxonomic rank. Vocal and morphological characters gave conflicting results. The rising calls of Magdalena Swallow are diagnosable in multidimensional space from White-winged Swallow, with slower pace, less change in frequency and overall lower frequency. Also, vocal repertoire differed, with 'buzz' calls predominating in Magdalena Swallow but 'rising' calls being more common in White-winged. Measured vocal differentiation exceeded that between Chilean and White-rumped Swallows and differences in repertoire were consistent. In contrast, Magdalena Swallow was non-diagnosably differentiated in plumage and biometrics from White-winged Swallow. Magdalena Swallows usually have a darker, navy-blue mantle (this usually being aquamarine or green-blue in White-winged Swallow but variable), and reduced white markings on the secondaries and wing coverts (which are typically extensive in White-winged Swallow but variable in both, with overlap). Biometrics largely overlap but Magdalena Swallows have an average shorter tarsus and bill. The molecular biology of Magdalena Swallow is unknown. Reported pairwise molecular differentiations between other species in the genus in previous published studies were tested for correlations against measured differentiation in all vocal and biometric variables. Magdalena Swallow achieved higher differentiation in vocal variables which correlated with molecular differentiation but low differentiation in biometric variables which also correlated with molecular differentiation, again a conflicting outcome. Considering differences in voice, plumage and biometrics, the Magdalena Swallow is here described as a new subspecies. The occurrence of dark-winged White-winged Swallows in western Colombia has led to dubious claims of both Mangrove Swallow and Bahama Swallow T. cyaneoviridis in this region, based on sight records. A 'Colombia' specimen of Mangrove Swallow was found in this study. Together with an intermediate specimen (showing a pale lore), there is evidence of occasional vagrancy of Mangrove Swallow to northern Colombia. There is some geographical variation in biometrics and plumage among Eastern populations of T. albiventer, but vocal differentiation is weak. The type locality of T. albiventer is in Cayenne, French Guyana, whilst western Amazonian birds were once named as aequatorialis due to their paler blue dorsal and more extensive white wing markings, but these variations are non-diagnostic. Populations of White-winged Swallow in the Atlantic and Cerrado regions of Brazil are taxonomically undescribed. These have a greenish mantle coloration (a rare morphotype in Amazonia) and often have a longer tail and white tail tips or remiges. Measured biometric and vocal differentiation of this population closely misses requirements for subspecies rank, but study of a larger sample of Brazilian specimens should be considered.