Investigating the movements of migratory thrushes Turdus merula and T. iliacus using intrinsic markers and morphology

Abstract

Stable isotopes of carbon (813C ), nitrogen (615N) and hydrogen (62H ) and microsatellite markers were used in conjunction with morphological data, to test whether breeding origins of two migratory passerines the Blackbird Turdus merula, and Redwing T iliacus, overwintering in the UK & Ireland could be predicted, based on data collected at European breeding sites. Geographical trends of feather 62H and 613C values of birds sampleda t Europeanb reeding sites were useful for determining breeding origins of Blackbirds at a broad geographical scale. Genetic markers were useful for discriminating between the two Redwing races iliacus and coburni. Conversely, no genetic structure was found within the nominate race iliacus and there was only weak genetic structure in Blackbird populations, suggesting relatively high gene flow. These results indicate that genetic markers are of limited value for population assigm-nenot f either species. Mean wing length of breeding Blackbirds differed significantly between breeding regions, and was positively correlated with latitude, suggesting that wing length is potentially a useful variable for discriminating between Blackbirds of different breeding origin. It was predicted that combining techniques would prove substantially more useful for assigning individuals to their most likely origin, than any one technique used in isolation. For Blackbirds, the combination of 62H and 813C values with wing length proved to be the most effective combination of variables (and was more effective than using either stable isotopes or wing length alone), allowing 72.2% to 76.3% of breeding Blackbirds to be correctly and consistently assigned to one of three broad geographic regions (UK, Fennoscandia and Continent). For Redwings, using DNA markers alone, 94.5% of birds were correctly assigned to either the coburni or iliacus; the addition of stable isotopes produced only a marginal improvement. Useful insights into the contrasting migratory strategies of Blackbirds and Redwings were revealed. The lack of genetic variation within the nominate Redwing race indicates a lack of migratory connectivity. In contrast, weak genetic population structure in the Blackbird, and differences in stable isotope i ratios and wing length between birds sampled at different wintering sites, suggests that different parts of the UK & Ireland may receive differing proportions of migrant Blackbirds originating from different breeding sites. This suggestst hat migratory connectivity might be strongeri n this species. The approach of using multiple techniques may prove useful for other species about which less is known regarding breeding origins, which may be particularly relevant for species of conservati on concern. However, the most useful variable/combination of different variables for a study of migratory connectivity will vary, both according to the species, its geographical range, and the scale of resolution required, and a clear understanding of the ecology and physiology of the study species is essential.