Asymmetric exchange between populations differing in habitat quality: A metapopulation study on the citril finch

Abstract

The citril finch ( Serinus citrinella ) is a Cardueline finch restricted to the high mountains of western Europe. Since 1991 we have captured-recaptured about 6000 birds in two contrasting subpopulations located on the same mountain but separated by 5 km in distance. Citril finches, at the north-facing locality (La Vansa), rely more on Pine trees ( Pinus uncinata ) as their main food source, than birds at the south-facing locality (La Bofia), which rely more on herb seeds, which are of lower energetic content. Birds at La Vansa had higher body mass and fat score than those at La Bofia, suggesting that La Vansa was a site of higher-quality than La Bofia. By the use of a metapopulation approach and multistate models, we found that citril finches at the high-quality locality (La Vansa) showed higher survival rates than those at the low-quality one (La Bofia) (Vansa adults: φ = 0.42 - 0.04, juveniles: φ = 0.34 - 0.05; Bofia adults: φ = 0.35 - 0.04, juveniles: φ = 0.28 - 0.05). Dispersal was also asymmetric and higher for juvenile birds, with movement rates for juvenile citril finches from the low-quality to the higher-quality locality (Bofia to Vansa: ‚ = 0.38 - 0.10) higher than the reverse (Vansa to Bofia: ‚ = 0.09 - 0.03). We also investigated time-specific factors (e.g. meteorological data and fructification rate of Pinus ) as potential predictors of overall mortality and dispersal patterns. The results do not allow strong conclusions regarding the impact of these factors on survival and movement rates. Patterns of movement found in the Citril Finch between localities document a new model for the dispersal of species from low to high quality habitats, which we label of 'sources and pools'. This contrasts with currently accepted models of 'sources and sinks', in which movement is from high to low quality habitats, and 'Ideal Free Distributions', in which there is a balanced dispersal between habitats of different quality.