Migratory connectivity in barn swallows and other hirundines

Abstract

Environmental indicators such as the Normalized Difference Vegetation Index (NDVI) (Pettorelli et al. 2005) provide important insights into spatial and temporal patterns of primary productivity and hence food availability for many different organisms, including barn swallows Hirundo rustica (Saino et al. 2004a, b). Based on this observation, Szep and Moller (2005) argued that when time series of survival estimates were available for a migratory bird population, this time series could be correlated with time series for environmental indicators such as NDVI, particularly in areas where the given population was wintering, provided that mortality mainly occurred outside the breeding season as is commonly the case in migratory birds (review in Szep and Moller 2005). Therefore, this novel approach to identification of potential wintering grounds of migratory bird populations opened up the possibility for further tests based on stable isotope, trace element or genetic marker profiles. A number of caveats and underlying assumptions of this approach were discussed in detail by Szep and Moller (2005). Recently, Szep et al. (2006) extended this approach in an attempt to identify potential wintering grounds for a Danish barn swallow population that had been subject to a long-term population study since 1984, thus providing information on annual survival rates. Szep et al. (2006) used correlations between NDVI and adult survival rates, but also information on the frequency of ringed barn swallows in different parts of South Africa, and the actual South African recovery sites of breeding birds from Denmark, to identify potential wintering grounds of the specific population. We are currently investigating the extent to which trace element and stable isotope profiles from barn swallows from different areas in South Africa match the profiles from breeding birds from the Danish breeding population. Underhill (2007; LGU hereafter) has criticized our approach on a number of different grounds, namely that: (1) we attempted to ‘‘demonstrate’’ that the Danish population wintered in Northern Cape; (2) we should have asserted that there is a fine-scale one-toone link between breeding areas and non-breeding areas; (3) ‘‘the fine-scale one-to-one link between breeding areas and non-breeding areas...does not change through the passage of decades’’; (4) there is no evidence of segregation of migratory populations of Communicated by F. Bairlein.