Switching Hemispheres: A New Migration Strategy for the Disjunct Argentinean Breeding Population of Barn Swallow (Hirundo rustica)

Abstract

BackgroundBarn Swallows (Hirundo rustica) breed almost exclusively in the Northern Hemisphere. However, since the early 1980's, a small disjunct breeding population has become established in eastern Argentina, presumably by birds previously derived from those breeding in North America. Currently, it is unknown where these individuals go following breeding and how they have adjusted to a reversal in phenology. Their austral wintering period corresponds to the breeding period of the northern ancestral population and so they can potentially return to these more traditional breeding sites or they may occupy other South American wintering regions left vacant by conspecifics returning to the Northern Hemisphere.Principal FindingsWe used a three-isotope (δ 13C, δ 15N, δ 2H) approach to investigate potential wintering areas in Central and South America of individuals breeding in Argentina. Feather isotope values differed from those expected and measured at local breeding sites in Argentina indicating molt after the austral breeding period and away from the breeding grounds. Potential molting origins were identified applying likelihood-based assignment methods to a δ 2H isoscape for South America and dichotomous prior information on the distribution of C3 and C4 vegetation types based on modeled vegetation-δ 13C values. Barn Swallows now breeding in Argentina have changed their migratory behavior but presumably use the same cues as those used by the ancestral population, molting their feathers during the austral winter, likely in north-eastern South America.

A multi‐isotope (δ2H, δ13C, δ15N) approach to establishing migratory connectivity of Barn Swallow (Hirundo rustica)

“…This situation provides a useful comparative paradigm whereby the strength of migratory connectivity can be investigated across regions with different population trends. Our approach was to assign individuals within swallow populations to regions of South America using a precipitationbased predicted feather d 2 H isoscape that was further constrained by a recently developed plant-d 13 C isoscape (García-Pérez et al 2013). …”

Connecting breeding and wintering grounds of Neotropical migrant songbirds using stable hydrogen isotopes: a call for an isotopic atlas of migratory connectivity

“…For establishing migratory connectivity in the North American Neotropical migration system, capturing birds at known wintering locations rather than at known breeding locations makes considerable sense because the feather deuterium isoscape (δ 2 H f ) for North America is much better known and structured than that in Central America, the Caribbean, or northern South America (but see Garcia‐Perez et al , García‐Pérez and Hobson ). Although there are important exceptions that include birds that molt primarily in wintering areas (e.g., flycatchers and swallows) or undergo a molt migration (see Discussion), most Neotropical migrant songbirds breeding in North America typically molt on their breeding grounds before migrating south, so feathers sampled in wintering areas can be used to infer breeding locations or natal origins in the north.…”

Barn Swallows keep expanding their breeding range in South America

“…Barn Swallows breeding in Argentina appear to have completely switched their moult phenology and developed their own migratory system; there is no evidence of breeding birds moulting actively in Argentina and no records of the species during winter (Garcia-Perez et al 2013). It is not clear where the birds go, although stable isotope analyses suggest that they may go to the north-east of South America (specifically north Brazil, French Guiana, Suriname, Guyana, and Venezuela; Garcia-Perez et al 2013).…”

“…It is not clear where the birds go, although stable isotope analyses suggest that they may go to the north-east of South America (specifically north Brazil, French Guiana, Suriname, Guyana, and Venezuela; Garcia-Perez et al 2013). The quality and extension of wintering grounds can have profound effects on the winter survival of a bird and, through carry-over effects, on their breeding and survival prospects in their next breeding attempt (Newton 2004;Norris et al 2004).…”