Long-Distance Range Expansion and Rapid Adjustment of Migration in a Newly Established Population of Barn Swallows Breeding in Argentina

Abstract

When bird populations spread, long-distance pioneering populations are often backfilled by a more slowly advancing front [1-3]. The Barn Swallow Hirundo rustica, a globally distributed passerine [4, 5], expanded its breeding range an exceptional 7,000 km when it began breeding 35 years ago in its regular wintering range in Argentina [6], subsequently expanding over 500 km from its starting point [7-11]. Trans-hemispheric breeding attempts have occurred previously in related swallows [12-14], but only this colonization has lasted. Comparative studies of birds show a remarkable diversity in patterns of change in migratory habits [15-21], and these Argentine-breeding swallows might retain ancestral patterns, breeding in Argentina but returning to North America for the austral winter. Feather isotopes from these birds are consistent with the alternative possibility that they migrate no farther than northern South America [22]. Because isotopic patterns cannot definitively distinguish these alternatives, we pursued a solar geolocator study [23, 24] to do so. Data from nine tagged birds show conclusively that Barn Swallows breeding in Argentina have rapidly changed their movements to migrate no farther north in austral winter than northern South America. The phenology of the annual cycles of molt, migration, and breeding for these Argentine-breeding swallows have all shifted by about 6 months, and we suggest that stimulatory day lengths and the proliferation of nesting substrates facilitated their colonization.

How the effects of latitude on daylight availability may have influenced the evolution of migration and photoperiodism

“…Just as species differences in photoperiodic mechanisms are expected to underlie species differences in migratory programme, between‐individual variation in photoperiodic mechanisms, which has long been known to occur (Dol'nik, 1963; Lofts & Murton, 1968), may, in some cases, be necessary to accommodate between‐individual variation in migratory programme (Fudickar, Greives, Atwell, Stricker, & Ketterson, 2016). Nonetheless, between‐individual variation seems small for at least the three species on which we were able to find data on many individuals' annual migratory programmes, barn swallows Hirundo rustica (Figures S3 and S4; Hobson et al., 2015; Winkler et al., 2017), Swainson's hawks (Figure S5; Kochert et al., 2011) and bobolinks (Figure S6; Renfrew et al., 2013).…”

“…But, despite experiencing spring and summer conditions potentially favourable to breeding in both hemispheres, to our knowledge no individual breeds in both hemispheres, raising questions regarding the proximate mechanisms that prevent two‐hemisphere breeding and adaptive significance of this limitation. Even species as a whole mostly follow these patterns, in that nearly all transequatorial migrant species restrict breeding entirely to one hemisphere (but see Winkler et al., 2017).…”

Endogenous Programs and Flexibility in Bird Migration

“…Differences between the species, in turn, may be due to the more flexible annual behaviors of Barn Swallows. It might be no coincidence that in Barn Swallows, recent cross-hemispheric colonization was observed, associated with complete inversion of the annual cycle (Winkler et al, 2017). Such an inversion is easy to envision if some individuals become sensitive to long and warm days while still on the winter grounds.…”

A long winter for the Red Queen: rethinking the evolution of seasonal migration

“…And, occasionally, a migratory population of birds does not come back to the original breeding grounds but rather begins breeding on its wintering grounds. This has been observed directly over the course of just a few generations (Winkler et al ., ), and there is phylogenetic evidence that such migratory ‘drop‐offs’ may have occurred regularly across macroevolutionary time scales (Kondo et al ., ; Winger, Lovette & Winkler, ; Rolland et al ., ). But these exceptions prove the rule, by highlighting that for the vast majority of the time, natural selection favours individuals that not only migrate away from their breeding grounds when seasonal conditions deteriorate, but also return again the next season to the same breeding regions and often the same specific site where they raised young the year before.…”