Filling latitudinal gaps in Tachycineta swallows: reproductive strategies in a South American species

Abstract

Chilean swallows (Tachycineta leucopyga) are the southernmost swallows in the Tachycineta genus, breeding from central Chile to southern Chile and Argentina. Previous studies, which focused on a population breeding at 54°S in Tierra del Fuego, Argentina, revealed differences in life-history traits compared to their Northern Hemisphere congeners. Notably, Chilean swallows exhibit smaller clutch size, and greater male nest attendance during the incubation period than their northern congeners, among other differences. It has been suggested that these variations could be adaptations for breeding in an extreme environment. To assess the consistency of these patterns for the species, we studied its breeding biology and parental care in north-western Argentine Patagonia (41°S). If the breeding traits observed in the austral population accurately reflect those of the species as a whole, we predict a smaller clutch size, higher female reproductive investment, and similar nest attendance by both sexes during incubation compared to other Tachycineta populations breeding at similar northern latitudes. Furthermore, we expect to observe a negative correlation between clutch size, egg size, and breeding success with laying date, consistent with the typical pattern observed in Tachycineta species, including Chilean swallows. Surprisingly, clutch and egg sizes did not differ between northern and southern Patagonian populations. We did not find evidence of a negative relationship between clutch or egg size and laying date (neither between first and second/replacement clutches). Male nest attendance during incubation was variable and low, but they fed nestlings at rates similar to those of females. These findings suggest that Chilean swallows adopt variable reproductive strategies in different areas. This may help this species adapt to different environments and climates, ranging from the shorter breeding seasons and lower summer mean temperatures of Tierra del Fuego to the longer breeding seasons and higher seasonal and daily thermal amplitudes at continental north Andean Patagonia.