Reproductive Success of a Tropical Barn Swallow Hirundo rustica Population Is Lower Than That in Temperate Regions

Simple Summary Barn Swallows (Hirundo rustica) mainly breed in temperate regions and parts of the tropics. These two regions have remarkably different environments, especially pertaining to their weather conditions. It is interesting to see how the breeding performances of Barn Swallows differ between the two regions and how the weather conditions affect the reproductive success of the Barn Swallow. We monitored the breeding activities of two Barn Swallow populations simultaneously in both tropical and temperate regions in China from 2019 to 2021 and investigated the effects of weather conditions on the reproductive success of the Barn Swallow in the study sites. We found that the reproductive success of the Barn Swallow at the tropical site was lower than that at the temperate site, i.e., swallows at the tropical site laid fewer eggs and had fewer nestlings/successful breeding attempts than swallows at the temperate site. Temperature, rather than rainfall, influenced the breeding performances of Barn Swallows at both sites. In particular, nestling survival decreased with increasing temperatures at the tropical site, but not at the temperate site. We suggest that researchers pay more attention to the reproduction and population changes of the Barn Swallow in the tropics in the context of climate warming. Temperate-tropical comparisons of avian life history traits are helpful to understand the different selective pressures placed on birds by different climate zones. Although there have been many comparative studies targeting multiple species in different regions, there are few comparative studies on the reproductive successes of the same species between tropical and temperate regions. In this study, we monitored the breeding activities of the Barn Swallow (Hirundo rustica) simultaneously at a single tropical site and a single temperate site in China, compared the breeding performances of the two populations, and investigated the effects of weather conditions on reproductive success separately. The clutch and brood sizes of the Barn Swallow at the topical site were significantly smaller than those at the temperate site. Furthermore, the breeding success of the Barn Swallow at the tropical site was significantly lower than that at the temperate site. The mean daytime temperature had a negative effect on the clutch size and brood size at both sites; it had a negative effect on nestling survival at the tropical site, but not the temperate site. This study will help us understand the adaptation strategies of widely distributed bird species in different environments, and how climate change will affect birds in different climate zones.