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The barn swallow (Hirundo rustica) poses a number of fascinating scientific questions, including the taxonomic status of postulated subspecies. Here, we obtained and assessed the sequence variation of 411 complete mitogenomes, mainly from the European H. r. rustica, but other subspecies as well. In almost every case, we observed subspecies-specific haplogroups, which we employed together with estimated radiation times to postulate a model for the geographical and temporal worldwide spread of the species. The female barn swallow carrying the Hirundo rustica ancestral mitogenome left Africa (or its vicinity) around 280 thousand years ago (kya), and her descendants expanded first into Eurasia and then, at least 51 kya, into the Americas, from where a relatively recent (<20 kya) back migration to Asia took place. The exception to the haplogroup subspecies specificity is represented by the sedentary Levantine H. r. transitiva that extensively shares haplogroup A with the migratory European H. r. rustica and, to a lesser extent, haplogroup B with the Egyptian H. r. savignii. Our data indicate that rustica and transitiva most likely derive from a sedentary Levantine population source that split at the end of the Younger Dryas (YD) (11.7 kya). Since then, however, transitiva received genetic inputs from and admixed with both the closely related rustica and the adjacent savignii. Demographic analyses confirm this species’ strong link with climate fluctuations and human activities making it an excellent indicator for monitoring and assessing the impact of current global changes on wildlife.

Environmental conditions experienced in early life may shape subsequent phenotypic traits including life history. We investigated how predation risk caused by domestic cats ( Felis silvestris catus ) and local breeding density affected patterns of reproductive and survival senescence in Barn Swallows ( Hirundo rustica ) breeding semicolonially in Denmark. We recorded the abundance of cats and the number of breeding pairs at 39 breeding sites during 24 years and related these to age-specific survival rate and reproductive senescence to test predictions of the life history theory of senescence. We found evidence for actuarial senescence for the first time in this species. Survival rate increased until reaching a plateau in midlife and then decreased later. We also found that survival rate was higher for males than females. Local breeding density or predation risk did not affect survival as predicted by theory. Barn Swallows with short lives did not invest more in reproduction in early life, inconsistent with expectations for trade-offs between reproduction and survival as theory suggests. However, we found that the rate of reproductive decline during senescence was steeper for individuals exposed to intense competition, and predation pressure accelerated the rate of reproductive senescence, but only in sites with many breeding pairs. These latter results are in accordance with one of the predictions suggested by the life history theory of aging. These results emphasize the importance of considering intraspecific competition and interspecific interactions such as predation when analyzing reproductive and actuarial senescence.

Context Urbanization has detrimental effects on biodiversity, yet how species respond to urban planning zoning outcomes and environmental changes at different spatial scales when selecting urban breeding habitats remains understudied. Mitigating such impacts on wildlife is instrumental to create biodiversity-friendly cities while accommodating urban development. Objectives We used Barn Swallow nesting site data (2017–2023) collected from a citizen science program to help identify the most influential factors affecting the presence of Barn Swallow nests at site and landscape scales. Methods We analyzed the relationship between Barn Swallow nest site selection in urbanized areas of southern China and land use data, including built-up percentages, cropland and waterbodies, as well as environmental factors such as heat-island effect (land surface temperature), noise pollution (road density and road simplicity), artificial light at night (ALAN) and nesting building attributes (year constructed, height and surroundings). Results Our findings revealed a positive association between Barn Swallow nest abundance and several anthropogenic factors, including land surface temperature at the site scale, and ALAN and road simplicity at the landscape scale. Our findings indicated the building year also had a negative impact on the Barn Swallow nests. Conclusions These results suggest that urban design and revitalization efforts can consider mitigating negative effects by implementing measures to regulate noise pollution and nighttime lighting schemes. Furthermore, urban planning could carefully consider the requirements for biodiversity-friendly architectural elements in new constructions and rezoning process of existing urban districts, such as old residential neighborhoods and urban fringes, to minimize impacts on declining nesting sites in urban areas.

Melanin is the main pigment in animal coloration and considerable variation in the concentrations of the two melanin forms (pheo- and eumlanin) in pigmented tissues exists among populations and individuals. Melanin-based coloration is receiving increasing attention particularly in socio-sexual communication contexts because the melanocortin system has been hypothesized to provide a mechanistic basis for covariation between coloration and fitness traits. However, with few notable exceptions, little detailed information is available on inter-individual and inter-population variation in melanin pigmentation and on its environmental, genetic and ontogenetic components. Here, we investigate melanin-based coloration in an Italian population of a passerine bird, the barn swallow (Hirundo rustica rustica), its sex- and age-related variation, and heritability. The concentrations of eu- and pheomelanin in the throat (brown) and belly (white-to-brownish) feathers differed between sexes but not according to age. The relative concentration of either melanin (Pheo:Eu) differed between sexes in throat but not in belly feathers, and the concentrations in males compared to females were larger in belly than in throat feathers. There were weak correlations between the concentrations of melanins within as well as among plumage regions. Coloration of belly feathers was predicted by the concentration of both melanins whereas coloration of throat feathers was only predicted by pheomelanin in females. In addition, Pheo:Eu predicted coloration of throat feathers in females and that of belly feathers in males. Finally, we found high heritability of color of throat feathers. Melanization was found to differ from that recorded in Hirundo rustica rustica from Scotland or from H. r. erythrogaster from North America. Hence, present results show that pigmentation strategies vary in a complex manner according to sex and plumage region, and also among geographical populations, potentially reflecting adaptation to different natural and sexual selection regimes, and that some coloration components seem to be highly heritable.

Nest site selection is critical for reproductive success in birds. Several bird species including Barn Swallow (Hirundo rustica gutturalis) typically nest near human settlements, presumably because this reduces their risk of predation. Here, we investigated the nesting habits of Barn Swallows in South Korea. We predicted that abandoned houses would not be chosen even if these houses provided similar nesting conditions compared to occupied houses. We conducted surveys across South Korea and recorded human occupancy, house structure, the number of active and old nests, and the deterioration condition of houses. Among the recorded active nests, 95% were located in inhabited houses (n = 38), whereas only 5% were observed in abandoned houses (n = 2). Interestingly, human presence was a significant factor in the occurrence of active nests, whereas house structure had no appreciable effect. Among the nest site characteristics, the number of old nests was significantly higher in inhabited houses than in abandoned houses. Further, ∼97% of inhabited houses were not damaged, whereas 50% of the abandoned houses were damaged, potentially affecting the nesting site selection of Barn Swallows. Our results are consistent with our hypothesis that Barn Swallows prefer to nest in human-inhabited houses. As house structures suitable for nesting are reduced due to rapid urbanization, and as swallow nests are actively eliminated by human residents, the reproductive success of Barn Swallows in South Korea can be significantly reduced.

We studied patterns of extra-pair maternity (EPM) in 245 nests (225 nests belonging to 120 females of known identity) of sexually promiscuous European barn swallows (Hirundo rustica rustica) over a 3-year period. At least one EPM nestling was identified in 54 nests (22.0 %), representing 5.7 % of a total of 1060 nestlings. Up to 28.3 % of all EPM nestlings resulted from quasi-parasitism (QP), whereby nestattending males sired parasitic offspring. Nests of quasiparasitic females were never in close proximity to the host nest. Our data thus indicate nonrandom QP patterns in our population suggesting that QP can be considered a third alternative reproductive strategy alongside extra-pair paternity (EPP) and intraspecific brood parasitism (IBP). Of several socioecological factors evaluated, only number of simultaneous egg-laying females in the population proved a good predictor for EPM occurrence. Whereas parasitic females produced more offspring per breeding attempt than was the population average, both QP and IBP affected host female reproductive output, being associated with a reduced number of her offspring produced from the nest. On the contrary, QP resulted in an increase in the number of offspring produced by nestattending males, suggesting that males may benefit from cooperating with parasitic females at the expense of their social partners.

Nest-switching is an important breeding strategy for multiple-brooded bird species. When deciding whether or not to switch nests for subsequent breeding attempts, pairs must weigh the costs and benefits of various factors related to the number of fledglings of the first breeding attempt, the likelihood of nest predation, and qualities of the nest environment, such as nest ectoparasites and the age of the nest. In this study, we analyzed the predictors and consequences of nest-switching behavior at 6 breeding sites of North American Barn Swallows (Hirundo rustica erythrogaster), where 60% of pairs that raised 2 broods within a season switched nests for a second breeding attempt. Pairs often reused existing (old) nests constructed during previous years, and pairs that settled in old nests for their first breeding attempt were the most likely to switch nests for a second breeding attempt. Contrary to previous studies, nest predation and nest ectoparasitism had no influence on whether or not pairs switched nests. Moreover, second breeding attempts overall had significantly more mites than first breeding attempts, but there was more variation in the change of mite intensities for those pairs that switched nests for a second breeding attempt compared to pairs that did not switch. Furthermore, pairs that switched from one old nest to another nest between breeding attempts decreased the time between first and second breeding attempts when compared to pairs that reused their first nest for a second breeding attempt. Because nest-switching led to greater fledging success for second breeding attempts compared to birds that reused their nests, our results suggest that switching between nests is an adaptive reproductive strategy for Barn Swallows.

It is widely assumed that colonizing species thrive because they lack natural enemies in their new range, increasing their survival and reproductive success. Barn Swallows (Hirundo rustica) started to breed in South America around 1980 and since then have dramatically increased their population size and geographic range, in stark contrast to the decline of the source population in North America. The reasons behind the growth of the South American population are unknown. However, because this species had never bred in this area in recorded times, the lack of natural predators, parasites, or pathogens could lead to higher breeding success, as predicted by the enemy release hypothesis. Here, we test whether breeding success is higher in the newly colonized range than in the native range to test the enemy release hypothesis. We studied the breeding biology of South American Barn Swallows quantifying 7 breeding parameters: clutch size, overall breeding success, offspring mortality by predation and ectoparasites, number of fledglings per breeding attempt, number of breeding attempts per breeding season, and total number of fledglings produced per pair per breeding season. Additionally, we compared these parameters with published information from North American populations using meta-analyses. We found that, while clutch size did not differ between North and South American populations, the southern population overall had lower breeding success, with higher mortality from predation and ectoparasites. An egg laid in South America was 2.6 times more likely to fail than one laid in North America, which resulted on average in 1.1 fewer fledglings per pair per breeding season for the South American breeding population. These results, demonstrating lower breeding success in a newly colonized range, do not support the enemy release hypothesis, and indicate that the growth of the South American Barn Swallow population is most likely caused by other demographic factors.

Pair formation is indispensable for breeding in monogamous species, generating selection for male traits that increase the probability of pairing success. Male ornamentation is one such trait, and several empirical studies have shown the importance of each of multiple ornaments. Still, it remains unclear how multiple ornaments in combination affect the probability of pair formation. Using the Japanese barn swallow, Hirundo rustica gutturalis, we studied the interplay of two sexually selected male traits, tail length and throat coloration, during pair formation. Independent of other morphological, ornamental and abiotic variables, the probability of pair formation was predicted by the negative interplay between tail length and plumage colour saturation: males possessing more-colourful plumage with shorter tails or males possessing less-colourful plumage with longer tails had a higher probability of pair formation than others. The current findings may explain spatiotemporal variation in ornamentation and sexual selection in this model species.

1. Melanin coloration is the most common type of colour in animals. Many species use melanin-based black, brown, grey or earth-toned colours as intraspecific signals of quality. 2. Melanin comes in two main forms in vertebrates -- eumelanin and phaeomelanin -- and these are said to create different colours, with eumelanin giving a darker black, brown or grey appearance and phaeomelanin a lighter reddish or buff hue. 3. However, the few studies that have examined the melanin content of animal colours, namely from bird feathers, have shown that both phaeomelanin and eumelanin are present in nearly all melanized patches. These pigment types also have different synthetic origins and may be differentially costly to display. 4. Thus, to fully understand the costs and benefits of melanic colours, we must first quantitatively determine the relationship between pigment composition and colour characteristics. 5. We studied melanin content and colour variation in the ventral chestnut-coloured plumage of male and female Barn Swallows (Hirundo rustica erythrogaster) from North America. 6. Total melanin concentration explained significant variation in the hue, saturation and brightness of male breast plumage and female breast plumage. Eumelanin is the predominant type in these feathers, constituting over 75% of total melanins. Eumelanin and phaeomelanin concentration were significantly positively correlated in the feathers of females but not males. 7. In both sexes, levels of both eumelanin and phaeomelanin concentrations were significantly and positively correlated with hue, saturation and brightness. In males but not females, however, the ratio of eumelanin-to-phaeomelanin was also an important predictor of colour, with smaller ratios conferring browner hues. Males also deposit more phaeomelanins, but not eumelanins or total melanins, in breast feathers than do females. These results suggest that phaeomelanin plays a unique role in the colouring strategy of male Barn Swallows.