Barn Swallow Populations Research Articles

Migration pattern of a population of Barn Swallows (Hirundo rustica) breeding in East Asian tropical region

Birds exhibit a high degree of migratory diversity, which is influenced by various ecological factors and life history strategies. Conducting studies on tropical bird migration, of which research is scarce, and comparing it with temperate birds can enhance our understanding of bird migration behaviour and its underlying mechanisms. In this study, we explored the migration behaviour of a breeding population of the Barn Swallow (Hirundo rustica) in Zhanjiang, southern China, a region located in the northern tropics, using light-level geolocators. From 2021 to 2023, we deployed geolocators on 92 breeding swallows and retrieved geolocators successfully from 23 individuals. These swallows all exhibited migratory behaviour, and wintering on various islands in Southeast Asia. They displayed sex differences in their wintering locations. All males concentrated in Borneo, while females primarily chose Borneo but also dispersed to the Philippines, South China Sea, and Vietnam for wintering. The studied swallow population adopted a seasonal migration pattern of “indirect in autumn, direct in spring”, bypassing the ecological barrier of the South China Sea in autumn and tending to directly cross it in spring migration. Moreover, the distance and duration of autumn migration was significantly longer than those of the spring migration. Compared to temperate Barn Swallows, the Barn Swallow population breeding in Zhanjiang adopts a pattern of “intra-tropical migration” and initiates autumn migration earlier. The formation of their migration pattern may be limited by ecological and physiological factors.

Multi-isotope (δ2H, δ13C, δ15N) feather profiles and morphometrics inform patterns of migratory connectivity in three species of North American swallows

Aerial insectivorous birds have suffered steep population declines in North America over the last 60 years. A lack of information on migratory connectivity between breeding and non-breeding grounds for these species limits our ability to interpret factors affecting their population-specific trends. We determined likely Latin American non-breeding regions of Bank (Riparia riparia), Barn (Hirundo rustica) and Cliff (Petrochelidon pyrrhonota) swallow from populations across their breeding ranges. We used predicted feather hydrogen (δ2Hf) and carbon (δ13Cf) isoscapes for winter-grown feathers to indicate areas of highest probability of moult origin and incorporated these results into a cluster analysis to determine likely broad non-breeding regions. We also assessed variation in wing length among populations to determine the potential for this metric to differentiate population moult origins. We then investigated patterns of multi-isotopic (δ2Hf, δ13Cf, δ15Nf) and wing-length niche occupancy by quantifying niche size and overlap among populations under the assumption that broad niches were consistent with low within-species migratory connectivity and narrow and non-overlapping niches with higher connectivity. Multivariate assignment identified different non-breeding regions and potential clusters of moult origin generally corresponding to Central America and northern South America, eastern and south-central South America, and the western and southern part of that continent, with variation within and among populations and species. Separate niche space indicated different wintering habitat or areas used by species or populations whereas niche overlap indicated only potential spatial similarity. Wing length varied significantly among populations by species, being longer in the west and north for Bank and Cliff Swallow and longer in eastern Canadian Barn Swallow populations. Barn Swallow occupied consistently larger isotopic and wing length niche space than the other species. Comparisons among populations across species showed variable isotopic and wing-length niche overlap generally being greater within breeding regions and lower between western and eastern breeding populations supporting a general North American continental divide for all species with generally low migratory connectivity for all species. We present a novel approach to assessing connectivity using inexpensive and broad isotopic approaches that provides the basis for hypothesis testing using more spatially explicit expensive techniques.

Brood size is associated with apparent telomere lengthening in nestling barn swallows

Early life for animals is often a time of rapid growth and development. In a resource-limited environment, life history theory predicts that there must be trade-offs between resource sinks in ways that optimize future survival and reproductive success. Telomeres have emerged as putative indicators of these early life trade-offs, but there are conflicting accounts as to how developmental traits and conditions impact telomere length and dynamics. For 2years, we studied the nestlings of a breeding population of barn swallows from day 6 to day 12 of life, measuring various ontogenetic factors to understand to what extent they explain variation in telomere length and dynamics. We unexpectedly found that telomeres lengthened between the two sampling points. Nestlings in large broods had shorter telomeres, but surprisingly, individuals that grew faster from day 6 to day 12 had longer telomeres and more telomere lengthening. Nestlings with higher mass relative to their nestmates on d6 had shorter telomeres, suggesting that the relatively fast growth barn swallows experience early in development is more costly than the relatively slower growth later in development. These effects were only found in the first year of study. Telomere lengthening may be due to the initiation of new hematopoietic cell lines during development or the expression of telomerase early in life. Favorable early life conditions and high parental investment could allow for more growth with little to no cost to telomere length or dynamics.

Barn Swallows in East Asia show strong tolerance against nest mites

Abstract Hosts defend against parasites through two main strategies: resistance and tolerance. These two strategies have different influences on the parasite load of the host and so have variable effects on the fitness of parasites. In this study, we investigated the strategy used by the Barn Swallow (Hirundo rustica) to defend against a hematophagous mite (Dermanyssus hirundinis) in East Asia. For the first time, we found Barn Swallows were parasitized by D. hirundinis in Asia and, in northeastern China, over 60% of broods were parasitized. We examined whether swallows with better body condition or the males with more extravagant sexually selected traits (longer tail streamers or redder ventral plumage) showed stronger resistance or tolerance to these parasites. The results showed that males with more extravagant sexual ornaments had weaker resistance (heavier brood mite load) than males with less extravagant traits; however, the heavier mite load was likely to be a byproduct of larger brood size. Moreover, the Barn Swallow population showed strong tolerance against the parasite (i.e. the brood mite load did not affect brood size or nestling body mass). It is worth noting that male swallows with redder breast plumage showed weaker tolerance (lower seasonal reproductive success) than other males. Further, we conducted experimental ectoparasite manipulations in 46 Barn Swallow nests to verify the potential effect of these parasites on the body condition of nestlings. Neither the body mass nor the hemoglobin concentration differed between the designed groups; however, the hemoglobin concentration of nestlings decreased with mite load. We concluded that the Barn Swallow population in East Asia used the tolerance strategy, rather than resistance to defend against nest mites, and the ectoparasitic mite did not appear to be a strong selective factor for reproductive success owing to the strong tolerance of the studied Barn Swallow population.

Egg Rejection and Nest Sanitation in an Island Population of Barn Swallows (Hirundo rustica): Probability, Response Latency, and Sex Effects.

Bird nests function as vessels for eggs and nestlings, and an environment for rearing offspring. However, foreign objects falling into bird nests and nestling eggshells may be harmful. Moreover, the smell of fecal sacs increases the risk of detection by predators. Many bird species have evolved nest sanitation to prevent damage to their nests. Furthermore, egg rejection evolved in some birds to thwart brood parasites that lay eggs in their nests. We studied 133 nests of barn swallows (Hirundo rustica) in an island population through a nest content manipulation experiment to determine nest sanitation and egg rejection behaviors and their relationship. Swallows rejected non-egg foreign objects more frequently (100% vs. 58.6%) and sooner than parasite eggs, which supports the hypothesis that nest sanitation is a pre-adaptation to egg rejection. However, nest sanitation did not increase egg rejection, either in probability or latency. Furthermore, both sexes incubated the eggs, cleaned the nests, and removed parasite eggs, implying that both are confronted with natural selection related to nest sanitation and brood parasitism. However, females invested more time in these behaviors than males. This provides evidence for the evolutionary relationship of nest sanitation and egg rejection behaviors in barn swallows.

Experimental manipulation of sexual traits in barn swallow populations-No evidence for divergent sexual selection.

Safran et al. (2016a) manipulated two sexual traits (ventral plumage coloration and tail streamer length) in male barn swallows (Hirundo rustica) and reported divergent effects on paternity change between two study populations, in Colorado and Israel. They concluded that geographical variation in the two phenotypic traits is maintained by divergent sexual selection. However, the response variable they used, the longitudinal change in paternity from a pre‐treatment clutch to a post‐treatment clutch, does not reflect an unbiased effect of the treatment. Here, I show that the magnitude of the change in paternity is influenced by variation in the initial paternity score among the treatment groups, which is presumably due to stochastic variation from low sample sizes in the treatment groups. When the bias was accounted for in re‐analyses of the Israeli dataset, the statistical significance of one of two treatment effects disappeared. Similar re‐analyses of the American population were not possible due to inaccessibility of raw data for individual clutches, but an assessment of the mean scores indicates that the two significant treatment effects in this population were similarly biased in their initial paternity scores. The conclusion of divergent sexual selection on male phenotypic traits between the two populations does not seem to be supported.

The evidence for divergent sexual selection among closely related barn swallow populations is strong.

Lifjeld's comment provides an opportunity to illustrate the intricacies of the "regression to the mean" (RTM) effect, to clarify the difficulty in teasing apart RTM from allocation bias, and to re-examine our results in relation to RTM and in the context of related evidence. Here, we show that (a) the correlations between paternity change and initial paternity are mathematically expected and can equally be produced when changes are caused by the experimental manipulation itself. (b) The approach taken by Lifjeld to control for RTM is overly conservative because it is based on the unrealistic assumption of zero correlation between individuals' repeated measurements. Yet, even when using this conservative method, the main effects we originally reported are still detectable. (c) The combined effect of color darkening and tail elongation in Israel is additionally supported by an increase in the number of extra-pair young in other nests and by three independent studies of this population. (d) The experimental effect of color darkening in North America has been replicated successfully and is consistent with multiple correlative studies. Thus, divergent sexual selection in barn swallow populations is supported by both a conservative reanalysis and multiple, independent analyses of experimental and observational datasets.

Telomere length is highly repeatable and shorter in individuals with more elaborate sexual ornamentation in a short-lived passerine.

Quantifying an individual's state as a fitness proxy has proven challenging, but accumulating evidence suggests that telomere length and attrition may indicate individual somatic state and success at self-maintenance, respectively. Sexual ornamentation is also thought to signal phenotypic quality, but links between telomeres and sexual ornamentation have been little explored. To address this issue, we examined whether telomere length and dynamics are predicted by the expression of a sexually selected ornament, the length of the outermost tail feathers (streamers), using longitudinal data from a population of European barn swallows (Hirundo rustica). In 139 adult individuals, each measured twice, we further assessed associations of telomere length with age, sex, breeding status and survival. Telomere length showed high individual repeatability (R=.97) across years while shortening with age in both sexes. Telomere length and dynamics were not significantly associated with survival to the next year, remaining lifespan or reproduction status (comparing breeding and nonbreeding yearlings). Tail streamer length, a sexually selected trait in barn swallows, was negatively associated with telomere length, independent of sex. Thus, telomere length may reflect the costs of carrying an elaborated sexual ornament, although ornament size did not significantly predict telomere shortening. In conclusion, telomere length in adult barn swallows is a highly consistent trait that shows a negative relationship with sexual ornamentation, suggesting a trade-off between sexual ornamentation and telomere length.

Integrating regional and site-level data to assess drivers of population decline in a threatened aerial insectivorous bird

Knowledge of demographic processes is essential for understanding population dynamics and developing appropriate conservation strategies under global change. However, it is often challenging to collect the necessary population and demographic data to understand population dynamics. Integrated population modeling (IPM) provides an opportunity to combine population survey and demographic data from different sources. We developed an IPM and applied it to jointly analyze 15 years (2004–2018) of regional North American Breeding Bird Survey and site-level capture-recapture and age-ratio data for a barn swallow (Hirundo rustica) population in Ontario, Canada. We aimed to identify key demographic parameters and tested hypothesized environmental drivers to understand population declines of this species. Our results showed that the study population had declined by about 2% annually during our study period, which was mainly contributed by changes in net immigration and adult apparent survival. In particular, low net immigration in a few years of our study period seemed to be the main reason of population decline. Furthermore, we found that warmer temperatures in April when barn swallows return to breeding grounds had a positive effect on adult survival, while warmer temperatures during the breeding period in May–June had a negative effect on productivity. Our results indicate vulnerability of the species to inclement weather during the breeding season, suggesting potential population recovery by adopting conservation strategies that buffer climatic threats. Moreover, our IPM provides a foundation for future monitoring programs that intend to combine and maximize the utility of large-scale population counts and fine-scale demographic data.

Regional variability in trajectories of Barn Swallow populations across Canada are not predicted by breeding performance

Regional variability in trajectories of Barn Swallow populations across Canada are not predicted by breeding performance

Speciation Associated with Shifts in Migratory Behavior in an Avian Radiation.

Migratory animals move up to thousands of kilometers every year [1]. Losses of migration (i.e., migratory drop-offs) occur when individuals of a migratory species stop migrating and establish founder sedentary populations, a phenomenon documented in birds [2-5] and butterflies [6]. In theory, losses-and also gains-of migration might promote speciation if sedentary and migratory populations become reproductively isolated [7-9]. Because migratory and sedentary strategies involve alternative physiological, behavioral, and morphological traits [10-13], divergence along multiple axes of organismal function is expected to accompany switches in migratory behavior, potentially accelerating speciation. We present evidence of speciation driven by a migratory drop-off in the fork-tailed flycatcher (Tyrannus savana) resulting in reproductive isolation likely driven by changes in breeding schedules (allochronic speciation [13-15]) and geographic isolation of breeding grounds (allopatric speciation [16]). Phylogenetic analyses across New World flycatchers (Tyrannidae) showed that an association between speciation and drop-offs is also observable at a macroevolutionary scale. Loss of migration was significantly more frequent than its gain, and speciation rates of migratory and partially migratory lineages (i.e., species having both migratory and sedentary populations) exceeded those of sedentary lineages. Models of trait evolution indicated that partial migration is an intermediate step between migratory and sedentary states in this family. Given that partial migration is widespread across migratory animals (e.g., of all migratory birds, ca. 51% are partially migratory [5]), speciation via switches in migratory behavior might be an important yet overlooked mechanism of animal diversification.

Lower breeding success in a new range: No evidence for the enemy release hypothesis in South American Barn Swallows

AbstractIt 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.

Selection on multiple sexual signals in two Central and Eastern European populations of the barn swallow.

Variation in intensity and targets of sexual selection on multiple traits has been suggested to play a major role in promoting phenotypic differentiation between populations, although the divergence in selection may depend on year, local conditions or age. In this study, we quantified sexual selection for two putative sexual signals across two Central and East European barn swallow (Hirundo rustica rustica) populations from Czech Republic and Romania over multiple years. We then related these differences in selection to variation in sexual characters among barn swallow populations. Our results show that tail length and ventral coloration vary between populations, sexes, and age classes (first‐time breeders vs. experienced birds). We found that selection on tail length was stronger in first‐time breeders than in experienced birds and in males than in females in the Romanian population, while these differences between age groups and sexes were weak in Czech birds. We suggest that the populational difference in selection on tail length might be related to the differences in breeding conditions. Our results show that ventral coloration is darker (i.e., has lower brightness) in the Romanian than in the Czech population, and in experienced birds and males compared with first‐time breeders and females, respectively. The sexual difference in ventral coloration may suggest sexual selection on this trait, which is supported by the significant directional selection of ventral coloration in first‐time breeding males on laying date. However, after controlling for the confounding effect of wing length and tarsus length, the partial directional selection gradient on this trait turned nonsignificant, suggesting that the advantage of dark ventral coloration in early breeding birds is determined by the correlated traits of body size. These findings show that ventral coloration may be advantageous over the breeding season, but the underlying mechanism of this relationship is not clarified.

Extra-pair paternity patterns in European barn swallows Hirundo rustica are best explained by male and female age rather than male ornamentation

Adaptive explanations for the evolution of extra-pair paternity (EPP) in birds often assume cuckolding males to be better-ornamented than cuckolded males. Several studies have confirmed that either male sexual ornamentation is associated with EPP or that phenotypes of cuckolded and cuckolding males differ. Expression of male ornamentation may change with age; however, a recent meta-analysis has identified age itself as an important factor that differed in cuckolding and cuckolded males. The age of social female partner may also affect EPP, though this has received little attention. Here, by using detailed data on age of individual barn swallows (Hirundo rustica rustica), we identified age as the major predictor of male and female promiscuity. Our results revealed that, whereas a male’s ability to obtain an extra-pair mate increased linearly with age, the only predictor of the probability of a male being cuckolded was the age of his social partner, with older females engaging more frequently in EPP. In contrast, male ornamentation was not significantly related to EPP pattern. Tarsus length was the sole significant phenotypic trait in comparison between cuckolding and cuckolded males. Our data provide little support for the hypothesis that extra-pair mate choice in our barn swallow population was ornament driven. This may indicate either a non-adaptive scenario for EPP, for example with older males better able to coerce females into copulation, or EPP mating based on other than absolute mate-choice criteria associated with the expression of male ornamentation. SIGNIFICANCE STATEMENT: We analysed patterns of extra-pair paternity (EPP) in barn swallows. Derived from observation of 160 nests, our results appear to differ from the findings of some previous studies that identified ornamental traits as being associated with extra-pair and within-pair paternity (WPP) in this iconic model taxon of sexual selection. In particular, tail streamer length had no detectable association with WPP or a male’s ability to obtain an extra-pair partner. Pairwise comparisons of cuckolded and cuckolding males, involving 76 mixed paternity nests, also supported the hypothesis that male ornamentation does not play a role in determining EPP patterns in the focal barn swallow population. When statistically controlled for clutch initiation date, the probability of obtaining an extra-pair partner only increased with increasing age of males, while WPP was only associated with the age of their female social partners. Our data provide little support for the hypothesis that extra-pair mate choice in our barn swallow population is ornament driven and indicated that age, rather than ornaments, would be a better predictor of paternity.

Small, Variable Sperm in a Barn Swallow Population with Low Extra-Pair Paternity

Sperm competition can theoretically affect sperm morphology; however, it remains unclear whether and how sperm morphology tracks the intensity of sperm competition in each population. The barn swallow Hirundo rustica is a model species used in the study of sexual selection, and exhibits considerable variation in extra-pair paternity (percentage extra-pair young, ca. 3-30%) among populations. In the Joetsu population of the barn swallow, extra-pair paternity is virtually absent (< 3%), providing a rare opportunity to study sperm morphology under limited sperm competition, and to compare it with those reported in populations with frequent extra-pair paternity (>15%). We found that head, midpiece, and total sperm length were significantly shorter in the Joetsu population than in populations with frequent extra-pair paternity. Moreover, the variability in total sperm length, measured as the coefficient of variation in the Joetsu population, was twice as high as that of populations with frequent extra-pair paternity. These results are consistent with a positive, directional, and stabilizing effect of sperm competition on sperm morphology. Together with previous studies in populations with frequent extra-pair paternity, the current study provides one of few sets of evidence to show a link between the intensity of sperm competition and the mean and variance of sperm morphology within a wild bird species.

IPM2: toward better understanding and forecasting of population dynamics

AbstractDynamic population models typically aim to predict demography and the resulting population dynamics in relation to environmental variation. However, they rarely include the diversity of individual responses to environmental changes, thus hampering our understanding of demographic mechanisms. We develop an integrated integral projection model (IPM2) that is a combination of an integrated population model (IPMpop) and an integral projection model (IPMind). IPM2 includes interactions between environmental and individual effects on demographic rates and can forecast both population size and individual trait distributions. First, we study the performance of this model using eight simulated scenarios with variable reproductive selective pressures on an individual trait. When the individual trait interacts with the environmental variable and the selective pressure on the individual trait is nonlinear, only IPM2 produces adequate predictions, because IPMind does not link predictions between the population level and observed data and because IPMpop does not include the individual trait. Second, we apply IPM2 to a population of barn swallows. The model accurately predicts trends of the barn swallow population while also providing mechanistic insights. High precipitation negatively influenced population dynamics through delaying laying dates, which lowered reproductive and survival rates. To predict the future of populations, we need to understand their individual drivers and thus include individual responses to their environment while following the entire population. As a consequence, IPM2 will improve our ability to test ecological and evolutionary hypotheses and improve the accuracy of population forecasting to aid management programs.

Barn swallow antipredator behavior covaries with melanic coloration and predicts survival

Conspecific individuals often consistently differ in their behavioral responses to specific exogenous stimuli. Such individual differences in have been shown to be heritable, suggesting that selection maintains variation in personality. However, survival selection on a major personality trait, antipredator behavior, and its sex-dependency, has been seldom measured in the wild. Here, we found that yearling barn swallows (Hirundo rustica) showed consistency in agitation upon repeated exposure to a restraint-handling protocol, probably reflecting antipredator behavior, and that males were more agitated than females. Females that exhibited larger antipredator behavior were more likely to survive until the next breeding season with no variation among 3 study years nor across 7 breeding colonies, suggesting that variation in antipredator behavior is not maintained by spatiotemporal variation in viability selection on antipredator behavior. In both sexes, the intensity of melanin-based ventral plumage coloration positively covaried with antipredator behavior, consistent with previous observations from diverse vertebrate species. In some barn swallow populations, male coloration is targeted by directional intersexual selection, suggesting that melanization signals antipredator behavior to prospecting females and that sexual selection for antipredator behavior, or other correlated personality traits, can drive the evolution of sexual color dimorphism in barn swallows. Thus, we showed that viability selection occurs on antipredator behavior and that spatiotemporal variation in selection has no major role in promoting variation in antipredator behavior. More melanized individuals were more agitated, implying that melanization may signal personality in a sexual communication context in a species where male coloration is an epigamic trait.

Migratory connectivity of barn swallows in South Africa to their Palaearctic breeding grounds

AbstractAimTo demonstrate that the population of barn swallow Hirundo rustica in South Africa in the austral summer is not a pan‐mixture of birds from their vast Palaearctic breeding range and to estimate proportions of barn swallows in four South African regions migrating to three breeding ground zones.LocationSouth Africa and Eurasia.MethodsWe analysed citizen science barn swallow ringing records, obtained between 1954 and 2011 in South Africa, and their recoveries made in Eurasia. We used the “division coefficient” method to model the spatial variability of the recovery process and to estimate proportions of barn swallows ringed in four regions of South Africa (KwaZulu‐Natal, Gauteng, eastern part of Northern Cape and the Western Cape) migrating to three zones in the Eurasian breeding grounds: western (west of 10°E), central (10°–60°E) and eastern (east of 60°E).ResultsThe percentages of barn swallows migrating from KwaZulu‐Natal to the eastern and central Palaearctic zones were estimated at 54.0% and 46.0%, respectively. The percentages from Gauteng to the central and western zones were 80.5% and 15.7%, from the Northern Cape to the central and western zones were 43.7% and 56.3% and from the Western Cape to the eastern, central and western zones were 30.8%, 35.4%, 33.8%. The estimate of the observer process suggested that reencounter rates per 10,000 swallows ringed increased from east to west: 32 reencounters per 10,000 in the eastern zone, 36 in the central zone and 50 in the western zone.Main conclusionsThese results provide the first quantitative estimates of the composition of barn swallow populations in four regions of South Africa, relative to their Palaearctic breeding zones.

Ventral colour, not tail streamer length, is associated with seasonal reproductive performance in a Chinese population of Barn Swallows (Hirundo rustica gutturalis)

Phenotypic differentiation is common among populations that have large geographic distributions. One proposed mechanism driving phenotype divergence is sexual selection, which predicts that trait differences among closely related populations are underlain by variation in reproductive performance. Here, we examined a population of Asian Barn Swallows (Hirundo rustica gutturalis) in northeastern China. We ask whether traits shown to be under divergent sexual selection in other Barn Swallow populations are under sexual selection in this north Asian population by determining whether two features of phenotype variation, tail streamer length and ventral plumage colouration (including both throat and belly regions), are sexually dimorphic, associated with patterns of assortative mating and predictive of reproductive success. In this population, the length of tail streamers did not correlate with ventral plumage colouration in either males or females. The length of tail streamers was sexually dimorphic, but we did not find assortative mating by tail streamer length. By contrast, we found no sexual dichromatism but we did find assortative mating by throat colouration. Our correlational results indicated that the breeding performance of male Barn Swallows was associated with differences in their ventral plumage colouration, suggesting that ventral plumage colouration is likely a target of sexual selection in this population. Our finding that tail streamer length is unlikely to be under sexual selection is consistent with studies of other H. rustica gutturalis populations. The result that ventral plumage colour is likely to be under sexual selection is partially consistent with previous studies on Japanese H. rustica gutturalis, in which male throat but not belly plumage colour is the sexually selected trait.

Cloacal microbiota of barn swallows from Northern Italy

Bird gut microbiota shows large variation among geographical populations of the same species – probably because, differently from mammals, gut microbiota of birds is largely affected by extrinsic factors such as diet and environmental conditions. We analysed the cloacal microbiota (CM) of 12 barn swallows (Hirundo rustica) from a colony in Northern Italy by high-throughput DNA sequencing of the 16S rRNA gene. The CMs, dominated by bacteria of the phyla Proteobacteria, Actinobacteria, Firmicutes and Bacteroidetes, did not significantly differ between adult females, adult males and fledglings. This first description of barn swallow CM in Italy, together with two previous studies from Europe, suggests that CMs of geographically different barn swallow populations are dominated by bacteria belonging to the same phyla but different genera. The intra-colony similarity of the CMs may be due to the exposure of individuals to the same local environmental conditions while on their breeding grounds.