Beauty alone is insufficient: female mate choice in the barn swallow

UV-induced feather color change reflects its porphyrin content

Pigmentary coloration is widespread in animals. Its evolutionary and ecological features are often attributed to the property of predominant pigments; therefore, most research has focused on predominant pigments such as carotenoids in carotenoid-based coloration. However, coloration results from predominant pigments and many other minority pigments, and the importance of the latter is overlooked. Here, we focused on porphyrin, an "uncommon" pigment found in bird feathers, and investigated its importance in the context of feather color changes in the barn swallow Hirundo rustica. We found that the "pheomelanin-based coloration" of the barn swallow faded after the irradiation of UV light, and this effect was particularly strong in the feathers of young swallows (nestlings and fledglings, here). We also found that it is not the predominant pigment, pheomelanin, but protoporphyrin IX pigment that showed the same pattern of depigmentation after the irradiation of UV light, particularly in the feathers of young swallows. In fact, the abovementioned age-dependent feather color change was statistically explained by the amount of porphyrin in the feathers. The current study demonstrates that a minority pigment, porphyrin, explains within-season dynamic color change, an ecological feature of feather coloration. The porphyrin-mediated rapid color change would benefit young birds, in which feather coloration affects the parental food allocation during a few weeks before independence, but not later. Future studies should not ignore these minor but essential pigments and their evolutionary and ecological functions.

Resource acquisition and pre-copulatory sexual selection

Sexual selection influences the evolution of phenotypic traits and contributes to patterns of biodiversity. In many animals, mating involves sequential steps. Often, individuals must secure resources that are essential for mating (nests, territories, food), and then after securing a resource, individuals engage in competition for access to limited opposite sex mates and gametes. A large body of empirical research and some verbal models have illustrated that resource acquisition can influence sexual selection. In general, though, we lack a priori predictions of when and how resource acquisition will influence sexual selection. Here, we use a mathematical framework to explore the link between resource acquisition and sexual selection on an advantageous mate‐acquisition trait across biologically relevant trade‐off scenarios. Our findings provide a set of testable predictions of how resource acquisition can influence sexual selection on mating traits. In general, selection on mate‐acquisition traits is expected to be heavily influenced by: (1) the episode of selection considered, and in particular, whether one considers selection associated with the mating pool only or selection associated with both the mating pool and pre‐mating pool; (2) whether resource‐acquisition and mate‐acquisition traits are positively associated or whether they trade off; and (3) the proportion of males with the resource‐ and mate‐acquisition traits.

Male Barn Swallows Tolerate Nestling-Like Courtship Calls of Rival Males

Animals often exhibit conspicuous, and sometimes curious, courtship traits, such as nestling-like courtship display in birds, though modern studies of nestling-like courtship display (and calls) are virtually lacking. An exception is previous experiments on the barn swallow Hirundo rustica, demonstrating that females are equally attracted to playback of two structurally similar calls, nestling-like male courtship calls and nestling food-begging calls. The experiments support the sensory trap hypothesis, i.e., that male signals mimic nestling stimuli to exploit female parental care for nestlings. However, female attraction might not be the sole function of nestling-like traits, and males might also have a sensory bias toward nestling-like traits, in which males would be less aggressive toward characteristics typical of immature individuals. Here, I conducted playback experiments to study the function of nestling-like male courtship calls in the context of male–male interactions. Playback of male courtship songs induced frequent approaches by neighbouring males, while nestling-like male courtship calls or nestling food-begging calls induced fewer approaches, though male responses to the latter two vocalisations increased when approaching the nestling period. The observed pattern indicates that, by mimicking immature individuals, males attract intended signal receivers (i.e., females) while avoiding interference from eavesdroppers (i.e., neighbouring males). This unique function can explain why species with parental care exhibit immature-like behaviour.

Nestling, but not adult male, barn swallows emit short calls in noisy environments

Some, but not all, animals cope with anthropogenic noise by changing the structure of their acoustic signals, possibly due to two interrelated factors: the design of the acoustic signals and the intrinsic biological state. Here, using the barn swallow (Hirundo rustica) that breed solitarily along streets, we studied male enticement calls and nestling food-begging calls in relation to ambient noise. The former calls are structurally quite similar to the latter, perhaps functioning as a sensory trap to exploit female parental care for nestlings. This provides a unique opportunity to study whether intrinsic biological state (i.e. developmental stage, here), virtually independent of acoustic design, matters in acoustic adaptation to anthropogenic noise. We found that the syllable length of nestling food-begging calls, but not of male enticement calls, decreased with increasing local noise levels within populations. The syllable lengths of the father’s enticement calls and those of the nestling’s food-begging calls were positively related, which explains why male enticement calls as well as nestling food-begging calls had shorter syllables in a noisy urban population compared to those in a quiet rural population. The current findings indicate that the intrinsic biological state of two acoustically similar sounds explains their differential relationships with ambient noise.

Opposing population trends of fork-tailed swallows and reddish-coloured swallows in our changing world

Sexual selection can in theory lead to positive and negative effect on population-level fitness and hence population increase/decline in our changing world, but the empirical evidence is scarce. Using a phylogenetic comparative approach, we examined whether and how different sexually selected ornaments affect recent population trends and extinction risk in swallows (Aves: Hirundininae). We found that population trends decreased with increasing depth of male tails, that is a well-known sexually selected trait, and increased with increasing score of reddish plumage coloration, another sexually selected ornament. Similar contrasting patterns were observed for extinction risk. These findings indicate ornament-specific population trends and extinction risk, perhaps due to the differential costs and benefits of ornamentation. Previous studies have mostly focused on the overall effects of sexual selection by combining different kinds of traits, presumed to be sexually selected. However, as predicted by theory, sexual selection would not be a process with the same universal effect on population dynamics as we found here. Divergent ecological consequences would occur through minor differences in sexual selection, which should not be dismissed in future studies.

Mineralogical and textural characteristics of nest building geomaterials used by three sympatric mud-nesting hirundine species

Many hirundine species construct their nests by carrying mud particles from adjacent areas. This study aimed to investigate for the first time the materials that mud-nesting hirundines choose for nest construction from a mineralogical and sedimentological perspective. For this purpose, we sampled nests of three sympatric species, namely the Barn Swallow (Hirundo rustica), the Red-rumped Swallow (Cecropis daurica) and the House Martin (Delichon urbicum), from southeastern Europe. Our results showed that all species tend to use clay minerals as a cement and especially smectite and illite and if these minerals are not present in the adjacent area, they use halloysite, kaolinite or chlorite. The amounts of clay minerals in the nests are generally low indicating that the studied species can accurately identify the properties of the nesting materials. Most of the non clay minerals that they use are the common, easily accessible colourless or white minerals with low specific gravity values such as quartz, feldspars and calcite. Grain size distribution analysis revealed that the amount of clay sized grains in the mud nests of all three species is relatively low, while the amount of larger grain particles decreases when the size of the non clay minerals is small. The Red-rumped Swallow showed an increasing preference for larger grain size particles and quartz, the Barn Swallow for finer grain size particles and calcite, and the preferences of the House Martin are in between the other two species. The three hirundine species present different nest building strategies and depending on the nest architecture, each of them seems to show preference for specific minerals and specific grain sizes.