Animals use acoustic signals for communication, implying that the properties of these signals can be under strong selection. The acoustic adaptation hypothesis predicts that species in dense habitats emit lower‐frequency sounds than those in open areas because low‐frequency sounds propagate further in dense vegetation than high‐frequency sounds. Signal frequency may also be under sexual selection because it correlates with body size and lower‐frequency sounds are perceived as more intimidating. Here, we evaluate these hypotheses by analysing variation in peak song frequency across 5,085 passerine species (Passeriformes). A phylogenetically informed analysis revealed that song frequency decreases with increasing body mass and with male‐biased sexual size dimorphism. However, we found no support for the predicted relationship between frequency and habitat. Our results suggest that the global variation in passerine song frequency is mostly driven by natural and sexual selection causing evolutionary shifts in body size rather than by habitat‐related selection on sound propagation.

Summary: 1. The phytohaemagglutinin (PHA) skin‐swelling test is widely used in immunoecology and ecotoxicology to estimate cell‐mediated immunity. Although often presumed, the involvement of T cells in generating an immune response to PHA in vivo remains unclear. 2. To investigate the mechanism triggering this response we have compared in zebra finch (Taeniopygia guttata) the immune responses to two PHA isolectins differing in their biological properties and one control protein. 3. In Experiment I, we applied PHA‐P (the commonly used L and E isolectin mixture) into one wing‐web and bovine serum allbumin (BSA) into the other. The swelling response to PHA‐P was significantly stronger than to BSA confirming that the reaction is governed by PHA‐specific properties. 4. In Experiment II, purified PHA‐L (the T‐cell‐stimulating isolectin) and PHA‐E (the erythroagglutinating isolectin) were compared. Contrary to our expectations, PHA‐E induced a significantly stronger swelling response than PHA‐L. Histological analysis revealed significantly higher quantities of erythrocytes and thrombocytes in PHA‐E‐treated patagia. Nevertheless, there was a positive correlation between the magnitudes of these swellings. 5. In Experiment III, we tested whether the results obtained by the application of PHA‐P and pure PHA‐L differ. Here, we failed to find any significant difference between these two preparations in their immunostimulatory activity. Magnitudes of the PHA‐L‐ and PHA‐P‐induced swellings were positively correlated. 6. To the best of our knowledge this is the first study to compare the biological activity of purified PHA fractions in vivo and also the first to show the importance of erythroagglutination in the development of an inflammatory response to PHA‐P. 7. Our results indicate that the skin‐swelling test using PHA‐P reliably mirrors the individual general proinflammatory potential. However, the immunological background of the test is highly complex and the test results cannot be interpreted as measurements of the adaptive immunity or T‐cell activity. This interpretational change importantly alters our view on the test results regarding the costs of the response or the evolutionary immunological adaptations.

Animal bodies are inhabited by a taxonomically and functionally diverse community of symbiotic and commensal microorganisms. From an ecological and evolutionary perspective, inter-individual variation in host-associated microbiota contributes to physiological and immune system variation. As such, host-associated microbiota may be considered an integral part of the host’s phenotype, serving as a substrate for natural selection. This assumes that host-associated microbiota exhibits high temporal stability, however, and that its composition is shaped by trans-generational transfer or heritable host-associated microbiota modulators encoded by the host genome. Although this concept is widely accepted, its crucial assumptions have rarely been tested in wild vertebrate populations. We performed 16S rRNA metabarcoding on an extensive set of faecal microbiota samples from an insectivorous, long-distance migratory bird, the barn swallow (Hirundo rustica). Our data revealed clear differences in faecal microbiota among juveniles and adults as regards taxonomic and functional composition, diversity and co-occurrence network complexity. Multiple faecal microbiota samples from the same juvenile or adult collected within single breeding seasons exhibited higher similarity than expected by chance, as did adult faecal microbiota samples over two consecutive years. Despite low effect sizes for faecal microbiota stability over time at the community level, we identified an adult faecal microbiota subset with relative abundances exhibiting significant temporal consistency, possibly inducing long-term effects on the host phenotype. Our data also indicate a slight maternal (but not paternal) effect on faecal microbiota composition in social offspring, though this is unlikely to persist into adulthood. We discuss our findings in the context of both evolution and ecology of microbiota vs. host interactions and barn swallow biology.