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.

1.Predation is an important selective pressure that can influence prey species in numerous ways. Predator-prey relationships are, however, poorly understood in taxa not typically associated with these interactions; this is especially the case when bats (Chiroptera) are the prey. 2. The main aim here is to review and synthesise global information on the predation of bats by birds of prey (Accipitriformes and Falconiformes) and other diurnal bird groups. 3. We compiled data on incidences of predation of bats by diurnal birds, through an extensive multilingual study of bibliographic and Internet-based sources. Scientific papers were found mainly via the Thomson Reuters (Web of Science and Zoological Record) and Scopus databases, Google Scholar, and Google Books. Additional data were found through Internet searches of trip reports, images and videos carried by Google, Google Images, Flickr, and YouTube. 4. In total, more than 1500 cases of bats being attacked by diurnal predatory birds were obtained. Reports were documented from every continent (except Antarctica) and from 109 countries, and were thus distributed globally between 70°N and 43°S. Overall, we found evidence for predation of bats by 143 species of diurnal raptors (Accipitriformes 107 spp. and Falconiformes 36 spp.) and by 94 non-raptor bird species from 28 families. At least 124 and 50 bat species were taken as prey by raptors and by other diurnal bird species, respectively. 5. Attacks on bats by diurnal raptors were found to be distributed globally and were present in the majority of extant raptor lineages. Attacks on bats by other diurnal birds were also occasionally recorded. Furthermore, the majority of extant bat families featured as prey. These results strongly suggest that predation by birds may act as a major factor affecting the scarcity of daytime activity in bats and as a driver in the evolution of bat nocturnality.

Fascination with animals and their behaviour is one the most prominent patterns persisting in all human cultures. During the last decades, however, technological development and public access to the Internet have increased the speed and the extent of information sharing at an unprecedented rate, in some cases even challenging the traditional methods used in science. In order to understand the extent of this influence, we focused on the behaviour of shrikes. Shrikes are an enigmatic group of songbirds with a unique behaviour of impaling prey. We employed an extensive Internet search on YouTube (YT), a very popular and increasingly important source of information worldwide, for videos recording shrikes. Our analyses revealed that the number of shrike videos on YT is strongly positively correlated with classical knowledge on shrikes from books and scientific articles. Our results also suggest that in some cases YT may provide an alternative source of information on shrike ecology and behaviour. YT videos may thus provide new insights into the study of certain species or subjects and help identify gaps in ecological studies, especially in poorly studied species.Electronic supplementary materialThe online version of this article (doi:10.1007/s00114-017-1470-8) contains supplementary material, which is available to authorized users.

Life-history theory predicts that current behaviour affects future reproduction, implying that animals should optimise their escape strategies to reflect fitness costs and benefits of premature escape. Both costs and benefits of escape may change temporally with important consequences for the evolution of escape strategies. Moreover, escape strategies of species may differ according to their positions on slow–fast pace of life gradients. We studied risk-taking in long-distance migratory animals, waders (Charadriiformes), during the annual cycle, i.e., breeding in Europe, stopover in the Middle East and wintering in tropical Africa. Phylogenetically informed comparative analyses revealed that risk-taking (measured as flight initiation distance, FID) changed significantly over the year, being lowest during breeding and peaking at stopover sites. Similarly, relationships between risk-taking and life-history traits changed among stages of the annual cycle. While risk-taking significantly decreased with increasing body mass during breeding, risk-taking–body mass relationship became marginally significant in winter and disappeared during migration. The positive trend of risk-taking along slow–fast pace of life gradient measured as adult survival was only found during breeding. The season-dependent relationships between risk-taking and life history traits suggest that migrating animals respond to fluctuating environments by adopting behavioural plasticity.

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