Abstract Many fresh and coastal waters are becoming increasingly turbid because of human activities, which may disrupt the visually mediated behaviours of aquatic organisms. Shoaling fish typically depend on vision to maintain collective behaviour, which has a range of benefits including protection from predators, enhanced foraging efficiency and access to mates. Previous studies of the effects of turbidity on shoaling behaviour have focussed on changes to nearest neighbour distance and average group‐level behaviours. Here, we investigated whether and how experimental shoals of three‐spined sticklebacks (Gasterosteus aculeatus) in clear (<10 Nephelometric Turbidity Units [NTU]) and turbid (~35 NTU) conditions differed in five local‐level behaviours of individuals (nearest and furthest neighbour distance, heading difference with nearest neighbour, bearing angle to nearest neighbour and swimming speed). These variables are important for the emergent group‐level properties of shoaling behaviour. We found an indirect effect of turbidity on nearest neighbour distances driven by a reduction in swimming speed, and a direct effect of turbidity which increased variability in furthest neighbour distances. In contrast, the alignment and relative position of individuals was not significantly altered in turbid compared to clear conditions. Overall, our results suggest that the shoals were usually robust to adverse effects of turbidity on collective behaviour, but group cohesion was occasionally lost during periods of instability.

The cover image is based on the Letter AVONET: morphological, ecological and geographical data for all birds by Tobias et al., https://doi.org/10.1111/ele.13898. The sword-billed hummingbird (Ensifera ensifera) is exquisitely adapted to its trophic niche as an aerial pollinator of flowerings plants (angiosperms) in the high Andes. A new global dataset of detailed ecomorphological traits for all birds offers a resource with numerous potential uses in ecology, evolution, and conservation biology. Image Credit: Cover Image © Oliver Krüger. Reproduced with permission.

Functional traits offer a rich quantitative framework for developing and testing theories in evolutionary biology, ecology and ecosystem science. However, the potential of functional traits to drive theoretical advances and refine models of global change can only be fully realised when species-level information is complete. Here we present the AVONET dataset containing comprehensive functional trait data for all birds, including six ecological variables, 11 continuous morphological traits, and information on range size and location. Raw morphological measurements are presented from 90,020 individuals of 11,009 extant bird species sampled from 181 countries. These data are also summarised as species averages in three taxonomic formats, allowing integration with a global phylogeny, geographical range maps, IUCN Red List data and the eBird citizen science database. The AVONET dataset provides the most detailed picture of continuous trait variation for any major radiation of organisms, offering a global template for testing hypotheses and exploring the evolutionary origins, structure and functioning of biodiversity.

Consistent inter-individual variation in behaviour within a population, widely referred to as personality variation, can be affected by environmental context. Feedbacks between an individual's behaviour and state can strengthen (positive feedback) or weaken (negative feedback) individual differences when experiences such as predator encounters or winning contests are dependent on behavioural type. We examined the influence of foraging on individual-level consistency in refuge use (a measure of risk-taking, i.e. boldness) in three-spined sticklebacks, Gasterosteus aculeatus, and particularly whether changes in refuge use depended on boldness measured under control conditions. In the control treatment trials with no food, individuals were repeatable in refuge use across repeated trials, and this behavioural consistency did not differ between the start and end of these trials. In contrast, when food was available, individuals showed a higher degree of consistency in refuge use at the start of the trials versus controls but this consistency significantly reduced by the end of the trials. The effect of the opportunity to forage was dependent on behavioural type, with bolder fish varying more in their refuge use between the start and the end of the feeding trials than shyer fish, and boldness positively predicted the likelihood of feeding at the start but not at the end of the trials. This suggests a state-behaviour feedback, but there was no overall trend in how bolder individuals changed their behaviour. Our study shows that personality variation can be suppressed in foraging contexts and a potential but unpredictable role of feedbacks between state and behaviour.Significance statementIn this experimental study, we examined how foraging influences consistency in risk-taking in individual three-spined sticklebacks. We show that bolder individuals become less consistent in their risk-taking behaviour than shyer individuals during foraging. Some bolder individuals reinforce their risk-taking behaviour, suggesting a positive feedback between state and behaviour, while others converge on the behaviour of shyer individuals, suggesting a negative feedback. In support of a role of satiation in driving negative feedback effects, we found that bolder individuals were more likely to feed at the start but not at the end of the trials. Overall, our findings suggest that foraging can influence personality variation in risk-taking behaviour; however, the role of feedbacks may be unpredictable.

Despite extensive interest in the dynamic interactions between individuals that drive collective motion in animal groups, the dynamics of collective motion over longer time frames are understudied. Using three-spined sticklebacks, Gasterosteus aculeatus, randomly assigned to 12 shoals of eight fish, we tested how six key traits of collective motion changed over shorter (within trials) and longer (between days) timescales under controlled laboratory conditions. Over both timescales, groups became less social with reduced cohesion, polarization, group speed and information transfer. There was consistent inter-group variation (i.e. collective personality variation) for all collective motion parameters, but groups also differed in how their collective motion changed over days in their cohesion, polarization, group speed and information transfer. This magnified differences between groups, suggesting that over time the ‘typical’ collective motion cannot be easily characterized. Future studies are needed to understand whether such between-group differences in changes over time are adaptive and represent improvements in group performance or are suboptimal but represent a compromise between individuals in their preferences for the characteristics of collective behaviour.

An organism’s ability to disperse influences many fundamental processes, from speciation and geographical range expansion to community assembly. However, the patterns and underlying drivers of variation in dispersal across species remain unclear, partly because standardised estimates of dispersal ability are rarely available. Here we present a global dataset of avian hand-wing index (HWI), an estimate of wing shape widely adopted as a proxy for dispersal ability in birds. We show that HWI is correlated with geography and ecology across 10,338 (>99%) species, increasing at higher latitudes and with migration, and decreasing with territoriality. After controlling for these effects, the strongest predictor of HWI is temperature variability (seasonality), with secondary effects of diet and habitat type. Finally, we also show that HWI is a strong predictor of geographical range size. Our analyses reveal a prominent latitudinal gradient in HWI shaped by a combination of environmental and behavioural factors, and also provide a global index of avian dispersal ability for use in community ecology, macroecology, and macroevolution.

Consistent inter-individual variation within a population, widely referred to as personality variation, can be affected by environmental context. Feedbacks between an individual’s personality and state can strengthen (positive feedback) or weaken (negative feedback) individual differences when experiences such as predator encounters or winning contests are dependent on personality type. We examined the influence of foraging on individual-level consistency in refuge use (a measure of risk-taking, i.e. boldness) in three-spined sticklebacks, Gasterosteus aculeatus , and particularly whether changes in refuge use depended on boldness measured under control conditions. In the control treatment trials with no food, individuals were repeatable in refuge use across repeated trials, and this behavioral consistency did not differ between the start and end of these trials. In contrast, when food was available, individuals showed a higher degree of consistency in refuge use at the start of the trials versus controls but this consistency significantly reduced by the end of the trials. The effect of the opportunity to forage was dependent on personality, with bolder fish varying more in their refuge use between the start and the end of the feeding trials than shyer fish. This suggests a state-behavior feedback, but there was no overall trend in how individuals changed their behavior, with some individuals spending more, and others less, time in the refuge area at the end than at the start of the trials. Our study shows that personality variation can be suppressed in foraging contexts and a potential but unpredictable role of feedbacks between state and behavior.

Animal groups vary in their collective order (or state), forming disordered swarms to highly polarized groups. One explanation for this variation is that individuals face differential benefits or costs depending on the group's order, but empirical evidence for this is lacking. Here we show that in three-spined sticklebacks (Gasterosteus aculeatus), fish that are first to respond to an ephemeral food source do so faster when shoals are in a disordered, swarm-like state. This is because individuals' visual fields collectively cover more of their environment, meaning private information is more readily available in disordered groups. Once social information becomes available, however, the arrival times of subsequent group members to the food are faster in more ordered, polarized groups. Our data further suggest that first responding individuals (those that benefit from group disorder) maintain larger differences in heading angle to their nearest neighbours when shoaling, thereby explaining how conflict over whether private or social information is favoured can drive dynamic changes in collective behaviour.

An organism’s ability to disperse influences many fundamental processes in ecology. However, standardised estimates of dispersal ability are rarely available, and thus the patterns and drivers of broad-scale variation in dispersal ability remain unclear. Here we present a global dataset of avian hand-wing index (HWI), an estimate of wingtip pointedness widely adopted as a proxy for flight efficiency and dispersal in birds. We show that HWI is correlated with geography and ecology across 10,391 (>99 %) bird species, increasing at higher latitudes and in migratory and/or non-territorial species. After controlling for these effects, the strongest predictor of HWI is temperature variability (i.e. seasonality), with secondary effects of diet and habitat type. Our analyses (1) reveal a prominent latitudinal gradient in HWI shaped by ecological and environmental factors, and (2) provide a global index of avian dispersal ability for wider use in community ecology, macroecology, and macroevolution.

Animal eyes are some of the most widely recognisable structures in nature. Due to their salience to predators and prey, most research has focused on how animals hide or camouflage their eyes [1]. However, across all vertebrate Classes, many species actually express brightly coloured or conspicuous eyes, suggesting they may have also evolved a signalling function. Nevertheless, perhaps due to the difficulty with experimentally manipulating eye appearance, very few species beyond humans [2] have been experimentally shown to use eyes as signals [3]. Using staged behavioural trials we show that Trinidadian guppies (Poecilia reticulata), which can rapidly change their iris colour, predominantly express conspicuous eye colouration when performing aggressive behaviours towards smaller conspecifics. Furthermore, using a novel, visually realistic robotic system to create a mismatch between signal and relative competitive ability, we show that eye colour is used to honestly signal aggressive motivation. Specifically, robotic 'cheats' (that is, smaller, less-competitive robotic fish that display aggressive eye colouration when defending a food patch) attracted greater food competition from larger real fish. Our study suggests that eye colour may be an under-appreciated aspect of signalling in animals, shows the utility of our biomimetic robotic system for investigating animal behaviour, and provides experimental evidence that socially mediated costs towards low-quality individuals may maintain the honesty of dynamic colour signals.

Sexual selection is proposed to be an important driver of speciation and phenotypic diversification in animal systems. However, previous phylogenetic tests have produced conflicting results, perhaps because they have focused on a single signalling modality (visual ornaments), whereas sexual selection may act on alternative signalling modalities (e.g. acoustic ornaments). Here, we compile phenotypic data from 259 avian sister species pairs to assess the relationship between visible plumage dichromatism—a standard index of sexual selection in birds—and macroevolutionary divergence in the other major avian signalling modality: song. We find evidence for a strong negative relationship between the degree of plumage dichromatism and divergence in song traits, which remains significant even when accounting for other key factors, including habitat type, ecological divergence and interspecific interactions. This negative relationship is opposite to the pattern expected by a straightforward interpretation of the sexual selection–diversification hypothesis, whereby higher levels of dichromatism indicating strong sexual selection should be related to greater levels of mating signal divergence regardless of signalling modality. Our findings imply a ‘trade-off’ between the elaboration of visual ornaments and the diversification of acoustic mating signals, and suggest that the effects of sexual selection on diversification can only be determined by considering multiple alternative signalling modalities.

Non-native animals can encounter very different environments than those they are adapted to. Functional changes in morphology, physiology and life-history following introduction show that organisms can adapt both fast and efficiently. It remains unclear, however, if female reproductive characters and male sexually selected behaviour show the same adaptive potential. Furthermore, the invasion success and evolutionary trajectory of non-native species might often depend on the ability of the sexes to coordinate shifts in characters associated with reproductive strategy. The common wall lizard, Podarcis muralis, has been repeatedly introduced from southern Europe to England over the past 80 years. Lizards in England experience a cool, seasonal climate that effectively restricts recruitment to the first clutch of the season, whereas in their native range up to three clutches per season recruit. As a consequence, both females and males in non-native populations should benefit from reducing or even eliminating their reproductive investment in second clutches. Using a combination of field data and experiments, we show that non-native females produce relatively larger and heavier first seasonal clutches and smaller and lighter second seasonal clutches compared to native females. In contrast, non-native and native males do not differ in their territorial and sexual behaviour later in the season. An adaptive shift in male seasonal reproductive investment may be constrained because males use breeding females as cues for sexual behaviour. If this is so, we expect a general pattern across climatic regimes whereby female reproductive investment evolves first, with responses in males lagging behind.

One of the major decisions individuals of many species make when deciding who to mate with is whether or not to inbreed. The prevailing theory is that individuals should avoid mating with closely related individuals because of the fitness costs associated with decreased offspring quality. However, theoretical work has suggested that levels of inbreeding depression need to be considerable for these costs to outweigh the possible, kin selected, benefits of inbreeding. While evidence for a preference for inbreeding exists in the literature, empirical results currently lag well behind theory. Here, we conducted an experiment to examine mate discrimination and preference with respect to relatedness in a natural population of the family living lizard, Liopholis whitii. We show that both male and female L. whitii distinguish between related and unrelated individuals and, furthermore, that both sexes preferentially associate with more closely related members of the opposite sex. These results correspond closely with patterns of pairing observed in the wild where both males and females have been shown to choose mating pairs that are more closely related to them than expected by chance. Combined, our findings suggest that mate choice in this system may represent an active preference for mating with related individuals rather than a result of passive constraints associated with population viscosity.

Expanded materials and methods detailing trait and model selection Avian functional traits.

Table showing diet and trait variation across bird species identified in communities.

Summary Divergence in sexually selected traits in allopatry should affect the degree and direction of hybridization. However, few studies have established the causes and targets of sexual selection during secondary contact. Common wall lizards (Podarcis muralis) from north‐central Italy have highly exaggerated male sexual traits compared to populations in Western Europe. Using experimental populations, we show that this creates asymmetries in male dominance, spatial habitat use and reproductive success upon secondary contact. Hybridization occurred almost exclusively between males of the Italian lineage and females of the Western European lineage. We provide evidence to suggest stronger ongoing selection on male sexual traits within the dominant Italian lineage. However, these same characters did not predict hybridization, and hybrid matings contributed little to variance in male reproductive success. Instead, most hybrid offspring were sired by Italian males displaying phenotypes associated with lower within‐lineage reproductive success. Thus, highly directional hybridization arises because some Italian males are out‐competed within their own lineage but remain competitive relative to males of the other lineage. This pattern of hybridization is consistent with the direction of introgression in natural contact zones, but our data suggest that sexual selection acting through hybridization may be weak at the leading edge of natural hybrid zones. A lay summary is available for this article.

Phenotypic divergence in allopatry can facilitate speciation by reducing the likelihood that individuals of different lineages hybridize during secondary contact. However, few studies have established the causes of reproductive isolation in the crucial early stages of secondary contact. Here, we establish behavioural causes of assortative reproduction between two phenotypically divergent lineages of the European wall lizard (Podarcis muralis), which have recently come into secondary contact. Parentage was highly assortative in experimental contact zones. However, despite pronounced divergence in male phenotypes, including chemical and visual sexual signals, there was no evidence that females discriminated between males of the two lineages in staged interactions or under naturalistic free-ranging conditions. Instead, assortative reproduction was driven by male mate preferences and, to a lesser extent, male-male competition. The effects were more pronounced when the habitat structure promoted high lizard densities. These results emphasize that assortative reproduction can occur in the absence of female choice and that male behaviour may play an important role in limiting hybridization during the initial stages of secondary contact.

Vertebrates perform key roles in ecosystem processes via trophic interactions with plants and insects, but the response of these interactions to environmental change is difficult to quantify in complex systems, such as tropical forests. Here, we use the functional trait structure of Amazonian forest bird assemblages to explore the impacts of land-cover change on two ecosystem processes: seed dispersal and insect predation. We show that trait structure in assemblages of frugivorous and insectivorous birds remained stable after primary forests were subjected to logging and fire events, but that further intensification of human land use substantially reduced the functional diversity and dispersion of traits, and resulted in communities that occupied a different region of trait space. These effects were only partially reversed in regenerating secondary forests. Our findings suggest that local extinctions caused by the loss and degradation of tropical forest are non-random with respect to functional traits, thus disrupting the network of trophic interactions regulating seed dispersal by forest birds and herbivory by insects, with important implications for the structure and resilience of human-modified tropical forests. Furthermore, our results illustrate how quantitative functional traits for specific guilds can provide a range of metrics for estimating the contribution of biodiversity to ecosystem processes, and the response of such processes to land-cover change.

Hybridisation is increasingly recognised as an important cause of diversification and adaptation. Here, we show how divergence in male secondary sexual characters between two lineages of the common wall lizard (Podarcis muralis) gives rise to strong asymmetries in male competitive ability and mating success, resulting in asymmetric hybridisation upon secondary contact. Combined with no negative effects of hybridisation on survival or reproductive characters in F1-hybrids, these results suggest that introgression should be asymmetric, resulting in the displacement of sexual characters of the sub-dominant lineage. This prediction was confirmed in two types of secondary contact, across a natural contact zone and in two introduced populations. Our study illustrates how divergence in sexually selected traits via male competition can determine the direction and extent of introgression, contributing to geographic patterns of genetic and phenotypic diversity.

Competitive interactions among species with similar ecological niches are known to regulate the assembly of biological communities. However, it is not clear whether such forms of competition can predict the collapse of communities and associated shifts in ecosystem function in the face of environmental change. Here, we use phylogenetic and functional trait data to test whether communities of two ecologically important guilds of tropical birds (frugivores and insectivores) are structured by species interactions in a fragmented Amazonian forest landscape. In both guilds, we found that forest patch size, quality, and degree of isolation influence the phylogenetic and functional trait structure of communities, with small, degraded, or isolated forest patches having an increased signature of competition (i.e., phylogenetic and functional trait overdispersion in relation to null models). These results suggest that local extinctions in the context of fragmentation are nonrandom, with a consistent bias toward more densely occupied regions of niche space. We conclude that the loss of biodiversity in fragmented landscapes is mediated by niche-based competitive interactions among species, with potentially far-reaching implications for key ecosystem processes, including seed dispersal and plant damage by phytophagous insects.