Relationship among vegetation structure and mixed-species flocks composition along the latitudinal gradient of the subtropical montane forest of the Yungas, Argentina

The structural and physiognomic characteristics of forests and mountain forest are fundamental aspects that influence the richness, abundance and composition in the bird community. The objective of the present work was to analyse how the vegetation structure could influence the species composition of mixed-species flocks (MSF), along the latitudinal gradient of the subtropical montane forest of the Yungas of Argentina. Eight sites were studied along the 700 km of distribution of the Argentine Yungas. Richness and abundance of MSF were determined. In addition, different variables of composition and structure of the vegetation were analysed. Multivariate analysis indicated that vertical strata coverage and litter depth were the main variables associated with changes in the species composition of MSF along the gradient. Variation in MSF composition within the Yungas was associated with the physiognomy of the subtropical montane forest, which could indicate that it is strongly linked to the condition of the local vegetation. Substantial changes in vegetation could drastically change the composition of the resident flocks. This article is part of the theme issue 'Mixed-species groups and aggregations: shaping ecological and behavioural patterns and processes'.

Optionality in animal communication: a novel framework for examining the evolution of arbitrariness

A critical feature of language is that the form of words need not bear any perceptual similarity to their function - these relationships can be 'arbitrary'. The capacity to process these arbitrary form-function associations facilitates the enormous expressive power of language. However, the evolutionary roots of our capacity for arbitrariness, i.e. the extent to which related abilities may be shared with animals, is largely unexamined. We argue this is due to the challenges of applying such an intrinsically linguistic concept to animal communication, and address this by proposing a novel conceptual framework highlighting a key underpinning of linguistic arbitrariness, which is nevertheless applicable to non-human species. Specifically, we focus on the capacity to associate alternative functions with a signal, or alternative signals with a function, a feature we refer to as optionality. We apply this framework to a broad survey of findings from animal communication studies and identify five key dimensions of communicative optionality: signal production, signal adjustment, signal usage, signal combinatoriality and signal perception. We find that optionality is widespread in non-human animals across each of these dimensions, although only humans demonstrate it in all five. Finally, we discuss the relevance of optionality to behavioural and cognitive domains outside of communication. This investigation provides a powerful new conceptual framework for the cross-species investigation of the origins of arbitrariness, and promises to generate original insights into animal communication and language evolution more generally.

Impacts of invasive plants on animal behaviour

The spread of invasive species is a threat to ecosystems worldwide. However, we know relatively little about how invasive species affect the behaviour of native animals, even though behaviour plays a vital role in the biotic interactions which are key to understanding the causes and impacts of biological invasions. Here, we explore how invasive plants - one of the most pervasive invasive taxa - impact the behaviour of native animals. To promote a mechanistic understanding of these behavioural impacts, we begin by introducing a mechanistic framework which explicitly considers the drivers and ecological consequences of behavioural change, as well as the moderating role of environmental context. We then synthesise the existing literature within this framework. We find that while some behavioural impacts of invasive plants are relatively well-covered in the literature, others are supported by only a handful of studies and should be explored further in the future. We conclude by identifying priority topics for future research, which will benefit from an interdisciplinary approach uniting invasion ecology with the study of animal behaviour and cognition.

Foraging ecology drives social information reliance in an avian eavesdropping community

Vertebrates obtain social information about predation risk by eavesdropping on the alarm calls of sympatric species. In the Holarctic, birds in the family Paridae function as sentinel species; however, factors shaping eavesdroppers' reliance on their alarm calls are unknown. We compared three hypothesized drivers of eavesdropper reliance: (a) foraging ecology, (b) degree of sociality, and (c) call relevance (caller-to-eavesdropper body-size difference). In a rigorous causal-comparative design, we presented Tufted Titmouse (Baeolophus bicolor) alarm calls to 242 individuals of 31 ecologically diverse bird species in Florida forests and recorded presence/absence and type (diving for cover or freezing in place) of response. Playback response was near universal, as individuals responded to 87% of presentations (N = 211). As an exception to this trend, the sit-and-wait flycatcher Eastern Phoebe (Sayornis phoebe) represented 48% of the nonresponses. We tested 12 predictor variables representing measures relevant to the three hypothesized drivers, distance to playback speaker, and vulnerability at time of playback (eavesdropper's microhabitat when alarm call is detected). Using model-averaged generalized linear models, we determined that foraging ecology best predicted playback response, with aerial foragers responding less often. Foraging ecology (distance from trunk) and microhabitat occupied during playback (distance to escape cover) best predicted escape behavior type. We encountered a sparsity of sit-and-wait flycatchers (3 spp.), yet their contrasting responses relative to other foraging behaviors clearly identified foraging ecology as a driver of species-specific antipredator escape behavior. Our findings align well with known links between the exceptional visual acuity and other phenotypic traits of flycatchers that allow them to rely more heavily on personal rather than social information while foraging. Our results suggest that foraging ecology drives species-specific antipredator behavior based on the availability and type of escape cover.

Social information drives ecological outcomes among competing species

Through its behavior, an organism intentionally or unintentionally produces information. Use of this "social information" by surrounding conspecifics or heterospecifics is a ubiquitous phenomenon that can drive strong correlations in fitness-associated behaviors, such as predator avoidance, enhancing survival within and among competing species. By eliciting indirect positive interactions between competing individuals or species, social information might alter overall competitive outcomes. To test this potential, we present new theory that quantifies the effect of social information, modeled as predator avoidance signals/cues, on the outcomes from intraspecific and interspecific competition. Our analytical and numerical results reveal that social information can rescue populations from extinction and can shift the long-term outcome of competitive interactions from mutual exclusion to coexistence, or vice versa, depending on the relative strengths of intraspecific and interspecific social information and competition. Our findings highlight the importance of social information in determining ecological outcomes.

Supplemental feeding may reduce responsiveness of Black-capped Chickadees (Poecile atricapillus) to avian mobbing calls during gap-crossing experiments

Deforestation creates gaps in forest habitat, which can limit the movements of many avian species. Increased predation risk associated with crossing open habitats is often considered the primary impediment to crossing gaps. However, other factors such as energetic reserves may also influence these decisions. We conducted playback experiments before and after supplemental feeding of Black-capped Chickadees (Poecile atricapillus (Linnaeus, 1766)) to investigate how energetic reserves influenced gap-crossing decisions. Black-capped Chickadees were less likely to respond to playbacks, whether conducted within forests or across gaps, after supplemental feeding. This suggests that energetic reserves may have less influence on gap-crossing decisions than on the willingness of birds to respond to mobbing calls in general. We recommend that future studies investigating gap-crossing decisions using playbacks account for differences in resource availability among sampling locations, especially when making comparisons across discrete habitats.

Can behavioural ecologists help establish protected areas?

Protecting wild places is conservation's most pressing task given rapid contemporary declines in biodiversity and massive land use changes. We suggest that behavioural ecology has a valuable, albeit limited, role to play in this agenda. Behaviourally based empiricism and modelling, especially of animal movements and habitat preferences have enjoyed wide applicability in delineating reserve boundaries. In protected areas that sanction exploitation, it may also be important to understand individuals' behavioural and life-history responses to management decisions. We also argue, however, that the in-depth studies of behavioural ecologists may have an important role in conservation by elevating species' status from mundane to charismatic and often sparking public empathy, and their mere presence in field generates local (or broader) intrigue. More generally behavioural ecologists will only be listened to, and their contributions considered of conservation importance, if they become more involved in decision-making processes as witnessed by several prominent examples that have supported the establishment of protected areas. This article is part of the theme issue 'Linking behaviour to dynamics of populations and communities: application of novel approaches in behavioural ecology to conservation'.

Effects of livestock grazing on flocks of seed-eating birds in the central Monte desert, Argentina

Animal populations often decline due to habitat disturbance, but the initial response of organisms to human-induced environmental change is usually behavioral. Intra- and inter-specific interactions can restrict or facilitate access to resources, resulting in changes to individual fitness, and resource depletion may affect the frequency and strength of interactions. In birds, it is often assumed that feeding in groups increases foraging efficiency. We assessed how the reduction of seed resources provoked by cattle grazing affected different properties of seed-eating bird flocks in woodlands having the same structural characteristics but differing in seed abundance. Under lower availability of grass seeds (i.e., under grazing), flocks were smaller and less rich and birds showed a lower flocking propensity. This pattern could be explained by three non-exclusive hypotheses. Food reduction caused by grazing (i) decreases the number of seed-eating birds and concomitantly generates smaller flocks; (ii) reduces the density of nuclear species, decreasing the group cohesion in large flocks; (iii) makes large flocks less attractive by increasing individual competence for food. Our results provide evidence that cattle grazing affect the interactions of seed-eating birds and suggest the importance of understanding flocking behavior to bring about management actions.

Flocking propensity by satellites, but not core members of mixed-species flocks, increases when individuals experience energetic deficits in a poor-quality foraging habitat

Mixed-species bird flocks are complex social systems comprising core and satellite members. Flocking species are sensitive to habitat disturbance, but we are only beginning to understand how species-specific responses to habitat disturbance affect interspecific associations in these flocks. Here we demonstrate the effects of human-induced habitat disturbance on flocking species' behavior, demography, and individual condition within a remnant network of temperate deciduous forest patches in Indiana, USA. Specifically, we characterized the following properties of two core species, Carolina chickadees (Poecile carolinensis) and tufted titmice (Baeolophus bicolor), across a secondary-forest disturbance gradient: foraging time budgets, home range size, fat scores, fledgling counts, survival rates, and abundance. We also report fat scores for two satellite species that flock with the core study species: white-breasted nuthatches (Sitta carolinensis) and downy woodpeckers (Dryobates pubescens). Finally, we assess mixed-species flock sizes and composition, in addition to avian predator call rates, across the disturbance gradient. Foraging time budgets and home range size were highest and fat scores were lowest for core species in the most-disturbed site. Fat scores of two satellite species followed the same pattern. Additionally, the number of tufted titmice fledglings and winter survival rate of Carolina chickadees were lowest at the most-disturbed site. These results suggest that core species in the most-disturbed site experienced energetic deficits. Moreover, cumulative calling rate of raptors was lowest at the most-disturbed site, and none of the individual raptor species call rates were higher at the most-disturbed site-suggesting that perception of predation risk does not contribute to these patterns. Surprisingly, the satellites continued associating with mixed species flocks through the breeding season at the most-disturbed site. Total flock size and interspecific association patterns were otherwise consistent across the gradient. The fact that satellites continued to flock with core species during the breeding season suggests foraging niche expansion resulting from mixed-species flocking is important in disturbed sites even beyond the winter season. Our study reveals mechanisms underlying flock composition of birds surviving in remnant forest and links the mechanisms to degradation of foraging habitat. These findings offer important insight into the relative impact potential of forest disturbance on mixed-species flocks in the North Temperate Zone.

Niche shift in three foraging insectivorous birds in lowland Malaysian forest patches

With the rapid growth of agricultural areas globally, forest birds increasingly encounter fragmented landscapes in which forest patches are surrounded by an agricultural plantation matrix, yet how birds respond behaviourally to this fragmentation is poorly understood. Information on microhabitat requirements of birds is scarce, but nevertheless essential to predicting adaptation of bird species to the patchy landscapes. We investigated foraging patterns of three tropical insectivorous birds, Green Iora Aegithina viridissima, Pin-striped Tit-Babbler Macronus gularis and Chestnut-winged Babbler Cyanoderma erythropterum, to determine whether they vary in foraging methods in different forest patches. Our study area encompassed old-logged lowland forest; one continuous forest and three forest patches. Observations were performed for 15 days every month for a period of 13 months. Information on foraging height, substrate, attack manoeuvres, and foliage density was collected independently for each foraging bird individual. All three species used different foraging substrates and attack manoeuvres in different habitat types. The Green Iora frequently used lower strata when foraging in forest patches as opposed to continuous forest, while the Pin-striped Tit-Babbler tended to forage in more dense vegetation in patches. Only Chestnut-winged Babbler displayed complete foraging plasticity across all study parameters. Different habitat features (e.g., edges, microclimates) between continuous forest and forest patches significantly influenced the foraging strategies of the study species. These changes in foraging strategies suggest that some Malaysian forest birds (e.g. generalist species) can respond behaviourally to fragmentation and habitat loss. Although continuous forest has critically important characteristics that need to be conserved, remnant forest patches are also important as ecological movement corridors and foraging grounds for birds.

Foraging patch selection in winter: a balance between predation risk and thermoregulation benefit

In winter, foraging activity is intended to optimize food search while minimizing both thermoregulation costs and predation risk. Here we quantify the relative importance of thermoregulation and predation in foraging patch selection of woodland birds wintering in a Mediterranean montane forest. Specifically, we account for thermoregulation benefits related to temperature, and predation risk associated with both illumination of the feeding patch and distance to the nearest refuge provided by vegetation. We measured the amount of time that 38 marked individual birds belonging to five small passerine species spent foraging at artificial feeders. Feeders were located in forest patches that vary in distance to protective cover and exposure to sun radiation; temperature and illumination were registered locally by data loggers. Our results support the influence of both thermoregulation benefits and predation costs on feeding patch choice. The influence of distance to refuge (negative relationship) was nearly three times higher than that of temperature (positive relationship) in determining total foraging time spent at a patch. Light intensity had a negligible and no significant effect. This pattern was generalizable among species and individuals within species, and highlights the preponderance of latent predation risk over thermoregulation benefits on foraging decisions of birds wintering in temperate Mediterranean forests.

Forest fragmentation and body condition in wintering black-capped chickadees

Winter represents a critical energy management challenge for northern birds. Subzero temperatures may persist almost without interruption for months, food supply steadily decreases, day length greatly limits time available for foraging, and they must endure long fasting at night. Body condition represents a potentially important concept to our understanding of how severe winters impact individual fitness and populations. Many researchers have addressed body condition in small wintering birds. However, studies published so far have rarely been placed into a landscape context, despite reported effects of landscape structure on the ecology of birds exposed to severe winters. We investigated whether forest cover affects body condition in a population of black-capped chickadees ( Poecile atricapillus (L., 1766)) during winter. We measured residual body mass, fat score, and rate of growth of induced feathers to assess body condition in 12 landscapes with forest cover ranging from 10% to 87%. Forest cover was not associated with any of these measures of body condition, suggesting that highly deforested and fragmented landscapes were not energetically more costly to chickadees. However, we may not rule out the possibility that this result was biased against by a higher emigration or mortality rate of individuals in poor body condition.

Effects of habitat degradation on mixed-species bird flocks in Indian rain forests

Habitat degradation affects mixed-species bird flocks (flock hereafter) through two mechanisms – changes in the bird community from which flocks are drawn and changes in the propensities of species to flock. We determined the relative influence of these two mechanisms by examining variation in flocks across nine rain-forest fragments (range 11–2600 ha) in a plantation landscape in the Western Ghats, India. We found differences between fragments in average number of species (range 10.8–15.2) and individuals (range 19.0–37.6) per flock, number of species that participated in flocks (range 34–59), encounter rates (range 0.5–2.4 flocks h−1) and flock composition. Multiple regression and randomization tests revealed that different mechanisms contributed to this variation. Three flock variables (open-forest individuals per flock, total open-forest species that participated in flocks in a fragment, flock composition) mainly reflected changes in the bird communities of fragments. Habitat structure strongly influenced three flock variables (open-forest species per flock, total and rain-forest individuals per flock) and flock composition to a lesser extent. Finally, flock encounter rate was strongly related to fragment area, but not to abundance of flock participants indicating habitat degradation-induced changes in propensities of species to flock.

Sex and sociality in a disconnected world: a review of the impacts of habitat fragmentation on animal social interactionsThis review is one of a series dealing with some aspects of the impact of habitat fragmentation on animals and plants. This series is one of several virtual symposia focussing on ecological topics that will be published in the Journal from time to time

Despite the extensive literature describing the impacts of habitat fragmentation on the distribution and abundance of species, fragmentation effects on life-history strategies have been relatively understudied. Social interactions are important life-history attributes that have fitness consequences for individuals and have been observed to differ among populations in relation to geographic and demographic variability. Therefore, habitat fragmentation is expected to affect social interactions, and these social impacts or responses may contribute to population viability and broad-scale patterns of distribution and abundance in fragmented landscapes. Here we review the emerging literature on this issue. We focus on the impacts of habitat fragmentation that are expected to, or have been observed to, affect social strategies. These include altered resource distribution (e.g., habitat quality, spatial configuration of patches), interspecific interactions (e.g., predator–prey and host–parasite dynamics, human disturbance), and sex (mate availability and inbreeding risk). The studies we cite identified altered social interactions in response to these influences, including changes to home-range overlap, territoriality, group size, and mating systems. The observed changes to social interactions include passive responses, whereby social interactions are affected by constraints introduced by habitat fragmentation, and adaptive social responses to a modified environment. We suggest that future research could focus on individual fitness benefits and on consequences for population viability of altered social interactions in fragmented environments.