Mixed-species Aggregations Research Articles

Variability and influence of fisheries acoustic echogram annotations on machine learning applications

High-frequency echosounders are the workhorse in fisheries and marine ecological surveys. Due to the inherent complexity of biological aggregations and ambiguity in interpreting echoes from species of similar size and anatomical compositions, echogram annotation typically requires combining spectral information referencing scattering physics, biological ground-truth from nearby net-trawls, and empirical school morphology of specific fish species. Here, we investigate the variability of echogram annotations and its influence on machine learning applications using data from the biennial Pacific hake acoustic-trawl survey. Compared to many other fish species, hake tend to possess less defined school boundaries with variable acoustic features and often form mixed-species aggregations in the mesopelagic. Nonnegative matrix factorization and hierarchical clustering of volume backscattering strength (Sv) distributions across the 18, 38, and 120 kHz channels revealed a spectrum of annotation region types that reflect differences in morphological and acoustic features as well as differences in annotator style. This variability likely contributes to the observed variable segmentation behavior of deep learning models trained using this dataset. These results highlight the importance of considering the diversity of echogram annotation, its connection to scattering physics and the underlying aggregation composition, and the incorporation of such information in developing machine learning models.

Inferring the decision rules that drive co-foraging affiliations in wild mixed-species parrot groups.

Animals gathered around a specific location or resource may represent mixed-species aggregations or mixed-species groups. Patterns of individuals choosing to join these groups can provide insight into the information processing underlying these decisions. However, we still have little understanding of how much information these decisions are based upon. We used data on 12 parrot species to test what kind of information each species may use about others to make decisions about which mixed-species aggregations to participate in. We used co-presence and joining patterns with categorization and model fitting methods to test how these species could be making grouping decisions. Species generally used a simpler lower-category method to choose which other individuals to associate with, rather than basing these decisions on species-level information. We also found that the best-fit models for decision-making differed across the 12 species and included different kinds of information. We found that not only does this approach provide a framework to test hypotheses about why individuals join or leave mixed-species aggregations, it also provides insight into what features each parrot could have been using to make their decisions. While not exhaustive, this approach provides a novel examination of the potential features that species could use to make grouping decisions and could provide a link to the perceptive and cognitive abilities of the animals making these minute-by-minute decisions. This article is part of the theme issue 'Mixed-species groups and aggregations: shaping ecological and behavioural patterns and processes'.

Material properties and broadband backscatter measurements of individual euphausiids from the California coastal ecosystem

During the 14–24 May 2019 leg of the NOAA juvenile rockfish survey, zooplankton and small nekton were sampled off the coast of Northern California using a midwater trawl. This talk will focus on three euphausiid species (E. pacifica, T. spinifera, N. difficilis) which were present in most catches. Krill morphology (size and shape) and animal density relative to seawater were measured for 175 live (or very recently expired) individuals. Roughly 150 individual krill were frozen and brought back to land for mass measurements. Soundspeed relative to seawater was measured for 15 mixed-species aggregations of krill. Broadband (38–73 and 130–210 kHz) and narrowband (38, 50, 70, 120, 150, and 200 kHz) backscatter measurements were made on 32 live (or very recently expired) individual krill. These empirical measurements were then compared to predictive backscatter measurements using scattering model parameters measured for those individuals (when available). Differences between predicted and measured backscatter were generally small, although there were individual krill where measurements at some frequencies varied significantly (i.e. more than 3dB). Understanding the variability in scattering model inputs among and within these species, and the accuracy of scattering model predictions provides context (i.e. uncertainty estimates) for acoustic measures (or indices) of krill biomass.

Multispecies collective waving behaviour in fish.

Collective behaviour is widely accepted to provide a variety of antipredator benefits. Acting collectively requires not only strong coordination among group members, but also the integration of among-individual phenotypic variation. Therefore, groups composed of more than one species offer a unique opportunity to look into the evolution of both mechanistic and functional aspects of collective behaviour. Here, we present data on mixed-species fish shoals that perform collective dives. These repeated dives produce water waves capable of delaying and/or reducing the success of piscivorous bird attacks. The large majority of the fish in these shoals consist of the sulphur molly, Poecilia sulphuraria, but we regularly also found a second species, the widemouth gambusia, Gambusia eurystoma, making these shoals mixed-species aggregations. In a set of laboratory experiments, we found that gambusia were much less inclined to dive after an attack as compared with mollies, which almost always dive, though mollies dived less deep when paired with gambusia that did not dive. By contrast, the behaviour of gambusia was not influenced by the presence of diving mollies. The dampening effect of less responsive gambusia on molly diving behaviour can have strong evolutionary consequences on the overall collective waving behaviour as we expect shoals with a high proportion of unresponsive gambusia to be less effective at producing repeated waves. This article is part of a discussion meeting issue 'Collective behaviour through time'.

Composition and Structure of Mixed-Species Foraging Aggregations of Waterbirds in Las Salinas, Ciénaga de Zapata, Cuba

Mixed-species foraging aggregations of waterbirds result from foraging and anti-predator behaviors, and understanding the dynamics of these aggregations is useful in planning the conservation of waterbirds species and understanding their ecological functions within the ecosystem. This paper describes the dynamics in composition and structure of mixed-species foraging aggregations during the wet and dry seasons in the lagoon system of Las Salinas, Ciénaga de Zapata, Cuba. We surveyed these aggregations monthly from 2009 to 2017. Aggregation structure was analysed through relative abundance per guild, percentage of presence, and participation weight and participation rate. Forty-two mixed-species foraging aggregations of waterbirds were observed throughout the study, with greater species richness (n = 12 species, confidence interval: 10–13) and number of individuals (n = 800 individuals, confidence interval: 565–1,055) in the wet season. During the dry season, the most prominent species in aggregation formation were Snowy Egrets (Egretta thula), American White Ibises (Eudocimus albus) and Roseate Spoonbills (Platalea ajaja), which have been identified as nuclear species in other studies. In the wet season, American White Pelicans (Pelecanus erythrorhynchos) appeared to be the nuclear species within aggregations, something that had not previously been described in mixed-species aggregations of waterbirds. Confirming these nuclear species at Las Salinas is an important next step, given that conservation efforts focused on nuclear species might benefit the maintenance and function of the entire assembly.

Any colour you like: fish interacting with bioinspired robots unravel mechanisms promoting mixed phenotype aggregations

Collective behaviours in homogeneous shoals provide several benefits to conspecifics, although mixed-species aggregations have been reported to often occur. Mixed aggregations may confer several beneficial effects such as antipredator and foraging advantages. However, the mechanisms promoting phenotypically heterogeneous fish aggregations have been poorly explored so far. Herein, the neon tetra Paracheirodon innesi was selected as the ideal model organism to test the role of visible phenotypic traits in promoting fish shoaling. Robotic fish replicas of different colours, but with a morphology inspired by P. innesi, were developed to test the affiliation behaviour of neon tetra individuals towards fish replicas with different phenotypic traits. P. innesi individuals showed a decreasing preference in shoaling with the biomimetic, the blue, the red, and the grey replicas. This could be due to the greater visibility of the blue colour even in dark conditions. Furthermore, an increased reddening of the livery is often caused by physiological processes related to a nonoptimal behavioural status. The time spent in shoaling with each fish replica was strongly influenced by different ecological contexts. The longest shoaling duration was observed when a biomimetic predator was present, while the shortest shoaling duration was recorded in the presence of food. This confirms the hypothesis that heterogeneous shoals are promoted by the antipredator benefits, and reduced by competition. This study allowed us to understand basic features of the behavioural ecology favouring heterogeneous aggregations in shoaling fish, and provided a novel paradigm for biohybrid robotics.

Fin, Humpback, and Minke Whale Foraging Events in the New York Bight as Observed from Aerial Surveys, 2017-2020

The fin whale (Balaenoptera physalus), humpback whale (Megaptera novaeangliae), and minke whale (Balaenoptera acutorostrata) are known to occur in the New York Bight (NYB). The primary North Atlantic feeding grounds for these large whale species are commonly recognized to be further north in waters of the Gulf of Maine, eastern Canada, West Greenland, and the eastern North Atlantic (e.g., Iceland, Norway, Ireland, Scotland). Although much is known about their feeding activities in the North Atlantic, relatively little is known about their occurrence and foraging behaviors in mid-Atlantic regions such as the NYB. Understanding how large whales utilize NYB waters is important to evaluate potential impacts from direct (e.g., offshore development, vessel strikes, entanglements) and indirect (e.g., rising ocean temperatures) anthropogenic sources. The New York State Department of Environmental Conservation funded a 3-year baseline monitoring program (2017-2020) which conducted monthly line-transect aerial surveys focused on large whales. Over 3 years, 36 surveys comprised of 263 flights and totaling 688.3 hours of observation time along 140,370 km of over-water flight path were completed. Aerial survey observers documented foraging events for the fin, humpback, and minke whales, including mixed-species aggregations, and analyzed other parameters such as distance from shore, distribution zones, and presence of fish schools. Foraging behavior was observed for 27% of the recorded fin whale sightings, 40% of the recorded humpback whale sightings, and 18% of the recorded minke whale sightings. Sighting rates of foraging whales were highest for humpback whales (4.4 whales/1,000 km of effort), followed by fin whales (0.6 whales/1,000 km effort) and minke whales (0.1 whales/1,000 km of effort), and varied by season, year, and distribution zone. In addition, nearly 5,700 fish schools were recorded with fish presence highest during summer and fall.

Benefits and Costs of Mixed-Species Aggregations in Harvestmen (Arachnida: Opiliones)

Many animals form aggregations with individuals of the same species (single-species aggregations, SSA). Less frequently, individuals may also aggregate with individuals of other species (mixed-species aggregations, MSA). Although the benefits and costs of SSA have been intensively studied, the same is not true for MSA. Here, we first review the cases of MSA in harvestmen, an arachnid order in which the records of MSA are more frequent than other arthropod orders. We then propose several benefits and costs of MSA in harvestmen, and contrast them with those of SSA. Second, using field-gathered data we describe gregariousness in seven species ofPrionostemmaharvestmen from Costa Rica. These species form MSA, but individuals are also found solitarily or in SSA. We tested one possible benefit and one possible cost of gregariousness inPrionostemmaharvestmen. Regarding the benefit, we hypothesized that individuals missing legs would be more exposed to predation than eight-legged individuals and thus they should be found preferentially in aggregations, where they would be more protected from predators. Our data, however, do not support this hypothesis. Regarding the cost, we hypothesized that gregariousness increases the chances of parasitism. We found no support for this hypothesis either because both mite prevalence and infestation intensity did not differ between solitary or aggregated individuals. Additionally, the type of aggregation (SSA or MSA) was not associated with the benefit or the cost we explored. This lack of effect may be explained by the fluid membership of the aggregations, as we found high turnover over time in the number of individuals and species composition of the aggregations. In conclusion, we hope our review and empirical data stimulate further studies on MSA, which remains one of the most elusive forms of group living in animals.

Mechanisms of collateral damage: heterospecific neighbor density mediates parasitism by eavesdroppers on hourglass treefrogs

Males often broadcast conspicuous signals from within mixed-species aggregations to attract mates. In addition to attracting females, however, these displays expose signalers to eavesdropping predators and parasites. For individuals in mixed-species aggregations, the balance between attracting mates and avoiding eavesdroppers is influenced both by the attractiveness of their calls and by the calling activity of conspecific and heterospecific neighbors. Through a process termed “collateral damage”, frogs signaling near heterospecific neighbors that are highly attractive to eavesdroppers can experience drastically increased parasitism compared to those signaling next to conspecifics. Here, we investigate whether the intensity of this collateral damage is influenced by the density of, or the call types produced by, heterospecific neighbors. In phonotaxis trials using hourglass treefrogs as the focal species and túngara frogs as the attractive heterospecific neighbor, we found no differences in the number of parasites attracted to hourglass treefrog calls played adjacent to simple versus complex túngara calls. By contrast, a higher density of neighboring túngara frogs decreased the collateral damage suffered by hourglass treefrogs by approximately 40%. Our results demonstrate that the density of nearby heterospecifics can modulate parasitism risk due to collateral damage in mixed-species aggregations. Thus, our conception of the dilution of eavesdropper risks with increasing group size may need to be augmented to include the effects of multiple prey or host types. HIGHLIGHTS ● Heterospecific call type does not change collateral damage on hourglass treefrogs ● However, parasitism decreases when calling next to high densities of heterospecifics

Baleen whale distribution, behaviour and overlap with anthropogenic activity in coastal regions of the New York Bight

ABSTRACT The New York Bight (NYB) is one of the busiest waterways along the US East Coast, and has recently seen an apparent increase in baleen whale presence. With growing anthropogenic pressures, a better understanding of their distribution and behaviour is needed. Encounter rates (sightings km−1) and behaviour of humpback (Megaptera novaeangliae), fin (Balaenoptera physalus) and minke (Balaenoptera acutorostrata) whales were obtained from dedicated non-systematic small vessel surveys conducted from May–November 2017–2019 in previously under-surveyed coastal areas of the NYB. Humpback whales were the most frequently sighted species, and their distribution in the NYB varied significantly by region and season. Within the mid-shelf region (10–60 km from shore), sightings of all species were generally concentrated spatially and temporally in mixed-species aggregations where feeding on sand lance (Ammodytes spp.) was observed. In the nearshore region (<10 km from shore), sightings of all species were more dispersed and typically occurred in close proximity to Atlantic menhaden (Brevoortia tyrannus). Humpback whales were commonly observed lunge feeding on these prey patches. The prevalence of foraging behaviour throughout the study area supports the hypothesis that the NYB may be an important supplementary feeding area for some species. Encounter rates were generally higher in areas overlapping with anthropogenic activity, suggesting that whales may be particularly vulnerable to anthropogenic impacts in the NYB. The observed species distribution patterns and importance of the NYB as a foraging habitat for baleen whales provides valuable information for the development of new baselines and best management practices within this heavily urbanized region.

Aggregations of Polistes Wasps Over-Wintering in Artificial Red-Cockaded Woodpecker Cavities

We document blockage of artificial Dryobates borealis (Red-cockaded Woodpecker) cavities by family Sphecidae (mud daubers) and large mixed-species aggregations of wintering Polistes (paper wasps) in cavity inserts at Great Dismal Swamp NWR in southeast Virginia. The large aggregations that we encountered are the only known cases of more than 2 paper wasp species cohabitating. Activity of Red-cockaded Woodpeckers within a cluster lessened the likelihood that paper wasps will aggregate in cavities, but not the likelihood that mud daubers will nest in cavities. The moist and saturated soils that predominate our field site may explain why these insects are more abundant compared to drier, upland habitats. Additionally, removal of hardwood trees may increase the breeding season habitat quality for paper wasps that hunt and nest in areas that are more open, while simultaneously limiting potential winter hibernacula habitat, typically found in mature hardwood tree cavities for Red-cockaded Woodpeckers. Common cavity management and translocation techniques may need to be altered to mitigate greater paper wasp and mud dauber activity at Red-cockaded Woodpecker cavity inserts in Pinus serotina (Pond Pine) pocosin habitat. We suggest using graduated rubber stoppers rather than screens that fail to exclude insect taxa or conducting translocations prior to the formation of winter aggregations of paper wasps.

Behavioural differences and interactions between two sessile bivalves forming mixed-species assemblages

The invasive zebra mussel, Dreissena polymorpha (ZM), established in Europe for a long time, has recently been joined and commonly outcompeted by a new invader, the quagga mussel, Dreissena rostriformis bugensis (QM). To identify factors contributing to this displacement, we studied behavioural differences between the species: aggregation, movement, and responses to conspecifics, congeners and their alarm cues. Compared to ZM, QM were more aggregated and less motile, crawling shorter distances for a shorter time at a slower speed. Conversely, QM exhibited more nonlocomotor movements. Both species aggregated and burrowed less and showed more nonlocomotor movements in response to conspecific and heterospecific alarm cues. They also moved shorter distances in the presence of conspecific alarm cues. ZM delayed their locomotion and nonlocomotor movements, whereas QM started locomotion earlier in the presence of both alarm cues. Mussel responses to living heterospecifics resembled those to alarm cues. In mixed-species aggregations, ZM attached to conspecifics more often than to QM shells, whereas QM were nonselective. To summarize, QM are less mobile, less selective regarding attachment site and more aggregated than ZM. This allows QM to perform better in mixed-species assemblages by spending less energy on relocation and overgrowing ZM to a higher extent than vice versa. Both species responded to heterospecific signals, which is helpful in mixed-species assemblages, particularly in novel areas occupied by these invasive species. Nevertheless, similar responses to alarm cues and living heterospecifics suggest a negative interaction between the congeners.

Mixed-Species Flock Fishing in Dalmatian Pelicans: Patterns and Benefits

In waterbirds, formation of mixed-species aggregations has been attributed to social facilitation and might also allow birds to exploit the ability of other species to make prey more accessible, to reduce their own time or effort spent in foraging and correspondingly increase foraging efficiency. However, some participants in mixed-species flocks may not benefit from the association. Dalmatian Pelicans often exhibit mixed-flock fishing with Great Cormorants, but there are no quantitative studies about the pattern and efficacy of mixed-flock fishing in Dalmatian Pelicans. Therefore, we studied the foraging behaviour of 328 Dalmatian Pelicans at the Lakes Mikri and Megali Prespa (Greece), during August 1993 and 2016. We examined the foraging patterns and the foraging success of Dalmatian Pelicans in relation to Cormorants' presence, water depth, Pelicans' age and flock size. Mixed-flock fishing with Cormorants was the dominant foraging behaviour of Pelicans, especially in deep waters and among adult Pelicans. Foraging effort was higher in adult Pelicans, in shallow waters, when Cormorants were absent and when occurring in larger flocks. Moreover, the number of successful dips per minute was higher in adults and when Cormorants were present. Finally, the number of successful prey captures per dip was higher in adults, in deeper waters and when Cormorants were present. However, in all cases interactions between the examined variables affected the observed differences between sexes and foraging conditions. These findings suggest that Dalmatian Pelicans can adjust their feeding tactics to the local habitat characteristics. We advocate that it is a case either of commensalism or of mutualism between the two species.

Social learning of acoustic anti-predator cues occurs between wild bird species.

In many species, individuals gather information about their environment both through direct experience and through information obtained from others. Social learning, or the acquisition of information from others, can occur both within and between species and may facilitate the rapid spread of antipredator behaviour. Within birds, acoustic signals are frequently used to alert others to the presence of predators, and individuals can quickly learn to associate novel acoustic cues with predation risk. However, few studies have addressed whether such learning occurs only though direct experience or whether it has a social component, nor whether such learning can occur between species. We investigate these questions in two sympatric species of Parids: blue tits (Cyanistes caeruleus) and great tits (Parus major). Using playbacks of unfamiliar bird vocalizations paired with a predator model in a controlled aviary setting, we find that blue tits can learn to associate a novel sound with predation risk via direct experience, and that antipredator response to the sound can be socially transmitted to heterospecific observers, despite lack of first-hand experience. Our results suggest that social learning of acoustic cues can occur between species. Such interspecific social information transmission may help to mediate the formation of mixed-species aggregations.

The Influence of Signaling Conspecific and Heterospecific Neighbors on Eavesdropper Pressure

The study of tradeoffs between the attraction of mates and the attraction of eavesdropping predators and parasites has generally focused on a single species of prey, signaling in isolation. In nature however, animals often signal from mixed-species aggregations, where interactions with heterospecific group members may be an important mechanism modulating tradeoffs between sexual and natural selection, and thus driving signal evolution. Although studies have shown that conspecific signalers can influence eavesdropper pressure on mating signals, the effects of signaling heterospecifics on eavesdropper pressure, and on the balance between natural and sexual selection, are likely to be different. Here, we review the role of neighboring signalers in mediating changes in eavesdropper pressure, and present a simple model that explores how selection imposed by eavesdropping enemies varies as a function of a signaling aggregation’s species composition, the attractiveness of aggregation members to eavesdroppers, and the eavesdroppers’ preferences to attack different member types. This approach can be used to model mixed-species signaling aggregations, as well as same-species aggregations, including those with non-signaling individuals, such as satellites or females. We discuss the implications of our model for the evolution of signal structure, signaling behavior, mixed-species aggregations, and community dynamics.

Social information cascades influence the formation of mixed-species foraging aggregations of ant-following birds in the Neotropics

Animals frequently make decisions based on social information obtained from other animals, which can influence interspecific interactions and affect individual fitness. For example, animals eavesdrop on other animals to find profitable food resources, yet the types of cues they use and how these cues influence decisions to approach a resource remain poorly understood. In tropical systems, arthropods inadvertently flushed by army ant, Eciton burchellii, swarms are an important food resource for many bird species, which form mixed-species foraging aggregations at swarms. Competition at swarms is intense and birds vocalize to defend foraging areas, inadvertently producing acoustic social information about the swarm's location. Eavesdropping birds may use these acoustic cues, which provide information about the bird aggregation (i.e. species participating in the aggregation, the size of the aggregation and/or diversity of the aggregation) to assess potential benefits (food resources) and costs (competition for food) of joining an aggregation. To test this hypothesis, we used an acoustic playback experiment to simulate aggregations of birds foraging at ant swarms and we measured community-wide and guild-specific responses of forest birds to playbacks. We included three types of acoustic social information in playbacks that potentially interact to affect an eavesdropping bird's probability of attraction to a swarm: (1) aggregation size, (2) aggregation species richness and (3) degree of specialization on ant swarms for food of birds vocalizing in the aggregation (hereafter ‘dependency’). Using Bayesian generalized linear mixed models, we found that playbacks of obligate ant-following species elicited greater community-wide responses (i.e. attracted more individuals and species) to simulated aggregations compared to playbacks of other, less dependent guilds. We also found that interactions between dependency, species richness and aggregation size influenced the overall community response to playbacks and that species from one guild generally responded to the guild above them (i.e. from less to more specialized). Our results suggest that species evaluate multiple types of acoustic cues representing the costs and benefits of foraging in a mixed-species aggregation at a swarm. We hypothesize that species change from information receivers to information producers upon joining a swarm, ultimately producing an information cascade that further affects the dynamics of feeding aggregations at swarms.

Observation of a mixed-sex, mixed-species aggregation of velvet ants, genus Timulla Ashmead, 1899 (Hymenoptera: Mutillidae) in the Brazilian Amazon, Roraima, with a new synonymy.

An aggregation of two males of Timulla bitaeniata (Spinola, 1841), three single males T. rufogastra (Lepeletier, 1845) and another two males T. rufogastra (Lepeletier, 1845) in copula with two females of T. eriphyla Mickel, 1938 is documented, described and discussed. A new synonymy is proposed for Mutilla rufogastra Lepeletier de Saint-Fargeau, 1845 [=Timulla (Timulla) eriphyla Mickel, 1938, syn. nov.] based on the mating pairs.

Mixed-species aggregations in arthropods.

This review offers the first synthesis of the research on mixed-species groupings of arthropods and highlights the behavioral and evolutionary questions raised by such behavior. Mixed-species groups are commonly found in mammals and birds. Such groups are also observed in a large range of arthropod taxa independent of their level of sociality. Several examples are presented to highlight the mechanisms underlying such groupings, particularly the evidence for phylogenetic proximity between members that promotes cross-species recognition. The advantages offered by such aggregates are described and discussed. These advantages can be attributed to the increase in group size and could be identical to those of nonmixed groupings, but competition-cooperation dynamics might also be involved, and such effects may differ between homo- and heterospecific groups. We discuss three extreme cases of interspecific recognition that are likely involved in mixed-species groups as vectors for cross-species aggregation: tolerance behavior between two social species, one-way mechanism in which one species is attractive to others and two-way mechanism of mutual attraction. As shown in this review, the study of mixed-species groups offers biologists an interesting way to explore the frontiers of cooperation-competition, including the process of sympatric speciation.

Male tree weta are attracted to cuticular scent cues but do not discriminate according to sex or among two closely related species

AbstractRecognition of conspecifics is an essential precursor of successful mating. Where related species coexist, species discrimination might be important, but because related species are similar, species signal recognition may actually be low. Chemical cues such as cuticular hydrocarbons (CHCs) are frequently used by insects to identify suitable sexual partners. We predicted that New Zealand tree weta (Hemideina spp.), a genus of nocturnal ensiferan Orthoptera that live both allopatrically and sympatrically, use chemical signals from either frass or CHCs to find mates. In a series of six laboratory trials using both H. thoracica and H. crassidens, we found that male tree weta, but not female tree weta, occupied cavities primed with female cuticular cues more often than cavities without. However, males did not discriminate between chemical cues of male and female conspecifics, or between conspecifics and heterospecifics. In field trials, tree weta did not occupy artificial cavities primed with either female frass or female cuticular cues more often than unscented cavities. However, in both trials weta preferentially returned to cavities that had already been occupied earlier in the trials. A final field trial confirmed the presence of mixed species harems during the mating season in one region of sympatry. Our results suggest that selection on sex and species specific chemical cues that could be used to find conspecific mates is weak. Mixed species aggregations suggest that identification of conspecific mating cues has not evolved to be species specific. We infer that for male tree weta, the cost of mating with heterospecifics is likely less than not mating at all.

Heterospecific attraction to the vocalizations of birds in mass-fruiting trees

A mixed-species aggregation refers to a group of individuals of different species attracted together by an environmental factor. Although they are traditionally considered not as structured as moving mixed-species groups, the composition and organization of aggregations can be influenced both by competition and by information flow between species. Frugivorous birds, for example, might move towards vocalizations of other species as cues to resources. If they differentially respond to a combination of other species’ vocalizations, an indication of a generalist fruiting tree, this could be a mechanism that creates nested structure in seed dispersal networks. We used both observational and experimental methods to study a community of frugivores in southwest China. Although the foraging niches of the birds highly overlapped, little interspecific aggression was found, suggesting a superabundant food resource. Birds were more attracted to playback of mass-fruiting trees, recorded when they were fruiting, than recordings made when those same trees were not fruiting. We compared attraction to the vocalizations of (a) an insectivorous species, (b, c, d) three species of frugivores, each presented separately, and (e) a combination of the frugivores in which their vocalizations were repeated sequentially. Response of heterospecific frugivores to the combination tape was significantly higher than to the insectivorous control, and tended (P = 0.068) to be higher than response to the frugivores. We suggest that this represents an indication that birds can use the diversity of vocalizations in a tree as a cue to understand the community of trees they interact with, as well as the community of birds. To our knowledge, although now used frequently with mixed-species bird flocks, playback has not been used to explore whether frugivorous birds could use heterospecific vocalizations to find fruit. We present the results of a simple, realistic experiment that shows that respondents approach trees in which birds, the majority of which are heterospecifics, are vocalizing. The results from a more complicated, controlled experiment suggest that birds may preferentially be attracted to a combination of frugivores’ sounds. We argue these results have relevance to the structure of seed dispersal networks, although we cannot demonstrate that specialist birds hone in on generalist trees by cuing on heterospecific vocalizations because of the rarity of specialists in our system.