Cooperative bird differentiates between the calls of different individuals, even when vocalizations were from completely unfamiliar individuals

Why don't all animals avoid inbreeding?

Individuals are expected to avoid mating with relatives as inbreeding can reduce offspring fitness, a phenomenon known as inbreeding depression. This has led to the widespread assumption that selection will favour individuals that avoid mating with relatives. However, the strength of inbreeding avoidance is variable across species and there are numerous cases where related mates are not avoided. Here we test if the frequency that related males and females encounter each other explains variation in inbreeding avoidance using phylogenetic meta-analysis of 41 different species from six classes across the animal kingdom. In species reported to mate randomly with respect to relatedness, individuals were either unlikely to encounter relatives, or inbreeding had negligible effects on offspring fitness. Mechanisms for avoiding inbreeding, including active mate choice, post-copulatory processes and sex-biased dispersal, were only found in species with inbreeding depression. These results help explain why some species seem to care more about inbreeding than others: inbreeding avoidance through mate choice only evolves when there is both a risk of inbreeding depression and related sexual partners frequently encounter each other.

Uniform Manifold Approximation and Projection for Clustering Taxa through Vocalizations in a Neotropical Passerine (Rough-Legged Tyrannulet, Phyllomyias burmeisteri)

Vocalizations from birds are a fruitful source of information for the classification of species. However, currently used analyses are ineffective to determine the taxonomic status of some groups. To provide a clearer grouping of taxa for such bird species from the analysis of vocalizations, more sensitive techniques are required. In this study, we have evaluated the sensitivity of the Uniform Manifold Approximation and Projection (UMAP) technique for grouping the vocalizations of individuals of the Rough-legged Tyrannulet Phyllomyias burmeisteri complex. Although the existence of two taxonomic groups has been suggested by some studies, the species has presented taxonomic difficulties in classification in previous studies. UMAP exhibited a clearer separation of groups than previously used dimensionality-reduction techniques (i.e., principal component analysis), as it was able to effectively identify the two taxa groups. The results achieved with UMAP in this study suggest that the technique can be useful in the analysis of species with complex in taxonomy through vocalizations data as a complementary tool including behavioral traits such as acoustic communication.

Individual vocal recognition across taxa: a review of the literature and a look into the future

Individual vocal recognition (IVR) has been well studied in mammals and birds. These studies have primarily delved into understanding IVR in specific limited contexts (e.g. parent-offspring and mate recognition) where individuals discriminate one individual from all others. However, little research has examined IVR in more socially demanding circumstances, such as when an individual discriminates all individuals in their social or familial group apart. In this review, we describe what IVR is and suggest splitting studies of IVR into two general types based on what questions they answer (IVR-singular, and IVR-multiple). We explain how we currently test for IVR, and many of the benefits and drawbacks of different methods. We address why IVR is so prevalent in the animal kingdom, and the circumstances in which it is often found. Finally, we explain current weaknesses in IVR research including temporality, specificity, and taxonomic bias, and testing paradigms, and provide some solutions to address these weaknesses. This article is part of the theme issue 'Signal detection theory in recognition systems: from evolving models to experimental tests'.

Acorn woodpeckers vocally discriminate current and former group members from nongroup members

In species with long-term social relationships, the ability to recognize individuals after extended separation and the ability to discriminate between former social affiliates that have died and those that have left the group but may return are likely to be beneficial. Few studies, however, have investigated whether animals can make these discriminations. We presented acorn woodpeckers (Melanerpes formicivorus), a group-living, cooperatively breeding bird, with playbacks of current group members, former group members still living nearby, former group members that had died or left the study area, and familiar nongroup members. Subjects responded more quickly to the calls of nongroup members than to the calls of current group members or former group members still living in the study area but did not discriminate between nongroup members and former group members that had died or disappeared. This suggests that acorn woodpeckers can vocally recognize both current group members and former group members that have dispersed to nearby groups and that they either forget former group members that no longer live in the vicinity or classify them differently from former group members that still live nearby. This study suggests an important role for vocal recognition in maintaining valuable relationships with social affiliates postdispersal.

Cooperative bird discriminates between individuals based purely on their aerial alarm calls

From an evolutionary perspective, the ability to recognize individuals provides great selective advantages, such as avoiding inbreeding depression during breeding. Whilst the capacity to recognize individuals for these types of benefits is well established in social contexts, why this recognition might arise in a potentially deadly alarm-calling context following predator encounters is less obvious. For example, in most avian systems, alarm signals directed toward aerial predators represent higher predation risk and vulnerability than when individuals vocalize toward a terrestrial-based predator. Although selection should favor simple, more effective alarm calls to these dangerous aerial predators, the potential of these signals to nonetheless encode additional information such as caller identity has not received a great deal of attention. We tested for individual discrimination capacity in the aerial alarm vocalizations of the noisy miner (Manorina melanocephala), a highly social honeyeater that has been previously shown to be able to discriminate between the terrestrial alarm signals of individuals. Utilizing habituation–discrimination paradigm testing, we found conclusive evidence of individual discrimination in the aerial alarm calls of noisy miners, which was surprisingly of similar efficiency to their ability to discriminate between less urgent terrestrial alarm signals. Although the mechanism(s) driving this behavior is currently unclear, it most likely occurs as a result of selection favoring individualism among other social calls in the repertoire of this cooperative species. This raises the intriguing possibility that individualistic signatures in vocalizations of social animals might be more widespread than currently appreciated, opening new areas of bioacoustics research.

The association between evidence of a predator threat and responsiveness to alarm calls in Western Australian magpies (Cracticus tibicen dorsalis)

Alarm calls are a widespread form of antipredator defence and being alerted to the presence of predators by the alarm calls of conspecifics is considered one of the benefits of group living. However, while social information can allow an individual to gain additional information, it can also at times be inaccurate or irrelevant. Such variation in the accuracy of social information is predicted to select for receivers to discriminate between sources of social information. In this study, we used playback experiments to determine whether Western Australian magpies (Cracticus tibicen dorsalis) respond to the predator information associated with alarm calls. Magpies were exposed to the alarm calls of two group members that differed in the threat associated with the alarm call: one call was played in the presence of a predator model while the other was not—in order to establish differences in the predator information provided by each caller. We then played back the alarm calls of the same group members in the absence of the predator model to determine whether magpies responded differently to signallers in response to the previous association between the alarm call and a predator threat. We found that receivers showed significantly greater levels of responsiveness to signallers that previously gave alarm calls in the appropriate context. Thus, the accuracy of threat-based information influenced subsequent receiver response.

Nestlings reduce their predation risk by attending to predator-information encoded within conspecific alarm calls

Predation is one of the main threats to altricial nestlings, with predators often locating nests via eavesdropping on begging signals. Nestlings may be able to adjust their begging based on the current level of risk by monitoring both intra- and interspecific alarm calls near the nest. We show that noisy miner (Manorina melanocephala) nestlings can differentiate between terrestrial and aerial alarm calls of their own species, as they suppressed begging behaviour for longer in response to terrestrial rather than aerial alarm calls. This differential response is potentially due to greater danger that terrestrial calls encode. In contrast, nestlings ignored alarm calls of the sympatric grey butcherbird (Cracticus torquatus) and continued to beg but reduced begging intensity in response to the non-alarm calls of a sympatric eastern rosella (Platycercus eximius), suggesting nestlings were likely responding based upon similarity to a known signal as opposed to expressing a learnt behaviour. Results show that nestlings respond adaptively to two different intraspecific alarm signals but have not learnt to respond to the alarm calls of sympatric species. These suggest that nestlings are able to take advantage of the complex vocal repertoire that adults produce, although discernment is an issue when filtering out irrelevant stimuli.

Can Nocturnal Flight Calls of the Migrating Songbird, American Redstart, Encode Sexual Dimorphism and Individual Identity?

Bird species often use flight calls to engage in social behavior, for instance maintain group cohesion and to signal individual identity, kin or social associations, or breeding status of the caller. Additional uses also exist, in particular among migrating songbirds for communication during nocturnal migration. However, our understanding of the information that these vocalizations convey is incomplete, especially in nocturnal scenarios. To examine whether information about signaler traits could be encoded in flight calls we quantified several acoustic characteristics from calls of a nocturnally migrating songbird, the American Redstart. We recorded calls from temporarily captured wild specimens during mist-netting at the Powdermill Avian Research Center in Rector, PA. We measured call similarity among and within individuals, genders, and age groups. Calls from the same individual were significantly more similar to one another than to the calls of other individuals, and calls were significantly more similar among individuals of the same sex than between sexes. Flight calls from hatching-year and after hatching-year individuals were not significantly different. Our results suggest that American Redstart flight calls may carry identifiers of gender and individual identity. To our knowledge, this is the first evidence of individuality or sexual dimorphism in the flight calls of a migratory songbird. Furthermore, our results suggest that flight calls may have more explicit functions beyond simple group contact and cohesion. Nocturnal migration may require coordination among numerous individuals, and the use of flight calls to transmit information among intra- and conspecifics could be advantageous. Applying approaches that account for such individual and gender information may enable more advanced research using acoustic monitoring.

Flight calls signal group and individual identity but not kinship in a cooperatively breeding bird

In many complex societies, intricate communication and recognition systems may evolve to help support both direct and indirect benefits of group membership. In cooperatively breeding species where groups typically comprise relatives, both learned and innate vocal signals may serve as reliable cues for kin recognition. Here, we investigated vocal communication in the plural cooperatively breeding superb starling, Lamprotornis superbus, where flight calls-short, stereotyped vocalizations used when approaching conspecifics-may communicate kin relationships, group membership, and/or individual identity. We found that flight calls were most similar within individual repertoires but were also more similar within groups than within the larger population. Although starlings responded differently to playback of calls from their own versus other neighboring and distant social groups, call similarity was uncorrelated with genetic relatedness. Additionally, immigrant females showed similar patterns to birds born in the study population. Together, these results suggest that flight calls are learned signals that reflect social association but may also carry a signal of individuality. Flight calls, therefore, provide a reliable recognition mechanism for groups and may also be used to recognize individuals. In complex societies comprising related and unrelated individuals, signaling individuality and group association, rather than kinship, may be a route to cooperation.