Avian vocal mimicry: a unified conceptual framework

Mimicry is a classical example of adaptive signal design. Here, we review the current state of research into vocal mimicry in birds. Avian vocal mimicry is a conspicuous and often spectacular form of animal communication, occurring in many distantly related species. However, the proximate and ultimate causes of vocal mimicry are poorly understood. In the first part of this review, we argue that progress has been impeded by conceptual confusion over what constitutes vocal mimicry. We propose a modified version of Vane‐Wright's (1980) widely used definition of mimicry. According to our definition, a vocalisation is mimetic if the behaviour of the receiver changes after perceiving the acoustic resemblance between the mimic and the model, and the behavioural change confers a selective advantage on the mimic. Mimicry is therefore specifically a functional concept where the resemblance between heterospecific sounds is a target of selection. It is distinct from other forms of vocal resemblance including those that are the result of chance or common ancestry, and those that have emerged as a by‐product of other processes such as ecological convergence and selection for large song‐type repertoires. Thus, our definition provides a general and functionally coherent framework for determining what constitutes vocal mimicry, and takes account of the diversity of vocalisations that incorporate heterospecific sounds. In the second part we assess and revise hypotheses for the evolution of avian vocal mimicry in the light of our new definition. Most of the current evidence is anecdotal, but the diverse contexts and acoustic structures of putative vocal mimicry suggest that mimicry has multiple functions across and within species. There is strong experimental evidence that vocal mimicry can be deceptive, and can facilitate parasitic interactions. There is also increasing support for the use of vocal mimicry in predator defence, although the mechanisms are unclear. Less progress has been made in explaining why many birds incorporate heterospecific sounds into their sexual displays, and in determining whether these vocalisations are functionally mimetic or by‐products of sexual selection for other traits such as repertoire size. Overall, this discussion reveals a more central role for vocal mimicry in the behavioural ecology of birds than has previously been appreciated. The final part of this review identifies important areas for future research. Detailed empirical data are needed on individual species, including on the structure of mimetic signals, the contexts in which mimicry is produced, how mimicry is acquired, and the ecological relationships between mimic, model and receiver. At present, there is little information and no consensus about the various costs of vocal mimicry for the protagonists in the mimicry complex. The diversity and complexity of vocal mimicry in birds raises important questions for the study of animal communication and challenges our view of the nature of mimicry itself. Therefore, a better understanding of avian vocal mimicry is essential if we are to account fully for the diversity of animal signals.     

Sound production in the Ponto‐Caspian goby Neogobius fluviatilis and acoustic affinities within the Gobius lineage: implications for phylogeny

The aim of this study was to describe the vocal repertoire of the Ponto‐Caspian goby Neogobius fluviatilis and to compare the acoustic properties of this species with those of other soniferous Mediterranean gobies belonging to the Gobius lineage. Vocalizations and associated behaviours were recorded under controlled aquarium conditions in female and male N. fluviatilis. Sound emission was elicited by means of ‘intruder tests’, using an individual of the same or opposite sex as an intruder, and recording sounds using a hydrophone placed 20 cm from the shelter used as a nest for the resident fish. Five acoustic properties, including spectral and temporal properties, were measured from 13 individuals. The vocal repertoire of the species consisted of sequences of short vocalizations during both agonistic and reproductive intraspecific interactions. The wave form of each sound resolved in a pure sine wave composed of rapidly repeated pulses. Sounds lasted about 200 ms, showing an average fundamental frequency of about 80 Hz. Sound properties did not differ between reproductive and the aggressive contexts, and the general structure of sounds was highly stereotyped. The individual means of three acoustic independent traits characterizing the sounds of seven species of the Gobius lineage, including N. fluviatilis, were then entered in a discriminant function analysis to assess how well species could be differentiated on the basis of acoustics, and their degree of affinities. The results suggested that the pulse repetition rate of the sounds, i.e. the relative tonal/pulsatile nature of the sounds, was the most important property in differentiating the species, and that this trait may contain a high level of phylogenetic signal, as the species producing tonal sounds clustered together, in line with the results of recent molecular phylogenic studies. The results were discussed in light of the geological and phylogeographical events believed to have driven the diversification of European gobies.     

Testing hypotheses of mitochondrial gene‐tree paraphyly: unravelling mitochondrial capture of the Streak‐breasted Scimitar Babbler (Pomatorhinus ruficollis) by the Taiwan Scimitar Babbler (Pomatorhinus musicus)

Species‐level paraphyly inferred from mitochondrial gene trees is a prevalent phenomenon in taxonomy and systematics, but there are several potential causes that are not easily explained by currently used methods. This study aimed to test the underlying causes behind the observed paraphyly of Streak‐breasted Scimitar Babbler (Pomatorhinus ruficollis) via statistical analyses of four mitochondrial (mtDNA) and nine nuclear (nuDNA) genes. Mitochondrial gene trees show paraphyly of P. ruficollis with respect to the Taiwan Scimitar Babbler (Pomatorhinus musicus), but nuclear genealogies support a sister‐group relationship. Predictive coalescent simulations imply several hypothetical explanations, the most likely being mitochondrial capture of P. ruficollis by P. musicus for the observed cyto‐nuclear incongruence. Further approximate Bayesian computation suggests a unidirectional introgression model with substantial level of gene flow from P. ruficollis to P. musicus during their initial divergence during the Late Pleistocene. This specific observation frames several potential causes for incongruent outcomes of mitochondrial and nuclear introgression in general, and on the whole, our results underscore the strength of multiple independent loci for species delimitation and importance of testing hypotheses that explain disparate causes of mitochondrial gene‐tree paraphyly.     

Dual Function of Chip Calls Depending on Changing Call Rate Related to Risk Level in Territorial Pairs of White‐Eared Ground‐Sparrows

Different mechanisms have been proposed for encoding information into vocalizations: variation of frequency or temporal characteristics, variation in the rate of vocalization production, and use of different vocalization types. We analyze the effect of rate variation on the dual function of chip calls (contact and alarm) produced by White‐eared Ground‐sparrows (Melozone leucotis). We conducted an acoustic playback experiment where we played back 1 min of four chip call rates (12, 36, 60, 84 calls/min). We measured the response of territorial pairs using behavioral responses, and fine structural features of calls produced in response to those playbacks. White‐eared Ground‐sparrows showed more intense behavioral responses to higher than lower call rate playbacks. Both individuals of the pair approached the source of the playback stimulus faster, produced the first vocalization faster, produced more vocalizations, and spent more time close to the stimulus in higher call rate than in lower call rate playbacks. Frequency and duration characteristics of calls (chip and tseet) were similar in response to all call rate playbacks. Our playback experiment elicited different intensity of behavioral responses, suggesting that risk‐based information is encoded in call rate. Our results suggest that variation in the rate of chip call production serves a dual function in this species; calls are used at lower rates for pair contact and at higher rates for alarm/mobbing signals.     

VIM‐1 carbapenemase‐producing Escherichia coli in gulls from southern France

Acquired carbapenemases currently pose one of the most worrying public health threats related to antimicrobial resistance. A NDM‐1‐producing Salmonella Corvallis was reported in 2013 in a wild raptor. Further research was needed to understand the role of wild birds in the transmission of bacteria resistant to carbapenems. Our aim was to investigate the presence of carbapenem‐resistant Escherichia coli in gulls from southern France. In 2012, we collected 158 cloacal swabs samples from two gull species: yellow‐legged gulls (Larus michahellis) that live in close contact with humans and slender‐billed gulls (Chroicocephalus genei) that feed at sea. We molecularly compared the carbapenem‐resistant bacteria we isolated through culture on selective media with the carbapenem‐susceptible strains sampled from both gull species and from stool samples of humans hospitalized in the study area. The genes coding for carbapenemases were tested by multiplex PCR. We isolated 22 carbapenem‐resistant E. coli strains from yellow‐legged gulls while none were isolated from slender‐billed gulls. All carbapenem‐resistant isolates were positive for bla_VIM‐1 gene. VIM‐1‐producing E. coli were closely related to carbapenem‐susceptible strains isolated from the two gull species but also to human strains. Our results are alarming enough to make it urgently necessary to determine the contamination source of the bacteria we identified. More generally, our work highlights the need to develop more bridges between studies focusing on wildlife and humans in order to improve our knowledge of resistant bacteria transmission routes.     

Interspecific signalling between mutualists: food-thieving drongos use a cooperative sentinel call to manipulate foraging partners

Interspecific communication is common in nature, particularly between mutualists. However, whether signals evolved for communication with other species, or are in fact conspecific signals eavesdropped upon by partners, is often unclear. Fork-tailed drongos (Dicrurus adsimilis) associate with mixed-species groups and often produce true alarms at predators, whereupon associating species flee to cover, but also false alarms to steal associating species'' food (kleptoparasitism). Despite such deception, associating species respond to drongo non-alarm calls by increasing their foraging and decreasing vigilance. Yet, whether these calls represent interspecific sentinel signals remains unknown. We show that drongos produced a specific sentinel call when foraging with a common associate, the sociable weaver (Philetairus socius), but not when alone. Weavers increased their foraging and decreased vigilance when naturally associating with drongos, and in response to sentinel call playback. Further, drongos sentinel-called more often when weavers were moving, and weavers approached sentinel calls, suggesting a recruitment function. Finally, drongos sentinel-called when weavers fled following false alarms, thereby reducing disruption to weaver foraging time. Results therefore provide evidence of an ‘all clear’ signal that mitigates the cost of inaccurate communication. Our results suggest that drongos enhance exploitation of a foraging mutualist through coevolution of interspecific sentinel signals.     

Functionally Referential Alarm Calls in Tamarins (Saguinus fuscicollis and Saguinus mystax) – Evidence from Playback Experiments

Studies on primate vocalisation have revealed different types of alarm call systems ranging from graded signals based on response urgency to functionally referential alarm calls that elicit predator‐specific reactions. In addition, alarm call systems that include both highly specific and other more unspecific calls have been reported. There has been consistent discussion on the possible factors leading to the evolution of different alarm call systems, among which is the need of qualitatively different escape strategies. We studied the alarm calls of free‐ranging saddleback and moustached tamarins (Saguinus fuscicollis and Saguinus mystax) in northeast Peru. Both species have predator‐specific alarm calls and show specific non‐vocal reactions. In response to aerial predators, they look upwards and quickly move downwards, while in response to terrestrial predators, they look downwards and sometimes approach the predator. We conducted playback experiments to test if the predator‐specific reactions could be elicited in the absence of the predator by the tamarins’ alarm calls alone. We found that in response to aerial alarm call playbacks the subjects looked significantly longer upwards, and in response to terrestrial alarm call playbacks they looked significantly longer downwards. Thus, the tamarins reacted as if external referents, i.e. information about the predator type or the appropriate reaction, were encoded in the acoustic features of the calls. In addition, we found no differences in the responses of S. fuscicollis and S. mystax whether the alarm call stimulus was produced by a conspecific or a heterospecific caller. Furthermore, it seems that S. fuscicollis terrestrial alarm calls were less specific than either S. mystax terrestrial predator alarms or either species’ aerial predator alarms, but because of the small sample size it is difficult to draw a final conclusion.     

Assembly and RNA‐free annotation of highly heterozygous genomes: The case of the thick‐billed murre (Uria lomvia)

Thanks to a dramatic reduction in sequencing costs followed by a rapid development of bioinformatics tools, genome assembly and annotation have become accessible to many researchers in recent years. Among tetrapods, birds have genomes that display many features that facilitate their assembly and annotation, such as small genome size, low number of repeats and highly conserved genomic structure. However, we found that high genomic heterozygosity could have a great impact on the quality of the genome assembly of the thick‐billed murre (Uria lomvia), an arctic colonial seabird. In this study, we tested the performance of three genome assemblers, ray /sscape , soapdenovo 2 and platanus , in assembling the highly heterozygous genome of the thick‐billed murre. Our results show that platanus , an assembler specifically designed for heterozygous genomes, outperforms the other two approaches and produces a highly contiguous (N50 = 15.8 Mb) and complete genome assembly (93% presence of genes from the Benchmarking Universal Single Copy Ortholog [BUSCO] gene set). Additionally, we annotated the thick‐billed murre genome using a homology‐based approach that takes advantage of the genomic resources available for birds and other taxa. Our study will be useful for those researchers who are approaching assembly and annotation of highly heterozygous genomes, or genomes of species of conservation concern, and/or who have limited financial resources.     

Current and future habitat availability for Thick‐billed and Maroon‐fronted parrots in northern Mexican forests

Thick‐billed Parrots (Rhynchopsitta pachyrhyncha) and Maroon‐fronted Parrots (Rhynchopsitta terrisi) are the only parrots in Mexico found in high‐elevation coniferous forests. Both species are critically endangered due to logging, and climate change is expected to further reduce their available habitat. Our objectives were to assess the present and future availability of a suitable habitat for these parrots using ecological niche models. Future climatic scenarios were estimated by overlaying the present distributions of these parrots on maps of projected biome distributions generated using a North American vegetation model. Our climatic scenarios revealed that the distribution of key habitats for both parrots will likely be affected as the climate becomes more suitable for xeric biomes. The climate associated with coniferous forests in the current range of Maroon‐fronted Parrots is predicted to disappear by 2090, and the climate associated with the key coniferous habitats of Thick‐billed Parrots may contract. However, our results also indicate that suitable climatic conditions will prevail for the high‐elevation coniferous biomes where Thick‐billed Parrots nest. The degree to which both species of parrots will be able to adapt to the new scenarios is uncertain. Some of their life history traits may allow them to respond with a combination of adaptive and spatial responses to climatic change and, in addition, suitable climatic conditions will prevail in some portions of their ranges. Actions needed to ensure the conservation of these parrots include strict control of logging and integration of rapid response teams for fire management within the potential foraging ranges of nesting pairs. A landscape with a greater proportion of restored forests would also aid in the recovery of current populations of Thick‐billed and Maroon‐fronted parrots and facilitate their responses to climate change.     

A Test of the Nonlinearity Hypothesis in Great‐tailed Grackles (Quiscalus mexicanus)

Highly aroused or scared animals may produce a variety of sounds that sound harsh and are somewhat unpredictable. These sounds frequently contain nonlinear acoustic phenomena, and these nonlinearities may elicit arousal or alarm responses in humans and many animals. We designed a playback experiment to elucidate whether specific nonlinear phenomena can elicit increased responsiveness in great‐tailed grackles (Quiscalus mexicanus). We broadcast two control sounds (a 0.5‐s, 3‐kHz pure tone and the song of tropical kingbirds (Tyrannus melancholicus) and three test sounds that all began with a 0.4‐s, 3‐kHz pure tone and ended with 0.1 s of either a 1‐ to 5‐kHz band of white noise, an abrupt frequency jump to 1 kHz, or an abrupt frequency jump to 5 kHz. In response to these three nonlinear phenomena, grackles decreased their relaxed behavior (walking, foraging, and preening) and increased looking. A second experiment looked at the rapidity of the time course of frequency change and found that the abrupt frequency jump from 3 to 1 kHz, as opposed to a gradual downward frequency modulation over the same bandwidth, was uniquely arousing. These results suggest that while nonlinear phenomena may be generally evocative, frequency jumps may be the most evocative in great‐tailed grackles. Future studies in other systems can evaluate this general hypothesis.     

Leaf wax n‐alkane patterns of six tropical montane tree species show species‐specific environmental response

It remains poorly understood how the composition of leaf wax n‐alkanes reflects the local environment. This knowledge gap inhibits the interpretation of plant responses to the environment at the community level and, by extension, inhibits the applicability of n‐alkane patterns as a proxy for past environments. Here, we studied the n‐alkane patterns of five Miconia species and one Guarea species, in the Ecuadorian Andes (653–3,507 m a.s.l.). We tested for species‐specific responses in the average chain length (ACL), the C₃₁/(C₃₁ + C₂₉) ratio (ratio), and individual odd n‐alkane chain lengths across an altitudinally driven environmental gradient (mean annual temperature, mean annual relative air humidity, and mean annual precipitation). We found significant correlations between the environmental gradients and species‐specific ACL and ratio, but with varying magnitude and direction. We found that the n‐alkane patterns are species‐specific at the individual chain length level, which could explain the high variance in metrics like ACL and ratio. Although we find species‐specific sensitivity and responses in leaf n‐alkanes, we also find a general decrease in “shorter” (<C₂₉) and an increase in “longer” (>C₃₁) chain lengths with the environmental gradients, most strongly with temperature, suggesting n‐alkanes are useful for reconstructing past environments.     

Occupancy of yellow‐billed and Pacific loons: evidence for interspecific competition and habitat mediated co‐occurrence

Interspecific competition is an important process structuring ecological communities, however, it is difficult to observe in nature. We used an occupancy modelling approach to evaluate evidence of competition between yellow‐billed (Gavia adamsii) and Pacific (G. pacifica) loons for nesting lakes on the Arctic Coastal Plain of Alaska. With multiple years of data and survey platforms, we estimated dynamic occupancy states (e.g. rates of colonization or extinction from individual lakes) and controlled for detection differences among aircraft platforms and ground survey crews. Results indicated that yellow‐billed loons were strong competitors and negatively influenced the occupancy of Pacific loons by excluding them from potential breeding lakes. Pacific loon occupancy was conditional on the presence of yellow‐billed loons, with Pacific loons having almost a tenfold decrease in occupancy probability when yellow‐billed loons were present and a threefold decrease in colonization probability when yellow‐billed loons were present in the current or previous year. Yellow‐billed and Pacific loons co‐occurred less than expected by chance except on very large lakes or lakes with convoluted shorelines; variables which may decrease the cost of maintaining a territory in the presence of the other species. These results imply the existence of interspecific competition between yellow‐billed and Pacific loons for nesting lakes; however, habitat characteristics which facilitate visual and spatial separation of territories can reduce competitive interactions and promote species co‐occurrence.     

The influence of various reef sounds on coral‐fish larvae behaviour

The swimming behaviour of coral‐reef fish larvae from 20 species of 10 different families was tested under natural and artificial sound conditions. Underwater sounds from reef habitats (barrier reef, fringing reef and mangrove) as well as a white noise were broadcasted in a choice chamber experiment. Sixteen of the 20 species tested significantly reacted to at least one of the habitat playback conditions, and a range of responses was observed: fishes were (1) attracted by a single sound but repelled by none (e.g. white‐banded triggerfish Rhinecanthus aculeatus was attracted by the barrier‐reef sound), (2) repelled by one or more sounds but attracted by none (e.g. bridled cardinalfish Pristiapogon fraenatus was repelled by the mangrove and the bay sounds), (3) attracted by all sounds (e.g. striated surgeonfish Ctenochaetus striatus), (4) attracted and repelled by several sounds (e.g. whitetail dascyllus Dascyllus aruanus was attracted by the barrier‐reef sound and repelled by the mangrove sound) and (5) not influenced by any sound (e.g. convict surgeonfish Acanthurus triostegus). Overall, these results highlight two settlement strategies: a direct selection of habitats using sound (45% of the species), or a by‐default selection by avoidance of certain sound habitats (35%). These results also clearly demonstrated the need to analyse the influence of sounds at the species‐specific level since congeneric and confamilial species can express different behaviours when exposed to the same sounds.     

Behavioral responses to conspecific mobbing calls are predator‐specific in great tits (Parus major)

When facing a predator, animals need to perform an appropriate antipredator behavior such as escaping or mobbing to prevent predation. Many bird species exhibit distinct mobbing behaviors and vocalizations once a predator has been detected. In some species, mobbing calls transmit information about predator type, size, and threat, which can be assessed by conspecifics. We recently found that great tits (Parus major) produce longer D calls with more elements and longer intervals between elements when confronted with a sparrowhawk, a high‐threat predator, in comparison to calls produced in front of a less‐threatening tawny owl. In the present study, we conducted a playback experiment to investigate if these differences in mobbing calls elicit different behavioral responses in adult great tits. We found tits to have a longer latency time and to keep a greater distance to the speaker when sparrowhawk mobbing calls were broadcast. This suggests that tits are capable of decoding information about predator threat in conspecific mobbing calls. We further found a tendency for males to approach faster and closer than females, which indicates that males are willing to take higher risks in a mobbing context than females.     

Variation in wing pattern and palatability in a female‐limited polymorphic mimicry system

Checkerspot butterflies in the genera Euphydryas and Chlosyne exhibit phenotypic polymorphisms along a well‐defined latitudinal and elevational gradient in California. The patterns of phenotypic variation in Euphydryas chalcedona, Chlosyne palla, and Chlosyne hoffmanni suggest a mimetic relationship; in addition, the specific patterns of variation in C. palla suggest a female‐limited polymorphic mimicry system (FPM). However, the existence of polymorphic models runs counter to predictions of mimicry theory. Palatability trials were undertaken to assess whether or not the different color morphs of each species were distasteful or toxic to a generalized avian predator, the European starling (Sturnus vulgaris). Results indicate that the black morph of E. chalcedona is distasteful, but not toxic, to predators, while the red morph is palatable. C. hoffmanni and both color morphs of C. palla are palatable to predators. Predators that learn to reject black E. chalcedona also reject black C. palla, suggesting that the latter is a FPM of the former. C. hoffmanni does not appear to be involved in this mimetic relationship.     

Woody Species Alpha‐diversity and Species Abundance Distributions in an African Semi‐deciduous Tropical Rain Forest

Understanding plant species diversity patterns and distributions is critical for conserving and sustainably managing tropical rain forests of high conservation value. We analyzed the alpha‐diversity, species abundance distributions, and relative ecological importance of woody species in the Budongo Forest, a remnant forest of the Albertine Rift in Uganda. In 32 0.5‐ha plots, we recorded 269 species in 171 genera and 51 families with stems of ≥2.0 cm in diameter at breast height (dbh). There were 53 more species with stems of ≥2.0 cm dbh than with stems of ≥10 cm dbh, of which 33 were treelets and 20 were multi‐stemmed shrubs. For both minimum stem diameter cut‐offs (i.e., ≥2 cm dbh vs. ≥10 cm dbh), the Fabaceae, Euphorbiaceae, Ulmaceae, and Meliaceae families and the species Cynometra alexandri, Lasiodiscus mildbraedii, and Celtis mildbraedii had the highest relative ecological importance. The relative ecological importance of some species and families changed greatly with the minimum stem diameter measured. Alpha‐diversity, species richness, and species abundance distributions varied across historical management practice types, forest community types, and as a function of minimum stem diameter. Species richness and Shannon–Weiner diversity index were greater for species with stems of ≥2.0 cm dbh than of ≥10 cm dbh. The decrease in species evenness with an increasing number of plots was accompanied by an increase in species richness for trees of both minimum diameters. This forest is characterized by a small number of abundant species and a relatively large proportion of infrequent species, many of which are sparsely distributed and with restricted habitats. We recommend lowering the minimum stem diameter measured for woody species diversity studies in tropical forests from 10 cm dbh to 2 cm dbh to include a larger proportion of the species pool.     

BIOACOUSTIC PUBLICATIONS, 1988

ABSTRACT

We examined vocalisations of a solitary subterranean rodent, the Silvery Mole-rat Heliophobius argenteocinereus (Bathyergidae). Seven true vocalisations and one mechanical vocalisation were identified. The main frequencies of the analysed sounds (0.34–13.17 kHz) match to a great extent the frequency range suitable for transmission in underground burrows. Due to the narrow frequency range of vocalisations, motivation is predominantly expressed by the rate of tonality rather than by frequency changes. The vocal repertoire of the Silvery Mole-rat perfectly matches to Morton's MS rules, i.e. that low-frequency and harsh vocalisations indicate hostility whereas high tonal calls signalise appeasement or fear. In comparison with social species, this solitary bathyergid produces fewer calls. It lacks contact and alarm calls, but has a rich repertoire of mating calls (mating ends with duet vocalization of male and female). Acoustic signals seem to play a major role in lowering natural aggressiveness during the mating period.     

Vocal mimicry in spotted bowerbirds is associated with an alarming context

Although the presence of vocal mimicry in songbirds is well documented, the function of such impressive copying is poorly understood. One explanation for mimicry in species that predominantly mimic alarm calls and predator vocal isations is that these birds use mimicry to confuse or deter potential threats or intruders, so these vocalisations should therefore be produced when the mimic is alarmed and be uncommon in other contexts. Male bowerbirds construct bowers to display to females and anecdotal reports from the Ptilonorhynchus genus suggest that males mimic alarm sounds when disturbed at their bowers. We quantified and compared the rate of mimicry during disturbance to the bower by a human and in naturally occurring social contexts in a population of spotted bowerbirds Ptilonorhynchus maculatus. Male bowerbirds produced mimicry more than thirty times more frequently in response to bower disturbance than they did in any other context. Neither conspecifics nor heterospecifics were attracted to the bower area by mimicry. These data are consistent with the hypothesis that the production of mimicry is associated with a response to an alarming situation. Additionally, the predominance of alarm mimicry by spotted bowerbirds raises the possibility that the birds learn these sounds when they experience alarming situations and they reproduce them in subsequent alarming situations.     

Response of Captive Raptors to Avian Mobbing Calls: the Roles of Mobber Size and Raptor Experience

The ‘move on’ hypothesis for avian mobbing proposes that mobbing induces stress in potential predators, thereby provoking them into moving elsewhere. We tested whether mobbing is stress inducing by subjecting captive owls, hawks, and falcons to the mobbing calls of four species of co‐occurring passerine birds that vary considerably in body size. Test subjects comprised 15 individuals of seven species of birds of prey that were housed at a wildlife rehabilitation center in northwestern Pennsylvania, USA. Playback treatments included mobbing calls of the black‐capped chickadee (Poecile atricapillus), blue‐headed vireo (Vireo solitarius), blue jay (Cyanocitta cristata), and American crow (Corvus brachyrhynchos), which vary in size from 12 g (chickadee) to 500 g (crow). A non‐mobbing vocalization (song of the black‐capped chickadee) was included in the playback treatments as a control. Playback treatment produced pronounced effects on three behavioral indicators of probable stress: head orientation toward the playback speaker, raising of feathers or spreading of wings, and changing of perch positions. Test subjects responded more vigorously to mobbing calls of the large passerines (jays and crows) than to those of the smaller passerines (chickadees and vireos). In addition, raptors that had entered the rehabilitation facility as adults generally responded more vigorously to the mobbing calls of jays and crows than did naive individuals that had entered the facility as fledglings or young juveniles. Our results are therefore consistent with the ‘move on’ hypothesis and suggest (1) that the mobbing calls of large passerines are more effective in provoking stress and altering the behavior of potential predators than are the mobbing calls of small passerines and (2) that a raptor's previous experience with mobbing in the wild can exaggerate the strength of its response, particularly to the mobbing calls of large passerines.     

Using RAD‐seq to recognize sex‐specific markers and sex chromosome systems

Next‐generation sequencing methods have initiated a revolution in molecular ecology and evolution (Tautz et al. 2010 ). Among the most impressive of these sequencing innovations is restriction site‐associated DNA sequencing or RAD‐seq (Baird et al. 2008 ; Andrews et al. 2016 ). RAD‐seq uses the Illumina sequencing platform to sequence fragments of DNA cut by a specific restriction enzyme and can generate tens of thousands of molecular genetic markers for analysis. One of the many uses of RAD‐seq data has been to identify sex‐specific genetic markers, markers found in one sex but not the other (Baxter et al. 2011 ; Gamble & Zarkower 2014 ). Sex‐specific markers are a powerful tool for biologists. At their most basic, they can be used to identify the sex of an individual via PCR. This is useful in cases where a species lacks obvious sexual dimorphism at some or all life history stages. For example, such tests have been important for studying sex differences in life history (Sheldon 1998 ; Mossman & Waser 1999 ), the management and breeding of endangered species (Taberlet et al. 1993 ; Griffiths & Tiwari 1995 ; Robertson et al. 2006 ) and sexing embryonic material (Hacker et al. 1995 ; Smith et al. 1999 ). Furthermore, sex‐specific markers allow recognition of the sex chromosome system in cases where standard cytogenetic methods fail (Charlesworth & Mank 2010 ; Gamble & Zarkower 2014 ). Thus, species with male‐specific markers have male heterogamety (XY) while species with female‐specific markers have female heterogamety (ZW). In this issue, Fowler & Buonaccorsi ( 2016 ) illustrate the ease by which RAD‐seq data can generate sex‐specific genetic markers in rockfish (Sebastes). Moreover, by examining RAD‐seq data from two closely related rockfish species, Sebastes chrysomelas and Sebastes carnatus (Fig.  1 ), Fowler & Buonaccorsi ( 2016 ) uncover shared sex‐specific markers and a conserved sex chromosome system.