A cross-species framework for classifying sound-movement couplings

Sound and movement are entangled in animal communication. This is obviously true in the case of sound-constituting vibratory movements of biological structures which generate acoustic waves. A little less obvious is that other moving structures produce the energy required to sustain these vibrations. In many species, the respiratory system moves to generate the expiratory flow which powers the sound-constituting movements (sound-powering movements). The sound may acquire additional structure via upper tract movements, such as articulatory movements or head raising (sound-filtering movements). Some movements are not necessary for sound production, but when produced, impinge on the sound-producing process due to weak biomechanical coupling with body parts (e.g., respiratory system) that are necessary for sound production (sound-impinging movements). Animals also produce sounds contingent with movement, requiring neuro-physiological control regimes allowing to flexibly couple movements to a produced sound, or coupling movements to a perceived external sound (sound-contingent movement). Here, we compare and classify the variety of ways sound and movements are coupled in animal communication; our proposed framework should help structure previous and future studies on this topic.

Feather function and the evolution of birds

The ability of feathers to perform many functions either simultaneously or at different times throughout the year or life of a bird is integral to the evolutionary history of birds. Many studies focus on single functions of feathers, but any given feather performs many functions over its lifetime. These functions necessarily interact with each other throughout the evolution and development of birds, so our knowledge of avian evolution is incomplete without understanding the multifunctionality of feathers, and how different functions may act synergistically or antagonistically during natural selection. Here, we review how feather functions interact with avian evolution, with a focus on recent technological and discovery-based advances. By synthesising research into feather functions over hierarchical scales (pattern, arrangement, macrostructure, microstructure, nanostructure, molecules), we aim to provide a broad context for how the adaptability and multifunctionality of feathers have allowed birds to diversify into an astounding array of environments and life-history strategies. We suggest that future research into avian evolution involving feather function should consider multiple aspects of a feather, including multiple functions, seasonal wear and renewal, and ecological or mechanical interactions. With this more holistic view, processes such as the evolution of avian coloration and flight can be understood in a broader and more nuanced context.

Morphology of migration: associations between wing shape, bill morphology and migration in kingbirds (Tyrannus)

Morphology is closely linked to locomotion and diet in animals. In animals that undertake long-distance migrations, limb morphology is under selection to maximize mobility and minimize energy expenditure. Migratory behaviours also interact with diet, such that migratory animals tend to be dietary generalists, whereas sedentary taxa tend to be dietary specialists. Despite a hypothesized link between migration status and morphology, phylogenetic comparative studies have yielded conflicting findings. We tested for evolutionary associations between migratory status and limb and bill morphology across kingbirds, a pan-American genus of birds with migratory, partially migratory and sedentary taxa. Migratory kingbirds had longer wings, in agreement with expectations that selection favours improved aerodynamics for long-distance migration. We also found an association between migratory status and bill shape, such that more migratory taxa had wider, deeper and shorter bills compared to sedentary taxa. However, there was no difference in intraspecific morphological variation among migrants, partial migrants and residents, suggesting that dietary specialization has evolved independently of migration strategy. The evolutionary links between migration, diet and morphology in kingbirds uncovered here further strengthen ecomorphological associations that underlie long-distance seasonal movements in animals.

Ways that Animal Wings Produce Sound

There are at least eight ways that wings potentially produce sound. Five mechanisms are aerodynamic sounds, created by airflow, and three are structural sound created by interactions of solid surfaces. Animal flight is low Mach (M), meaning all animals move at <30% of the speed of sound. Thus in aerodynamic mechanisms the effects of air compressibility can be ignored, except in mechanism #1. Mechanism #1 is trapped air, in which air approaches or exceeds Mach 1 as it escapes a constriction. This mechanism is hypothetical but likely. #2 is Gutin sound, the aerodynamic reaction to lift and drag. This mechanism is ubiquitous in flight, and generates low frequency sound such as the humming of hummingbirds or insect wing tones. #3 is turbulence-generated atonal whooshing sounds, which are also widespread in animal flight. #4 are whistles, tonal sounds generated by geometry-induced flow feedback. This mechanism is hypothetical. #5 is aeroelastic flutter, sound generated by elasticity-induced feedback that is usually but not always tonal. This is widespread in birds (feathers are predisposed to flutter) but apparently not bats or insects. Mechanism #6 is rubbing sound (including stridulation), created when bird feathers or insect wings slide past each other. Atonal rubbing sounds are widespread in bird flight and insects; tonal stridulation is widespread in insects. #7 is percussion, created when two stiff elements collide and vibrate, and is present in some birds and insects. Mechanism #8 are tymbals and other bistable conformations. These are stiff elements that snap back and forth between two conformations, producing impulsive, atonal sound. Tymbals are widespread in insects but not birds or bats; insect cuticle appears predisposed to form tymbals. There are few examples of bat wing sounds: are bats intrinsically quiet, or just under-studied? These mechanisms, especially Gutin sound, whooshes, and rubbing (#2, #3, and #6) are prominent cues in ordinary flight of all flying animals, and are the "acoustic substrate" available to be converted from an adventitious sound (cue) into a communication signal. For instance, wing sounds have many times evolved into signals that are incorporated into courtship displays. Conversely, these are the sounds selected to be suppressed if quiet flight is selected for. The physical mechanisms that underlie animal sounds provide context for understanding the ways in which signals and cues may evolve.

Introduction to the Symposium: Bio-Inspiration of Quiet Flight of Owls and Other Flying Animals: Recent Advances and Unanswered Questions

Animal wings produce an acoustic signature in flight. Many owls are able to suppress this noise to fly quietly relative to other birds. Instead of silent flight, certain birds have conversely evolved to produce extra sound with their wings for communication. The papers in this symposium synthesize ongoing research in "animal aeroacoustics": the study of how animal flight produces an acoustic signature, its biological context, and possible bio-inspired engineering applications. Three papers present research on flycatchers and doves, highlighting work that continues to uncover new physical mechanisms by which bird wings can make communication sounds. Quiet flight evolves in the context of a predator-prey interaction, either to help predators such as owls hear its prey better, or to prevent the prey from hearing the approaching predator. Two papers present work on hearing in owls and insect prey. Additional papers focus on the sounds produced by wings during flight, and on the fluid mechanics of force production by flapping wings. For instance, there is evidence that birds such as nightbirds, hawks, or falcons may also have quiet flight. Bat flight appears to be quieter than bird flight, for reasons that are not fully explored. Several research avenues remain open, including the role of flapping versus gliding flight or the physical acoustic mechanisms by which flight sounds are reduced. The convergent interest of the biology and engineering communities on quiet owl flight comes at a time of nascent developments in the energy and transportation sectors, where noise and its perception are formidable obstacles.