RESUMEN

Two hypotheses have been proposed for the evolution of structures that reduce flight sounds in birds. According to the stealth hypothesis, flying quietly reduces the ability of other animals (e.g., prey) to detect the animal's approach from its flight sounds. This hypothesis predicts that animals hunting prey with acute hearing evolve silencing features. The self-masking hypothesis posits that reduced flight sounds permit the animal itself to hear better (such as the sounds of its prey, or its own echolocation calls) during flight. This hypothesis predicts that quieting features evolve in predators that hunt by ear, or in species that echolocate. Owls, certain hawks, and nightbirds (nocturnal Caprimulgiformes) have convergently evolved a sound-reducing feature: a velvety coating on the dorsal surface of wing and tail feathers. Here we document a fourth independent origin of the velvet, in the American kestrel (Falco sparverius). Among these four clades (hawks, falcons, nightbirds, and owls), the velvet is longer and better developed in wing and tail regions prone to rubbing with neighboring feathers, apparently to reduce broadband frictional noise produced by rubbing of adjacent feathers. We tested whether stealth or self-masking better predicted which species evolved the velvet. There was no support of echolocation as a driver of the velvet: oilbird(Steatornis caripensis) and glossy swiftlet (Collocalia esculenta) each evolved echolocation but neither had any velvet. A phylogenetic least squares fit of stealth and self-masking (to better hear prey sounds) provided support for both hypotheses. Some nightbirds (nightjars, potoos, and owlet-nightjars) eat flying insects that do not make much sound, implying the velvet permits stealthy approach of flying insects. One nightbird clade, frogmouths (Podargus) have more extensive velvet than other nightbirds and may hunt terrestrial prey by ear, in support of self-masking. In hawks, the velvet is also best developed in species known or suspected to hunt by ear (harriers and kites), supporting the self-masking hypothesis, but velvet is also present in reduced form in hawk species not known to hunt by ear, in support of the stealth hypothesis. American kestrel is not known to hunt by ear, and unlike the other falcons sampled, flies slowly (kite-like) when hunting. Thus the presence of velvet in it supports the stealth hypothesis. All owls sampled (n = 13 species) had extensive velvet, including the buffy fish-owl (Ketupa ketupu), contrary to literature claims that fish-owls had lost the velvet. Collectively, there is support for both the self-masking and stealth hypotheses for the evolution of dorsal velvet in birds.

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RESUMEN

We raise and explore possible answers to three questions about the evolution and ecology of silent flight of owls: (1) do owls fly silently for stealth, or is it to reduce self-masking? Current evidence slightly favors the self-masking hypothesis, but this question remains unsettled. (2) Two of the derived wing features that apparently evolved to suppress flight sound are the vane fringes and dorsal velvet of owl wing feathers. Do these two features suppress aerodynamic noise (sounds generated by airflow), or do they instead reduce structural noise, such as frictional sounds of feathers rubbing during flight? The aerodynamic noise hypothesis lacks empirical support. Several lines of evidence instead support the hypothesis that the velvet and fringe reduce frictional sound, including: the anatomical location of the fringe and velvet, which is best developed in wing and tail regions prone to rubbing, rather than in areas exposed to airflow; the acoustic signature of rubbing, which is broadband and includes ultrasound, is present in the flight of other birds but not owls; and the apparent relationship between the velvet and friction barbules found on the remiges of other birds. (3) Have other animals also evolved silent flight? Wing features in nightbirds (nocturnal members of Caprimulgiformes) suggest that they may have independently evolved to fly in relative silence, as have more than one diurnal hawk (Accipitriformes). We hypothesize that bird flight is noisy because wing feathers are intrinsically predisposed to rub and make frictional noise. This hypothesis suggests a new perspective: rather than regarding owls as silent, perhaps it is bird flight that is loud. This implies that bats may be an overlooked model for silent flight. Owl flight may not be the best (and certainly, not the only) model for "bio-inspiration" of silent flight.

Proponemos y exploramos posibles respuestas a tres preguntas sobre la evolución y ecología del vuelo silencioso en lechuzas: (1) ¿Las lechuzas vuelan silenciosamente por sigilo o para reducir el auto-enmascaramiento?. La evidencia actual favorece levemente la hipótesis del auto-enmascaramiento, pero éste tema permanece irresuelto. (2) Dos de las características derivadas de las alas que aparentemente evolucionaron para suprimir el sonido del vuelo son los flecos del vexilo y la felpa dorsal de las alas de las lechuzas. Estas características ¿suprimen el ruido aerodinámico (sonido generado por el flujo de aire) o reducen en cambio el ruido estructural, tal como el ruido friccional de las plumas frotándose durante el vuelo? La hipótesis del ruido aerodinámico carece de apoyo empírico. Por el contrario, varias líneas de evidencia apoyan la hipótesis de que la felpa y el fleco reducen los sonidos friccionales, incluyendo: la posición anatómica del fleco y felpa, esta última mejor desarrollada en regiones del ala y cola propensos a frotación, y no tanto en áreas expuestas a flujo de aire ; la signatura acústica de frotación, que es de banda ancha e incluye ultrasonido, está presente en el vuelo de otras aves pero no en lechuzas; y la aparente relación entre la felpa y las bárbulas de fricción presentes en las remiges de otras aves. (3) ¿Evolucionó el vuelo silencioso en otros animales? Las características de las alas de las aves nocturnas (miembros nocturnos de Caprimulgiformes) sugieren que podrían haber evolucionado independientemente para volar de forma relativamente silenciosa, tal como ocurre en más de un gavilán diurno (Accipitriformes). Hipotetizamos que el vuelo de las aves es ruidoso porque las plumas alares están intrínsecamente predispuestas a frotarse y producir ruido friccional. La hipótesis sugiere una nueva perspectiva; en vez de considerar a las lechuzas como silenciosas, tal vez es que el vuelo de las aves es ruidoso. Esto implica que los murciélagos podrían representar un modelo ignorado de vuelo silencioso. El vuelo de las lechuzas podría no ser el mejor (y ciertamente no el único) modelo para la "bio-inspiración" del vuelo silencioso.Palabras clave: Acústica, Aeroacústica, sonidos inducidos por locomoción, Strigiformes, ala.