Biomechanics illuminates form–function relationships in bird bills

Abstract

The field of comparative biomechanics examines how form, mechanical properties and environmental interactions shape the function of biological structures. Biomechanics has advanced by leaps and bounds as rapid technological progress opens up new research horizons. In this Review, I describe how our understanding of the avian bill, a morphologically diverse multifunctional appendage, has been transformed by employing a biomechanical perspective. Across functions from feeding to excavating hollows in trees and as a vocal apparatus, the study of the bill spans both solid and fluid biomechanics, rendering it useful to understand general principles across disciplines. The different shapes of the bill across bird species result in functional and mechanical trade-offs, thus representing a microcosm of many broader form–function questions. Using examples from diverse studies, I discuss how research into bird bills has been shaped over recent decades, and its influence on our understanding of avian ecology and evolution. Next, I examine how bill material properties and geometry influence performance in dietary and non-dietary contexts, simultaneously imposing trade-offs on other functions. Following an examination of the interactions of bills with fluids and their role as part of the vocal apparatus, I end with a discussion of the sensory biomechanics of the bill, focusing specifically on the bill-tip mechanosensory organ. With these case studies, I highlight how this burgeoning and consequential field represents a roadmap for our understanding of the function and evolution of biological structures.

Body size shapes song in honeyeaters

“…However, that study did not consider influences of habitat in these models, which was acknowledged as a limitation because of the complex relationships between beak size and shape, foraging ecology, and the environment [39]. Beak morphology in general is understood to act as a mechanical constraint on song and influence movement [14, 16], the extent of which is underpinned by environmental conditions and beak specialisation for dietary preferences. For example, mechanics behind small and slender beaks of insectivores prioritise closing velocity for catching insects, whereas curved and narrow beaks of nectarivores prioritise nectar transfer [14], and these factors can influence song.…”

“…Beak morphology in general is understood to act as a mechanical constraint on song and influence movement [14, 16], the extent of which is underpinned by environmental conditions and beak specialisation for dietary preferences. For example, mechanics behind small and slender beaks of insectivores prioritise closing velocity for catching insects, whereas curved and narrow beaks of nectarivores prioritise nectar transfer [14], and these factors can influence song. Ultimately, this complex relationship between beak morphology, the environment, and song highlights the importance of exploring effects of habitat and morphology altogether.…”

“…However, only a few past studies have explicitly acknowledged this statistical concern [19, 30] and caution is required when interpreting findings which might not have fully accounted for body size because any effect of beak shape on song variables could be mistakenly attributed [19, 20, 63]. Beyond statistical concerns, fully understanding the influence of beak shape on song is not entirely straightforward and requires more attention because behaviours such as adjusting gape and coordinated movements of the beak with the tongue also alter amplification and resonance of song [14].…”

Body size shapes song in honeyeaters

“…In birds, beak morphology influences acoustic signals too. The avian bill is used in coordination with tongue and vocal tract movements to modify sounds [14] and is thought to act as a final modulator of song, influencing both its frequency and temporal components [15][16][17][18][19]. These effects are complicated, however, by the allometric relationship between body size and beak size [19,20], and by interactions among the biotic and abiotic factors that are thought to influence acoustic signalling.…”

“…Such complexity, caused by the effects of ecological selection on signal production, characterizes both the acoustic adaptation and morphological constraint hypotheses. For example, environmental conditions not only influence body size and beak shape but they also affect habitat productivity, complexity, and food availability, which, because of dietary preferences, influence beak shape [ 14 , 25 ] and, presumably, song characteristics [ 14 , 26 , 27 ]. This may account for mixed outcomes when studies focus on just one of the hypotheses, with particularly contradictory results for the AAH [ 28 , 29 ].…”