Morphological innovation and biomechanical diversity in plunge‐diving birds

Abstract

Innovations in foraging behavior can drive morphological diversity by opening up new ways of interacting with the environment, or limit diversity through functional constraints associated with different foraging behaviors. Several classic examples of adaptive radiations in birds show increased variation in ecologically relevant traits. However, these cases primarily focus on geographically narrow adaptive radiations, consider only morphological evolution without a biomechanical approach, or do not investigate tradeoffs with other non‐focal traits that might be affected by use of different foraging habitats. Here, we use X‐ray microcomputed tomography, biomechanical modeling, and multivariate comparative methods to explore the interplay between foraging behavior and cranial morphology in kingfishers, a global radiation of birds with variable beaks and foraging behaviors, including the archetypal plunge‐dive into water. Our results quantify covariation between the shape of the outer keratin covering (rhamphotheca) and the inner skeletal core of the beak, as well as highlight distinct patterns of morphospace occupation for different foraging behaviors and considerable rate variation among these skull regions. We anticipate these findings will have implications for inferring beak shapes in fossil taxa and inform biomimetic design of novel impact‐reducing structures.

Swimming, flying, and diving behaviors from a unified 2D potential model

“…Some animals plunge-dive into water at high speeds: aquatic animals 11 and aerial birds 14 , 41 – 43 . Most animals have a stream-lined body like a spheroidal head front for aquatic animals or a conical beak for birds, which might help reducing the likelihood of injury under high dynamic loadings while diving.…”

“…The other type are birds plunge-diving into water from air. Several bird species exhibit high-speed diving into water as a hunting mechanism 12 – 14 . These plunge-diving birds are not very common, but are widely spread in the phylogeny; the Sulidae family species (e.g., Northern Gannet, Brown Booby) and other species (e.g., Brown Pelican, Terns, and Kingfishers).…”

Trends of avian locomotion in water – an overview of swimming styles

“…Positive buoyancy moves the body upwards in the water column. Penetrating under the surface is problematic for the highly buoyant volant taxa, which must exert force, or gain extra momentum in order to counteract the upward-directed force of water, dive, and reach their foraging depth (Hustler 1991, Eliason et al 2020. The most common way for aquatic vertebrates to decrease the effect of the buoyant force is to increase the density of the body.…”

Plunge diving by Brown Pelicans resembles a Split-S Turn

“…The pursuit divers can be grouped into those that start the underwater dive while at the water surface such as anhingas, grebes, penguins, and puffins, and those such as boobies, gannets, kingfishers, pelicans, terns, and tropicbirds that are plunge divers, i.e., they begin the dive headfirst from the air before water entry. Among the plunge divers, all but the Brown Pelican enter the water upright, i.e., the dorsum is up and the venter is down, and we can find no report that describes a body rotation leading to an inverted entry (Machovsky-Capuska et al 2012, Chang et al 2016, Holbech et al 2018, Crandell et al 2019, Eliason et al 2020. Upon water entry, these plunge divers that enter the water upright actively swim after their prey and where they surface will depend solely on prey pursuit and has little correlation to where or how they entered the water.…”