A sneak peek into the bird’s beak

Beaks are an appendage diverse in shape, size and function. Bird beaks are technically referred to as bills — composite structures that lack teeth. Beaks are bony extensions of the upper and lower mandibles (jaw) covered by a layer of keratinised tissue called ‘rhamphotheca’.

The keratinised tissue (from which our fingernails and hair are also made) may be pigmented, giving rise to brilliantly colourful beaks. Bird beaks have hollow air pockets that make them lightweight. Birds use beaks for many functions such as preening, catching prey, tearing into flesh and feeding the offspring, drinking nectar from flowers, digging wood or probing soft soil for worms.

Bird beaks have fascinated evolutionary biologists right from the time of Charles Darwin, who studied a group of sparrow-like birds called Finches on the islands of the Galapagos. He found that many of the islands had many kinds of finches that all differed in their beak shape and size. He attributed this variation to ‘natural selection’ where certain traits perform better under certain conditions.

Several decades later, Peter and Rosemary Grant studied the Galapagos Finches and found that the beak size changed rather rapidly during droughts which altered the food resource availability. Birds that had thicker beaks and could crack harder seeds survived better. This rapidly shifted the mean beak size of the population within subsequent generations.

Trade-offs

Beaks serve several mechanical functions including pecking the wood. Many birds such as Woodpeckers chisel dead wood for food or nesting. Upon impact with the beak, the skull experiences a force that can cause internal and physical damage.

Logically, many birds have adaptations that reduce damage due to impact. However, in nature, there are tradeoffs. While strong beaks are useful for a bird that chisels, the extra strength may add more weight, affecting its ability to fly. These trade-offs then dictate the morphology and functions of beaks.

Comparative studies, where a particular aspect such as beak strength is compared across a group of closely and distantly related organisms, have shed crucial insights into how the tradeoffs manifest and affect life history.

Usage of biomechanics

Biomechanics is a branch of science that studies the structure, function, and motion of the mechanical aspects of biological systems. Evolutionary biologists from India and the United States of America resorted to biomechanics to simulate how beaks behave across species.

They visited multiple museums in the United States over several years to study specimens of 15 species of Barbets, a group of primarily fruit-eating birds that use their bills to excavate nesting hollows in trees.

They scanned the specimens using microcomputed tomography (μCT scan) and obtained the volume of the beak, a digital surface mesh of the beak separately for the bony structure and the rhamphotheca. Then they used finite element analysis, commonly used in materials engineering, to predict and understand how beaks perform under the physical conditions of pecking and gouging.

“Field observations suggest that barbet beaks experience two kinds of forces, one from the impact of pecking, and the other from torsion, caused by gouging action. We resorted to the use of biomechanics to understand how the excavation performance changes with bill shapes,” said Vaibhav Chhaya, the lead author of the paper that was published in the Proceedings of the Royal Society, London.

“We found that the peak points of stress differed for the two physical conditions. Controlling for beak length, deeper and wider beaks had greater impact resistance from pecking, but narrower beaks had a higher resistance to the gouging action. We found further support for this trade-off by modelling beaks as an approximation of an elliptical beam and determined the buckling stress as well as shear stress,” explains Vaibhav. They also confirmed earlier findings of how composite structures perform better than bony or keratinised structures alone.

Evolutionary origins

The bird beak, made of composite material, appears to have been an ingenious solution to overcome the trade-off of form and function by being both strong and lightweight. Birds that we see today, are a lineage of now extinct Dinosaurs.

“Along the evolutionary trajectory, the ancestors of the present-day birds lost their teeth and instead, evolved keratinised beaks. This process, known as edentulism, was likely due to shifting to a herbivorous diet. However, the keratinisation of the beak led to a great diversification of shapes and a few lineages have specialised in the use of beaks for cavity excavation,” explains Anand Krishnan, a professor at IISER Bhopal who supervised the work.

Anand recently reviewed the use of biomechanics to understand the form-function trade-off in birds, published separately in the centenary issue of the Journal for Experimental Biology.

Museums helping science

Museums have a central role in science and society though they are seen as vestiges of the colonial past and a waste of public funds. “Comparative studies using museum samples have helped resolve some of the long-standing questions in evolution such as how did species evolve to be so different?” says Sushma Reddy, Professor, and Curator of the Bell Museum of Natural History, University of Minnesota, and senior author on the paper.

“The face of museums is also changing as they are making specimens more accessible,” she adds. The authors hope that their work opens up many doors for further research into the form-function trade-off among birds.

(The author is an ecologist and a faculty at ATREE)