Research Stories – X-raying birds at Manchester Museum

Manchester Museum has a large collection of over 18,000 bird skins, representing a huge diversity of species. Many specimens are over 100 years old making this historic collection an important resource for scientific research.

Figure 1: A drawer of bird skins from the Manchester Museum zoology collections.
© Manchester Museum, The University of Manchester

Scientists from the School of Earth and Environmental Sciences at the University of Manchester have been studying birds from the collections, looking closely at their claws. By x-raying and measuring the shape of the toes and claws they can learn more about how birds move, whether they live on the ground or prefer to perch on trees.

Birds evolved from a group of bipedal carnivorous dinosaurs called Theropods. When an asteroid hit the earth 66 million years ago there was mass extinction; many dinosaurs did not survive, but avian dinosaurs did. There are nearly 10,000 different species of bird and they are all descended from dinosaurs. Studying modern birds can help us to gain a greater understanding of how extinct birds and feathered dinosaurs lived in the past.

Previous studies have found the curvature of the keratinous sheath of the claw correlates with life style of extant birds. This relationship has been used to predict lifestyles for extinct avian dinosaurs, such as Archaeopteryx. However, often the keratinous sheath is not preserved, so it is not possible to use this method to predict the lifestyle of dinosaurs alone. New research examining toe bones hopes to provide a more accurate measure.

Figure 2: Set up for x-raying museum bird skins using the Nomad Pro Radiography Unit.

A scientist from the University of Manchester measured over 100 birds at Manchester Museum, World Museum Liverpool and the National Museum of Scotland. Each bird was carefully x-rayed using the same technology used by dentists to examine teeth. Key features of the claw and toe bones were measured to gather mathematical data describing the shape of claws. Different species were compared and a relationship was found between the curvature of the ungual bone within the claw and four modes of life, ground-dwelling, predatory, perching and climbing.
Claws are involved in a number of behaviours including locomotion and prey capture. As a result animals evolve claw morphologies that enable these functions. Ground dwelling birds have flat claws, whereas birds living in trees have curved claws to enable grip.

Figure 3: X-ray scans of Manchester Museum bird skins. (A) Black bittern Ixobrychus flavicollis – ground-dwelling. (B) Black woodpecker Dryocopus martius – climbing. (C) Greater sooty owl Tyto tenebricosa – predatory. (D) Common raven Corvus corax – perching.

Figure 4: Diagram of how the fossil claws were measured, this example is an Archaeopteryx claw.

Images of fossil avian dinosaurs and fossil bird claws were analysed and compared to the claws of museum bird specimens. 36 fossil claws were studied including Microraptor, Archaeopteryx and Confuciusornis.

The results suggested many of the measured fossil taxa were capable of climbing; the majority had similar shaped claws to extant perching birds. Many fossil taxa that grouped with perching birds lack an opposable hallux (or big toe) and would have been incapable of perching in the style of modern birds. However, these results could suggest that Mesozoic Theropods such as Microraptor were capable of climbing trees, but further study of the fossil is required to see if they possess adaptations consistent with arboreal habits such as extensive hindlimb feathering and elongate forelimbs. Archaeopteryx claws were classed as either perching or predatory, and analysis of the claws found it was unlikely they were ground-dwelling. The results could suggest they lived in arboreal habits.

Figure 5: Fossil cast of a Archaeopteryx lithographica, Jurassic period, Eichstatt, Bavaria, Germany.
© Manchester Museum, The University of Manchester

The study found that reconstructing behaviour from the curvature of the toe bone is not particularly reliable, as behaviour is complex and most animals use their claws for multiple functions. Predictions of lifestyle for fossil taxa based on claw curvature alone should be considered alongside additional evidence to improve reliability of predictions.

In conclusion the curved pedal claws, in conjunction with other features such as elongated forelimbs and flight ability, represent adaptations to living in trees. The large collection of birds at Manchester museum will continue to provide opportunities for studying many different species, enabling a better understanding of how extinct birds and feathered dinosaurs lived in the past.

Further reading

Burnham, D.A., Feduccia, A., Martin, L.D. and Falk, A.R., 2011. Tree climbing–a fundamental avian adaptation. Journal of Systematic Palaeontology, 9(1), pp.103-107.

Cobb, S.E. and Sellers, W.I., 2020. Inferring lifestyle for Aves and Theropoda: a model based on curvatures of extant avian ungual bones. Plos one, 15(2), p.e0211173.

Manning, P.L., Payne, D., Pennicott, J., Barrett, P.M. and Ennos, R.A., 2006. Dinosaur killer claws or climbing crampons?. Biology Letters2(1), pp.110-112.

Pavid, K., 2021. How dinosaurs evolved into birds. Natural History Museum London [Online] [Accessed on 3rd February 2021] https://www.nhm.ac.uk/discover/how-dinosaurs-evolved-into-birds.html

Yalden, D.W., 1985. Forelimb function in Archaeopteryx. The beginnings of birds, pp.91-97.

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