On the heel-pad in certain Malaysian birds
F.N. Chasen’s 1923 article in the first volume of the Journal of the Malaysian Branch of the Royal Asiatic Society examines the heel-pad—a thickened, sometimes tuberculated pad on the metatarsal joint of nestling birds—across Malaysian species, arguing that the structure is a morphological consequence of the plantigrade posture adopted by altricial nestlings whose ankle joints are not yet functionable. Drawing on specimens from the Raffles Museum in Singapore, Chasen documents the persistence of well-developed pads in adult Barbets and their early shedding in Woodpeckers, while also cataloguing incipient pads in a wide range of other families.
Summary
Chasen situates his work within a growing European ornithological literature on heel-pads, beginning with Gunther’s 1890 description in the Wryneck and Pycraft’s prediction that the structure would prove widespread among birds reared in unlined cavities. The most comprehensive prior study was Gyldenstolpe’s examination of the Stockholm Museum collection, which identified pads across sixteen families, mostly within the Orders Piciformes and Coraciiformes. Chasen’s contribution is to extend this survey to Malaysian material, focusing particularly on the Barbets (Capitonidae) and Woodpeckers (Picidae), where the pads are most elaborately developed.
The central argument of the paper is that the heel-pad is not primarily a locomotory aid (as Seth-Smith had suggested for toucans) nor merely a protective cushion (as Collingwood Ingram proposed), but rather a direct consequence of the plantigrade condition in which altricial nestlings rest. Chasen demonstrates that in ripe embryos and newly hatched Passerines, the ankle joint is not functionable—the tarsus springs back when forced into a right angle with the crus—making the plantigrade posture inevitable rather than a result of mere physical weakness. The pad thus represents a thickening of the skin at the point of friction where the nestling rests on its tarsi.
The paper proceeds through specific taxonomic groups. In Barbets, Chasen finds that well-developed tubercular pads persist in fully adult birds of several species, a fact he finds puzzling and suggests might relate to roosting or brooding posture. In Woodpeckers, the pointed tubercles are shed almost immediately after the bird leaves the nest, though slight swellings and enlarged scales remain visible in adults of some species. A final section surveys incipient pads in Broadbills, Nightjars, Swifts, Weaver-birds, Sunbirds, Crows, and Owls, noting that in the small Owls (Scops lempiji, Ninox scutulata) the pads function as protective cushions on which the nestling rests with its feet in the air.
Key Findings
- In Mesobucco duvauceli, six of seventeen skins in the Raffles Museum collection (all young or subadult) possessed well-developed tubercular pads approximately 6 mm long, with up to 11 conical tubercles whose points were directed upward (p. 242).
- In Chotorhea versicolor, three of twenty adults (one a “finest old male”) retained heel-pads consisting of an elliptical ring of ten to twelve large pointed scales with six smaller papillae within; the remaining seventeen showed no trace (p. 243).
- In Chotorhea chrysopogon, a subadult male displayed an extraordinarily well-developed and complete pad with an elliptical row of large modified pointed scales, the two posterior scales being fused, larger, and with hollow projections (p. 243).
- Across approximately two hundred Malaysian Woodpecker skins examined, pointed tubercles were absent in all adults; only slight swellings or enlarged scales persisted, most notably in Dryocopus javensis where “an unmistakable pad” was visible even in old, badly prepared skins (p. 244).
- In a nestling Scops lempiji about eight days old, kept in captivity, Chasen observed the bird spending most of its time resting on smooth, flattened, hard pads with feet and toes held in the air, confirming the cushion function (pp. 245–246).
- In ripe Passerine embryos, the tarsus, when forced down to form a right angle with the crus, “flies back when released, like the arm of a spring,” demonstrating that the ankle joint is morphologically non-functionable in the nestling (p. 241).
Conclusion
Chasen concludes that the heel-pad is best understood as a morphological adaptation to the obligatory plantigrade posture of altricial nestlings, whose ankle joints are not yet differentiated enough to permit digitigrade locomotion. While the pad may secondarily serve protective or locomotory functions in specific taxa—most dramatically in the tubercular pads of Barbets and Woodpeckers—its origin lies in the mechanical necessity of resting on a hard, unlined surface during the first weeks of life. He calls for careful field observation of living birds to resolve remaining questions about the precise functional role of the more elaborate tubercular forms.
Context
- Primary material drawn from the Raffles Museum collection, Singapore, supplemented by references to the Stockholm Museum (Gyldenstolpe) and the Norwich Museum (British Redbreast series).
- The paper represents an early contribution to the comparative anatomy of avian nestlings from a Malayan collection, extending Gyldenstolpe’s European-centric survey to the region’s rich Piciform and Coraciiform fauna.