Stilts and Avocets

Visit a healthy wetland in many parts of the world and you may be able to see boldly patterned, lightly built birds with remarkably long legs and bills wading through the shallows. These are the members of the Recurvirostridae, commonly known as the stilts and avocets.

American avocets Recurvirostra americana, from here.

About a dozen species of recurvirostrid are currently recognised, depending on the exact classification scheme in play. They are divided between three genera with the avocets forming the genus Recurvirostra and the stilts divided between Himantopus and Cladorhynchus. The most obvious distinction between the two subgroups is in the shape of the bill: that of stilts is straight but avocets have a distinct upwards curve towards the end of theirs. A fourth genus has often been included in the Recurvirostridae for the ibisbill Ibidorhyncha struthersii, a striking-looking inhabitant of the upland rivers of the Himalayan plateau, but uncertainty about this bird's phylogenetic position has led most recent authors to exclude it from the family.

The recurvirostrids feed mostly on small aquatic invertebrates such as brine shrimp or insect larvae. Their long legs, among the longest relative to body size of any bird, allow them to wade in deeply in search of prey. Stilts actively probe the waters and underlying sediment whereas avocets tend to forage by sweeping their bill through the water side to side. Avocets and the banded stilt Cladorhynchus leucocephalus of Australia prefer brackish waters such as lagoons and estuaries, with the banded stilt congegrating around the great salt lakes of inland Australia. Breeding is conducted by monogamous pairs that share the duty of incubating their simple nest on the ground near water. These nests may be gathered into loose colonies; the banded stilt forms particularly large colonies in which the chicks are herded into communal creches of several hundred.

Pied stilt Himantopus leucocephalus, copyright JJ Harrison.

The majority of recurvirostrids are patterned with black or dark brown and white. The red-necked avocet Recurvirostra novaehollandiae has the head and neck coloured reddish-brown as does the American avocet R. americana during the breeding season. The banded stilt has a broad reddish-brown band across the top of the breast. There is also the black stilt Himantopus novaezelandiae of New Zealand, which is somewhat self-explanatory. Beaks are black in all species; the legs are grey in avocets and red in stilts.

Four geographically distinct species of avocet occupy the modern world: the American avocet in North America, the red-necked avocet in Australia, the pied avocet Recurvirostra avosetta in Eurasia and Africa, and the Andean avocet R. andina in South America. The Andean avocet is a bird of high altitudes, occupying shallow, alkaline lakes in the upper Andes. Cladorhynchus includes only the banded stilt. The most varied taxonomy concerns the genus Himantopus. Historically, all the black-and-white stilts (and sometimes also the black stilt) have been recognised as a single near-cosmopolitan species. In more recent years, the trend has been towards recognition of five or six distinct species in the genus. Most of these species are well separated geographically except for in New Zealand where the black stilt shares its range with the pied stilt Himantopus leucocephalus, a more recent immigrant from Australia. The breeding range of the black stilt is currently restricted to a relatively small area of New Zealand's South Island, and the species is considered endangered due to factors such as habitat alteration and the threat of hybridisation with the more abundant pied stilt*.

*It's worth spending some thought on the role of hybridisation as a conservation risk. Some observers may express concern that regarding hybridisation as a threat per se carries uncomfortable intonations of "racial purity", and that limiting the available gene pool may do more harm than good. After all, it's not as if the black stilt heritage of hybrid individuals is just gone (hybrids between the two species are, I believe, fully fertile and able to produce offspring of their own). The question is, I suppose, do the black stilt genes actually persist in the mixed population? Or does selection and/or drift winnow them out over time? This would be a difficult question to answer, and not without risk to find out.

Banded stilts Cladorhynchus leucocephalus and red-necked avocets Recurvirostra novaehollandiae, copyright Ed Dunens.

Phylogenetically, it is reasonably well established that recurvirostrids form a clade with the ibisbill and oystercatchers. This clade is in turn closely related to the plovers of the Charadriidae; indeed, many recent phylogenies have indicated that the recurvirostrid-oystercatcher clade may even be nested within the plovers as generally recognised. Considering the relatively small number of species in each clade, it might seem reasonable to suggest the recurvirostrids be reduced to a subfamily of the Charadriidae, but bird taxonomists being bird taxonomists, there seems to be more of a push to divide the Charadriidae up instead.

The fossil record of the Recurvirostridae is limited. A handful of species have been assigned to this family from the Eocene, but all are known from limited remains and their position is questionable. Coltonia recurvirostra is known from part of a wing from Utah; it was a relatively large bird, appearing to be more than one-and-a-half times the size of any living recurvirostrid. Fluviatilavis antunesi was described from a femur, humerus and radius from Portugal but was described as exhibiting some primitive features not found in modern recurvirostrids. It is also worth noting that its original description (Harrison 1983) compared it most favourably with the ibisbill, so if that species is not to be regarded as a recurvirostrid, probably neither is Fluviatilavis.

REFERENCE

Harrison, C. J. O. 1983. A new wader, Recurvirostridae (Charadriiformes), from the early Eocene of Portugal. Ciências da Terra 7: 9–16.

The Wingless Penguin

A couple of weeks ago, I put up a page on the 'terrestrial penguin' Cladornis pachypus, described from the Oligocene of Patagonia by the Argentine palaeontologist Florentino Ameghino. As it happens, Cladornis wasn't the only unusual penguin recognised from the Patagonian fossil record by Ameghino nor was it even necessarily the most unusual. That title should probably go to another species, the wingless Palaeoapterodytes ictus.

Anterior (left) and posterior view of humerus of Palaeoapterodytes ictus, from Acosta Hospitaleche (2010). Scale bar = 10 mm.

Like Cladornis, Palaeoapterodytes was based on only a single bone, in this case a humerus (upper wing bone) from the Early Miocene. And also like Cladornis, Ameghino's description of this bone indicated a truly remarkable bird. The distal part of the humerus lacked any sign of the facets that would normally articulate with the succeeding wing bones and, as a result, Ameghino concluded that the wing skeleton had been reduced to the humerus only. The crest and pits on the humerus marking the attachment of the wing muscles were also reduced. Ameghino's Palaeoapterodytes presumably had wings reduced to the merest nubs, effectively functionless and probably of little mobility. Nevertheless, the humerus of Palaeoapterodytes remained relatively robust, its breadth little less than that of other penguins.

I am not aware of any other bird with a wing structure anything like this. In other birds without functional wings, the entire wing skeleton becomes reduced, not simply truncated. Perhaps the closest approximation I have found is the wing of Hesperornis, which also lacks known wing bones beyond the humerus. However, the Hesperornis humerus is slender and gracile, and even without direct indication of the presence of more distal bones, it still looks to retain some remnant of the ancestral articulation. Also, the whole concept of a wingless penguin is decidedly problematic. Hesperornis derived its main propulsion in swimming from its feet and so its wings became reduced because they served little function. Penguins, on the other hand, get most of their propulsion from their wings, swimming in a manner that has been compared to flying underwater. Despite being flightless, penguins retain a wing skeleton that is, if anything, even more well developed than that of their flying relatives. For Palaeoapterodytes to have lost functional wings, it would have somehow had to change its mode of propulsion.

Reconstruction of the Palaeoapterodytes humerus with missing sections restored, from Acosta Hospitaleche (2010).

As a result, even while authors were cautiously considering Ameghino's interpretation of Cladornis, they treated Palaeoapterodytes with more scepticism. This scepticism was eventually concerned when the humerus was re-examined by Acosta Hospitaleche (2010). The reason for the lack of structure at its distal end was very simple: the original distal end had been broken off. The apparent lack of development of the muscle attachment structures was the result of erosion, not any indication of the bone's original appearance. When alive, Palaeoapterodytes had probably been very similar to, if not identical with, one of the several other penguin species known from around the same time and place. Unfortunately, the state of preservation of the type humerus is so poor that its exact identity cannot be determined, and Palaeoapterodytes ictus has been cast into the taxonomic limbo of nomen dubium. Ameghino's Cladornis may remain an intriguing mystery, but his Palaeoapterodytes is just a red herring.

REFERENCES

Acosta Hospitaleche, C. 2010. Taxonomic status of Apterodytes ictus Ameghino, 1901 (Aves; Sphenisciformes) from the Early Miocene of Patagonia, Argentina. Neues Jahrbuch für Geologie und Paläontologie—Abhandlungen 255 (3): 371–375.