Boobies

The northern gannet (Morus bassanus). Photo by Alan Wilson.

The Taxon of the Week post is a day late this week because, of course, yesterday was a public holiday here in Australia (as far as I can make it out, it seems a bunch of poms were so glad to see land after six months in a leaky boat [trying hard to keep afloat] that they've been celebrating ever since - the people already here may have had a different view of matters, but nobody ever asked them). Not only is it late, but it'll be short, too, because the Taxon of the Week is the Sulidae, which has been covered only recently by Darren Naish (here and here) with all the innuendo that is unavoidable when dealing with a group of birds going by the common name of "boobies" (though he did omit mentioning the close connection between boobies and shags).

The Sulidae are the boobies and gannets, a small but fairly cosmopolitan assortment of seabirds. Among the living sulids, the gannets form the genus Morus, and most boobies belong to the genus Sula. The exception is Abbott's booby (Papasula abbotti) which was originally included in Sula, and is still commonly referred to as such (at least in popular sources), but sits on the gannet rather than the booby side of the divergence between the two main genera (Friesen & Anderson, 1997). The distinctions between the three genera are not huge, and some authors in the past have referred to all living sulids as Sula. In the recent fauna there is a clear geographical division between gannets and boobies - Morus is found in the northern and southern temperate zones while Sula and Papasula are tropical or subtropical - but this does not appear to have always been so in the past. The Pliocene Pisco Formation of Peru has provided species assigned to both Sula and Morus (Stucchi & Urbina, 2004).


A sulid on the Galapagos Islands. This photo comes from here, where it is identified as a masked booby (Sula dactylatra). I don't think it is - the orange beak identifies it as a Nazca booby (Sula granti), a species that was only distinguished from S. dactylatra in 1998 (Pitman & Jehl, 1998). Don Roberson's site has a photo by Bob Pitman of the two species standing side by side to show the difference.

Eight fossil genera have been assigned to Sulidae (as well as fossil species of Sula and Morus*) - the Eocene Masillastega and Eostega, the Oligocene Empheresula, the Miocene Microsula (now a synonym of Morus), Miosula, Enkurosula and Sarmatosula, and the Pliocene Palaeosula and Ramphastosula. The Cretaceous Elopteryx did a stint as a close relative of the sulids, but is now regarded as a dinosaur of the Troodontidae and not even a bird (for most definitions of the word "bird"). Eostega lebedinskyi is known from a single mandible that was recently redescribed by Mlíkovský (2007), who reasserted its sulid nature (past authors have disagreed). Masillastega rectirostris is known from a skull from the famed Messel formation, and differs from living sulids in having a comparatively long beak (Mayr, 2002). It is also distinct in having seemingly inhabited a freshwater environment, while all modern species are exclusively marine. Mayr (2002) only tentatively regarded Masillastega as a sulid, and it may be a stem-member of the family (Mayr, 2005). Mlíkovský (2007) synonymised the two Eocene genera on the basis of a lack of significant differences between them.

*Apart from a single taxon described as a subspecies of the modern species (Papasula abbotti costelloi [ha ha]), there don't appear to be any fossil species assigned to Papasula. Considering the only recent distinction of Papasula from Sula, one wonders whether any fossils of the former are masquerading as the latter.

Empheresula arvernensis is represented by a pelvis from France (the original material also included a sternum, but Lambrecht later indicated that the sternum was not even sulid - Mlíkovský, 2002). The French Oligocene also provided Sula ronzoni, which, if correctly assigned (which seems to be debatable*), would suggest that the living sulid lineages had diverged by that point (Friesen & Anderson, 1997, used a molecular clock to estimate a divergence time for Sula vs. Morus/Papasula of 23 million years ago, which is also consistent with this, but the age calculation methods used by Friesen & Anderson can only be described as [ahem] dated). The European Miocene genera Enkurosula and Sarmatosula are both known from isolated humeri, and are both doubtfully distinct from Morus (Nelson, 2006; Olson, 1984, suggests that Microsula (=Enkurosula) pygmaea may be conspecific with the contemporaneous Microsula avita of Maryland in the United States, which has itself been since reassigned to Morus), as are the Californian genera Miosula and Palaeosula.

*I'm rather confused here. Nelson (2006) notes that Sula ronzoni has four notches on the sternum, and indicates that this would place it on the sulid stem. However, Mlíkovský (2002, 2007) states that the type material of S. ronzoni is an incomplete pelvis, so what is Nelson talking about?



The wierdest of all sulids, though, is the Pliocene Peruvian Pisco Formation's Ramphastosula ramirezi. The genus name means "toucan-booby" and is undeniably appropriate as Ramphastosula, instead of having a dagger-like straight beak like all other sulids, had a deep beak with a distinct arch as shown in the reconstruction above from Stucchi & Urbina (2004). The skull of Ramphastosula is also more robust than in other sulids, seemingly to support the enlarged beak. Ramphastosula was obviously pursuing a different lifestyle to other sulids, as it looks as if it would be ill-suited to catching fish by plunge-diving. Stucchi & Urbina (2004) suggest that its robust skull indicates greater diving ability than other sulids, so perhaps Ramphastosula was more inclined to pursue its prey underwater than its modern relatives. Unfortunately, no post-cranial material is known as yet for this species.

REFERENCES

Friesen, V. L., & D. J. Anderson. 1997. Phylogeny and evolution of the Sulidae (Aves: Pelecaniformes): a test of alternative modes of speciation. Molecular Phylogenetics and Evolution 7 (2): 252-260.

Mayr, G. 2002. A skull of a new pelecaniform bird from the Middle Eocene of Messel, Germany. Acta Palaeontologica Polonica 47 (3): 507-512.

Mayr, G. 2005. The Paleogene fossil record of birds in Europe. Biological Reviews 80: 515-542.

Mlíkovský, J. 2002. Cenozoic Birds of the World. Part 1: Europe. Ninox Press: Praha.

Mlíkovský, J. 2007. Taxonomic identity of Eostega lebedinskyi Lambrecht, 1929 (Aves) from the middle Eocene of Romania. Annalen des Naturhistorischen Museums in Wien 109A: 19-27.

Nelson, J. B. 2006. Pelicans, Cormorants, and Their Relatives: The Pelecaniformes. Oxford University Press.

Olson, S. L. 1984. A brief synopsis of the fossil birds from the Pamunkey River and other Tertiary marine deposits in Virginia. In Stratigraphy and Paleontology of the Outcropping Tertiary Beds in the Pamunkey River Region, Central Virginia Coastal Plain: Guidebook for the Atlantic Coastal Plain Geological Association 1984 field trip (L. W. Ward & K. Krafft, eds.) pp. 217-223. Atlantic Coast Plain Geological Association.

Pitman, R. L., & J. R. Jehl, Jr. 1998. Geographic variation and reassessment of species limits in the "masked" boobies of the eastern Pacific Ocean. Wilson Bulletin 110: 155-170.

Stucchi, M., & M. Urbina. 2004. Ramphastosula (Aves, Sulidae): a new genus from the early Pliocene of the Pisco. Journal of Vertebrate Paleontology 24 (4): 974–978.

The Stately Herons

Taxon of the Week this week may overstep its bounds a little. This is because of a somewhat surprising amount of disagreement about what exactly the taxon in question covers, despite being familiar to people the world around. Prepare to meet the Ardeinae, the herons.

Herons belong to the family Ardeidae, which also includes the bitterns. When I was young and reading Ausich (1961), the division of this family was simple - the bitterns formed the subfamily Botaurinae, while everything else fell into Ardeinae. Since then, however, the picture has become a bit more complicated. The bitterns are almost certainly nested within this broad picture of Ardeinae, and most authors have tended to restrict Ardeinae to birds more closely related to the genus Ardea than to the bitterns. Unfortunately, because different authors have found differing positions for the bitterns within heron phylogeny (McCracken & Sheldon, 1998), this has resulted in differing contents for Ardeinae.

One point that most authors have agreed on is that the family Ardeidae can be divided into four main groups, whatever their inter-relationships might be. These groups are the day-herons (Ardea and its relatives), night-herons, bitterns and tiger-herons. There are also two single-species genera of more uncertain relationships, Cochlearius and Agamia. The South American tiger-herons have been regarded in the past as closely related to the day-herons on the basis of osteological data (Payne & Risley, 1976), but DNA-DNA hybridisation and vocal data position them as the basalmost group in the Ardeidae (McCracken & Sheldon, 1998). Unfortunately, heron phylogeny does not appear to have been given much attention since the DNA-DNA hybridisation days, and the only study I found referred to that used (barely) more advanced molecular methods (Chang et al., 2003) seems to have not included tiger-herons. Payne & Risley (1976) took a conservative approach that referred to each of the four groups as separate subfamilies, while Kushlan & Hancock (2005) included both the night-herons and day-herons in the Ardeinae and placed the other two groups in separate subfamilies. Kushlan & Hancock (2005) also recognised a separate subfamily each for Cochlearius and Agamia, but I suspect this more reflects their uncertain relationships rather than any positive idea about their positions.



The day-herons (Ardeinae proper or tribe Ardeini, depending on whom you ask - Kushlan & Hancock, 2005, divide them into two tribes Ardeini and Egrettini, but that isn't an approach I've seen elsewhere) are the best-known of the groups, and include what most people associate with the name "heron" - long-necked, long-legged, stately birds. The image at the top of this post (from Wikimedia) shows a fairly typical example, the white heron or great egret (Casmerodius albus), while the photo just above (from here) shows the Chinese pond-heron (Ardeola bacchus). As well as the herons of the genus Ardea, this group also includes the egrets (Egretta) and the pond-herons in Ardeola and Butorides. As the common name indicates, the day herons are largely diurnal. The males of a number of day heron species (most notably members of the genus Egretta) produce long decorative plumes in the breeding season, as can be seen in the photo of Casmerodius.



The night herons (Nycticoracini or Nycticoracinae) of the genera Nycticorax and Gorsachius are generally shorter, stouter birds than the day herons, with relatively shorter beaks, as well as (obviously) being nocturnal or crepuscular. Osteological data suggest that the night herons are closely related to the bitterns, while molecular data would place them closer to the day herons (McCracken & Sheldon, 1998). One night heron genus, the American Nyctanassa, is included by Kushlan & Hancock (2005) among the day herons as opposed to with the other Old World night herons.



The boat-billed heron (Cochlearius cochlearius - shown above in an photo stolen from Brian Switek) and the agami heron (Agamia agami - photo below from Arthur Grosset) are both South American oddballs that have been particularly difficult to place among the herons. In the case of Cochlearius, it was regarded as distinct enough that Wetmore placed it in its own separate family. Cochlearius differs from other herons in its unique beak structure and the number of powder-down patches on the chest (four as opposed to three). However, Cracraft (1967) claimed that, except for features directly connected with the beak, Cochlearius was little different osteologically from Nycticorax, and in fact resembled Nycticorax more closely than the other night-heron genus Gorsachius did! While osteological data might indicate that Cochlearius is simply a very specialised night heron, DNA-DNA hybridisation data indicated a more basal position, around the level of the tiger-herons (though unresolved as to which of the two was the basalmost clade - McCracken & Sheldon, 1998). Whichever is the true position, it is clear that the boat-billed heron is highly specialised, though we have little idea what, in fact, it is specialised for - Biderman & Dickerman (1978) found little apparent difference in diet and foraging behaviour of boat-billed herons from more typical heron species, and were only able to suggest somewhat half-heartedly that the oversized beak might be related to courtship displays.



The agami heron (Agamia agami) seems to be a specialist bank feeder (Payne & Risley, 1976). In proportions, it is much like a day heron, and osteological data also associates it with that group. However, if it is a day heron, it differs in a number of characteristics from the other members of that group. As can be seen in the photo above, it is a particularly colourful bird, and it is distinct from the day herons in many features of its adult and juvenile plumage. It also has a particularly slender, needle-like bill. The relationships of Agamia do not seem to have yet been investigated molecularly.

REFERENCES

Austin, O. L., Jr. 1961. Birds of the World: A survey of the twenty-seven orders and one hundred and fifty-five families. Paul Hamlyn: London.

Biderman, J. O., & R. W. Dickerman. 1978. Feeding behavior and food habits of the boat-billed heron (Cochlearius cochlearius). Biotropica 10 (1): 33-37.

Chang Q., Zhang B.-W., Jin H., Zhu L.-F. & Zhou K.-Y. 2003. Phylogenetic relationships among 13 species of herons inferred from mitochondrial 12S rRNA gene sequences. Acta Zoologica Sinica 49 (2): 205-210.

Cracraft, J. 1967. On the systematic position of the boat-billed heron. The Auk 84 (4): 529-533.

Kushlan, J. A., & J. Hancock. 2005. Herons. Oxford University Press.

McCracken, K. G., & F. H. Sheldon. 1998. Molecular and osteological heron phylogenies: sources of incongruence. The Auk 115 (1): 127-141.

Payne, R. B., & C. J. Risley. 1976. Systematics and evolutionary relationships among the herons (Ardeidae). Miscellaneous Publications, Museum of Zoology, University of Michigan 150: 1-115.