Monday, April 20, 2020

The Lost Mousebird Dynasty

In sub-Saharan Africa there live six species of small, sociable birds called the mousebirds or colies. Classified into two genera (Colius and Urocolius), they are found in semi-open habitats such as savannah and woodlands, where they flit from tree to tree and feed on fruits, leaves, and flowers. Mousebirds climb with great agility, scurrying along tree branches in almost mouse-like fashion (hence "mousebirds").

Facilitating the arboreal activities of mousebirds are their unusual feet. Both the outermost and innermost toe on each foot can swivel to direct either forward or backward. As a result, mousebirds can adopt not only the anisodactyl toe arrangement of most modern birds, in which each foot has three toes pointing forward and one pointing back, but also zygodactyl (two toes forward, two toes back) and pamprodactyl (all four toes facing foward) configurations as well. Given this versatility, mousebirds can grasp and cling to surfaces in a variety of ways, as well as use their feet to handle food items. They often dangle from perches in a distinctive posture, with their feet held up at chest level and their very long, stiff tail feathers propped against a nearby surface.

A speckled mousebird in the characteristic dangling pose frequently adopted by mousebirds, photographed by Bob McDougall, under CC BY-NC 4.0.

Despite their specializations for climbing, mousebirds seem to have no trouble walking and running on the ground. They rarely drink water, but when they do, they are among the few birds that can drink by suction, and thus don't need to tip their head back to swallow each mouthful. (Among birds, similar abilities are also found in pigeons and some estrildid songbirds.) When food is scarce, mousebirds may enter torpor at night, dramatically lowering their metabolism to conserve energy.

Genetic studies have consistently placed mousebirds within Telluraves, a very diverse clade of mainly arboreal birds that also includes songbirds, parrots, woodpeckers, the various groups of birds of prey, and many, many more. Although some researchers have noted anatomical similarities between mousebirds and parrots, genetic studies generally don't favor a close relationship between these two groups within Telluraves. Instead, the most comprehensive recent analyses recover mousebirds as the closest living relatives of Cavitaves, a clade that unites woodpeckers, kingfishers, hornbills, trogons, and the Malagasy courol. Even so, uncertainty remains, as some types of genetic material strongly favor placing mousebirds as more distant relatives of cavitavians than are hawks and owls.

Phylogeny showing the phylogenetic position of mousebirds (Coliiformes) recovered by the most recent large-scale genetic studies (though see main text).

Mousebirds are generally unfamiliar to people outside of Africa today. However, during the Eocene Epoch (56-33.9 million years ago), the close extinct relatives of mousebirds were likely some of the most common birds in the forests of Europe and North America, as evidenced by the discovery of numerous fossil specimens. (Eocolius from the Eocene London Clay Formation in England probably wasn't one of them though. Known from a partial skeleton, it was originally described as a mousebird, but subsequent studies have found that it exhibits no convincing mousebird traits.)

A number of early mousebirds belonged to the extinct group Sandcoleidae, named by Peter Houde and Storrs Olson in 1992. Some sandcoleids, such as Botauroides, Eobucco, and the two species of Uintornis from the Bridger Formation of North America, are only known from feet. In contrast, Sandcoleus from the Willwood Formation and Anneavis from the Green River Formation of North America are known from nearly complete skeletons, as is Eoglaucidium from Geisel Valley and the Messel Formation in Germany.

In addition to Eocene fossil deposits, sandcoleids are also known from rocks dating to the preceding epoch, the Paleocene. In fact, ~62.5-million-year-old Tsidiiyazhi, known from a partial skeleton discovered in the Nacimiento Formation of New Mexico, represents the oldest well-corroborated fossil of any kind of neoavian bird, indicating that modern-type birds had already diversified greatly by the early Paleocene.

A specimen of Eoglaucidium preserved with feathers, from Mayr (2018).

Sandcoleids had "generalist", thrush-like beaks, suggesting that they weren't particularly specialized for feeding on any specific type of food. However, seeds have been found as gut contents in some specimens. These seeds had been swallowed intact rather than crushed, perhaps implying that sandcoleids swallowed fruits whole at least on occasion, similar to modern mousebirds.

Some specimens of Anneavis and Eoglaucidium have been found with preserved feathers. Like extant mousebirds, they had short, rounded wings, but at least Eoglaucidium differed from them in lacking a head crest. The tail feathers of sandcoleids were fairly long, but not elongated to the extent seen in modern mousebirds. This, along with a pygostyle (fused vertebrae at the end of the tail) that was not as broad as it is in living mousebirds, indicate that sandcoleids did not habitually prop their tail against surfaces.

The feet of sandcoleids were more robust than those of extant mousebirds, further implying that they did not typically use a dangling perching posture. They were probably still skilled climbers and clingers, as was proposed by Houde and Olson, and later supported in a functional study by Nikita Zelenkov and Gareth Dyke in 2008. Sandcoleid feet bore curved claws, which have been likened to those of raptors. Although sandcoleids seem to otherwise lack raptorial adaptations, perhaps this indicates that they used their feet for manipulating food items. Like modern mousebirds, sandcoleids appear to have had flexible toes that could alternate between several different toe configurations.

Intriguingly, the raptor-like claws of sandcoleids may be consistent with the hypothesis that the last common ancestor of all telluravians was a raptorial bird. Might this also explain the apparently convergent similarities between mousebirds and parrots, in that their shared features could have both resulted from the adoption of a fruit-eating ecology by a raptorial ancestor? Food for thought. (Maybe someone should keep an eye on the palm-nut vulture...)

The feet of Eoglaucidium showing large, curved claws, from Mayr (2018).

Formerly thought to be a sandcoleid was Selmes from the Messel Formation. (Its genus name is an anagram of "Messel".) Originally named in 1999, a better-preserved specimen described by Gerald Mayr in 2001 exhibited several features, including slender feet and a broad pygostyle, that suggested it was more closely related to modern mousebirds than sandcoleids were. In addition, these characteristics suggest that it often perched in the dangling pose of modern mousebirds. Selmes was also initially interpreted as having toes that were permanently fixed in a pamprodactyl arrangement, but Mayr argued that its feet more likely functioned similarly to those of sandcoleids and living mousebirds. Like sandcoleids, Selmes has been found with seeds preserved as gut contents. A mousebird foot from the Eocene Quercy Phosphorites may represent a French record of Selmes.

A specimen of Selmes, from Mayr (2001). The black arrow points to the broad pygostyle.

There is, however, another Messel mousebird that might have actually had permanently pamprodactyl feet. That mousebird is Masillacolius, known from three specimens that all have the toes preserved in a pamprodactyl posture. Other than mousebirds, the only extant birds that adopt a pamprodactyl foot configuration with any regularity are some types of swifts (though they don't do so as frequently as sometimes implied by popular texts), which use this ability to cling to vertical surfaces. Perhaps Masillacolius did the same.

A seed is associated with one specimen of Masillacolius; this may represent yet another example of ingested material preserved in an extinct mousebird, though in this case the seed is not directly preserved in the body cavity. Masillacolius is also one of several fossil mousebirds (as we shall see) known to have had long bony projections on the back of its lower jaw. These projections are relatively reduced in living mousebirds, but are often well developed in birds that need to open their jaws widely and forcefully.

A specimen of Masillacolius, from Mayr (2015). An associated seed is marked as "sd".

Among fossil mousebirds, such well-developed projections were first noted in Chascacocolius. The first species of Chascacocolius to be named was C. oscitans from the Willwood Formation. It was described as a sandcoleid by Houde and Olson (1992), but more recent studies suggest that it was more closely related to extant mousebirds. Although a lower jaw is known for C. oscitans, the rest of its skull has not been found.

However, a second species from the Messel Formation, C. cacicirostris, was named by Mayr in 2005, and it is known from a complete skull as well as some neck vertebrae. And what an unexpected skull it had! Its long, pointed beak more closely resembles those of New World blackbirds than that of any other kind of mousebird, living or extinct. In fact, Mayr noted that if it weren't for his knowledge of a privately-owned specimen that preserves a complete skeleton (figured in his paper), it would have been difficult to identify C. cacicirostris as a mousebird at all! (Unfortunately, the current location of the complete skeleton is unknown.) New World blackbirds use their enhanced gaping abilities and pointed bills to pry open crevices in search of prey and to open up large fruits so they can lap up the juices inside. Chascacocolius may have done so as well. Zelenkov and Dyke (2008) proposed that Chascacocolius had a woodpecker-like, trunk climbing ecology, though a later study by Dan Ksepka and Julia Clarke in 2010 was unable to verify the presence of features that purportedly supported this hypothesis.

The skull of Chascacocolius cacicirostris, from Mayr (2005).

Ksepka and Clarke (2010) also named one of the most unusual fossil mousebirds that have been described so far, Celericolius from the Green River Formation. Celericolius is known from a complete skeleton preserved with traces of the wing and tail feathers. These feather remains indicate that it had very long, pointed wings, quite unlike the short, rounded wings of modern mousebirds and sandcoleids. Such wings are commonly found in birds that specialize in catching insects in flight, such as swallows and swifts. Was Celericolius a mousebird version of a swallow? The only known specimen has a poorly-preserved skull and no associated gut contents, so it's hard to say.

Holotype of Celericolius, from Ksepka and Clarke (2010). The arrows indicate preserved wing and tail feathers.

The youngest known mousebird from North America was Palaeospiza from the late Eocene Florissant Formation. It is known from a partial skeleton preserved with feathers, and appears to have been closely related to modern mousebirds. However, it still differed from living mousebirds in details of the feet and forelimb bones. Possibly similar to Palaeospiza were the two species of Primocolius that lived at around the same time in what would become France, but a detailed comparison is difficult because Primocolius is only known only from isolated bones.

In Europe, mousebirds continued to persist beyond the Eocene. Two species of the appropriately named Oligocolius have been identified, both of which lived during the Oligocene in what would become Germany. The slightly older O. brevitarsus is known from a partial skeleton, whereas the younger O. psittacocephalon is known from a nearly complete skeleton, including a skull.

O. psittacocephalon is yet another extinct mousebird known to have had elongated projections behind the lower jaw. However, its beak was short and stout, more similar to those of extant mousebirds than that of Chascacocolius. It also had a marked hinge between its upper jaw and the rest of its skull, which likely allowed the upper jaw more freedom to flex up and down. As it happens, O. psittacocephalon was found with several large seeds preserved in its throat region. Its well-developed lower jaw projections and hinge at the base of the upper jaw may have both allowed it to swallow large fruits.

Holotype of Oligocolius psittacocephalon, from Mayr (2013). Note the large seeds preserved in the throat region.

Mousebirds remained part of the European avifauna during the first half of the Miocene, and two genera, Limnatornis and Necrornis, have been named for European mousebird fossils from this time. Yet no mousebird fossils younger than the mid-Miocene have been discovered in Europe, nor anywhere else in the Northern Hemisphere.

What happened to the northern mousebird dynasty? In 2019, Erin Saupe and colleagues showed that climatic conditions in North America and Europe during the early Eocene were similar to those favored by living mousebirds, matching the known occurrences of mousebird fossils. However, the cooling and drying of global climate throughout the Cenozoic gradually restricted such favorable conditions to regions near the equator. Saupe et al. also found similar patterns for several other bird groups that have geographically restricted distributions in modern times.

It is not clear whether mousebirds already lived in Africa before climate change limited them to that continent. However, the oldest known African mousebird is Colius hendeyi from the Pliocene of South Africa, postdating mousebird fossils from North America and Europe. Although assigned to the extant mousebird genus Colius, it is uncertain whether C. hendeyi really belonged to the modern radiation of mousebirds.

Mousebirds today are greatly diminished not only in geographic range but also in ecological diversity. The six living species only differ from one another in relatively minor details. Yet despite occasionally falling victim to persecution (due to their feeding on fruiting trees), pesticides, and collisions with vehicles, none of them are under immediate threat of extinction. In fact, mousebirds have benefited from some human activity, as gardens, orchards, and fruit plantations provide suitable habitat for them. As long as such conditions are maintained, it is likely that these curious little birds will continue the legacy of their incredible former diversity.

My amateur restorations of some fossil mousebirds.


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