Bermuteo: the extinct hawk of Bermuda

Bermuteo avivorus was an extinct hawk endemic to Bermuda. Its combination of accipitrid features was unusual enough for Storrs Olson to name a separate genus in 2008. The most striking preserved feature is not a skull or hooked beak, but a very robust lower leg on a bird that could still fly.

The description rests on only seven bones from at least six individuals, so some parts of the animal are much better known than others. Legs reveal unusual strength; the head, feet and plumage are incomplete or absent. Birds were probably important prey on an island without native land mammals, but no prey remains are directly associated with Bermuteo. Its uncertain place in Bermuda's history makes the extinct bird catalogue a useful comparison with island species whose last records are better established.

Quick facts

A hawk named from one thigh bone

Olson formally described Bermuteo avivorus in 2008. The holotype is a complete left femur, Smithsonian specimen USNM 531932, found by Olson and Paul Harty on 15 October 2000 in Admirals Cave, Hamilton Parish. It is held by the National Museum of Natural History in Washington. The genus name combines Bermuda with Buteo, the familiar name for buzzards or hawks; avivorus means bird-eating. That second part records a proposed ecology, not a meal found in the cave.

The femur did not match the studied bones of any living hawk genus. Its head is relatively small and somewhat flattened, its neck joins the shaft in a distinctive way, and the outer condyle has a reduced wing-like projection. These details, together with other limb characters, support a member of Buteoninae, the hawk lineage that includes Buteo and related genera. The classification is based on a diagnostic mosaic, not on a complete skeleton.

Seven bones and several caves

Besides the holotype, the sample includes a complete right humerus, one complete tibiotarsus and three incomplete tibiotarsi. These bones came from Admirals Cave, Sibley's Cave, Fern Sink Cave, the Walsingham cave system and a locality whose exact collection data were lost. Three of the seven bones are complete. They represent at least six birds, but they cannot be joined into individuals.

The holotype femur is 55.4 millimetres long. The complete humerus measures 84.8 millimetres, and the longest complete tibiotarsus is 89.6 millimetres. Another ulna from Admirals Cave was deliberately excluded from the species: Olson judged it to belong to a larger hawk, perhaps a visiting Red-tailed Hawk. That distinction matters. A bone from the same island or cave does not automatically belong to Bermuda's endemic species.

Predators are naturally less numerous in a food web than many of the animals they hunt, and hawk remains do not necessarily enter caves. Seven bones therefore cannot tell us the living population density. Nor do they establish that the bird was rare. They describe the fossil sample, shaped by preservation and discovery as well as by the animal's abundance.

Dates that are not all equally secure

The holotype came from a deposit assigned to marine isotope stage 5e, the last interglacial roughly 120,000 years ago. Some specimens from lower pockets of Sibley's Cave may be younger, but their proposed age depends partly on calcite coatings. A bone found on the surface at Fern Sink Cave was considered possibly Holocene because the cave contains both glacial-age and younger deposits. It has no direct radiocarbon date reported in the description.

One tibiotarsus bears marine worm tubes in the carbonate that coated it. The bone may have rested in seawater after a low cave flooded. That observation was used to infer a broad minimum age around the preceding glaciation or the start of the last interglacial. It is a taphonomic argument about the specimen's history, not a precise date. Another lightly coloured, uncoated bone was also tentatively considered Holocene. The youngest proposed ages should therefore remain qualified.

A short, powerful lower leg

The tibiotarsus is especially unusual: it is short and massive, with a shaft broader than expected for hawks of comparable general dimensions. Olson's comparative collection contained no other accipitrid with the same degree of robusticity. This is direct evidence about the bone. It suggests resistance to substantial loads during landing or prey handling, but it cannot by itself calculate toe strength. The tarsometatarsus and toe bones needed for that assessment are missing.

The shoulder and femur also combine traits not seen together in one living genus. The proportions differ from the longer, more slender lower legs of Buteogallus and Parabuteo, while the shoulder crest and femur differ from those of Accipiter. The complete humerus and other limb material are consistent with an ordinary functional wing. Bermuteo was not a flightless island rail or a ground-running predator, even though its legs were unusually strong.

How large was the Bermuda hawk?

The holotype femur is close in size to the bones of small male Roadside Hawks, whose living mass is around 244–268 grams. The humerus suggests a bird larger than a female Broad-winged Hawk, which may exceed 459 grams, and some of the massive lower legs may have belonged to still heavier individuals. These are comparisons, not direct weighings of Bermuteo.

Olson proposed pronounced sexual size dimorphism as one explanation. Many hawks that take birds have larger females, and the largest Bermuteo bones could represent females more than twice the mass of the smallest males. The hypothesis has limits: the bones are isolated, their ages are not all equally secure, and future discoveries could show that the sample includes temporal variation or another form. No individual skeleton documents a male and female pair. A single exact mass would therefore imply more certainty than the fossils allow.

Likely prey on a small island

Before people arrived, Bermuda had no native terrestrial mammals other than bats. Seabird colonies, rails and other resident birds offered potential prey, while migrants visited the islands seasonally. In that ecological setting, birds are the strongest candidate for an important part of the hawk's diet. Its exceptionally robust legs are compatible with taking substantial prey, but the bones do not reveal whether it attacked birds on the ground, in flight or near nesting caves.

Bats, lizards or carrion could also have contributed. No stomach contents, pellet or identifiable prey bone proves any of these. Other extinct island birds, including the dodo and the Great Auk, had their own histories of ecological change and extinction. They are comparisons for island vulnerability, not evidence that Bermuteo behaved in the same way.

The contrast with the soaring scavenger Teratornis and the large South American Argentavis is also informative. All were large birds, but their anatomy and likely feeding strategies differed. A strong leg is not proof that Bermuteo was a specialist on any one prey species.

Could Bermuteo have survived into human history?

In 1603 the Spanish navigator Diego Ramírez described abundant, approachable hawks on Bermuda that could be killed with sticks. William Strachey also mentioned hawks in 1610, although historical English names could refer to more than one raptor. By 1623 John Smith described only occasional migrating falcons and ospreys. The sequence allows a hypothesis that a resident island hawk disappeared early in the seventeenth century.

No historical specimen connects those accounts securely to Bermuteo avivorus, and no Holocene bone has a direct date near 1600. The reported tameness fits an island bird unfamiliar with people, but does not identify a species. Hunting, forest clearance and introduced animals could all have mattered if the endemic hawk persisted. Their separate effects cannot be measured from the small fossil sample. The species may have survived into the human era, but that remains an interpretation rather than a confirmed last record.

What a reconstruction can safely show

A defensible illustration can show a small-to-medium flying hawk on a Bermuda coast, with sturdy legs. The accipitrid relationships, flight capability and exceptional tibiotarsus are supported by fossils. Exact body proportions are less certain because no complete skeleton exists. Feather colour, tail pattern, the shape of the soft-covered bill and a particular attack are not preserved.

A scene of the bird taking a seabird is plausible, not documented. The word “bird-eating” in the species name should not be mistaken for gut contents or a fossilised chase. The strongest portrait keeps the robust leg visible while leaving the animal's detailed appearance and final history open to revision.