Papagaios, Life & Times on a Speculative Archipelago - Cavadors

Text taken from Clancy A., Galvin D. 2018. ‘Biomechanical modelling of beak and foot strength suggests the Oligocene cavador Rhamphosfinus was fossorial.’ Lethaia.

Cavadors - the ground-dwelling, insectivorous birds of the Papagaios Archipelago – are of all birds, perhaps the most specially adapted for a fossorial, burrowing lifestyle. Living in burrows ranging from single-chamber cavities to multi-chambered setts, cavadors are often considered equivalent to badgers in lifestyle and ecology. So obvious is this fact that the group name is derived from the Portuguese ‘pássaro escavador’ or ‘digging bird’.

The origins of this lifestyle are much debated. The ancestral marqunihet auks lost their flight around 25 - 30 million years ago, and their sister group, the cavadors, lost the trait at a similar time. The transition from flightless ground bird to burrower is less understood. That is what this paper seeks to address.

We used biomechanical modelling to better understand the stresses and strengths of the beaks and feet of various extant bird species (including cavadors, puffins, kiwis, grouse) to cavadors known from the fossil record to determine the point at which burrowing became a major part of their lifestyle, and determined that the earliest known cavador adapted to dig extensively was Rhamphosfinus milagresensis, from the late Oligocene, 25 million years ago.

[…]

Even without the modelling data, Rhamphosfinus has many traits that had been argued were of use in a fossorial lifestyle. Its namesake wedge-shaped beak (Rhampho (ράμφος) – beak, Sfina (σφήνα) - wedge) is well-shaped to both break hard earth without injury and carve out tunnels and chambers. Likewise its wide, flat feet, which retained the webbing of their seaborne ancestors, would be able to push back large volumes of dirt in a single sweep. The proportionally large head and short, thick neck would have supported muscles that again aided in digging.

The skull possesses large orbits, implying large eyes and a nocturnal or crepuscular lifestyle, with the animal sleeping in its burrow during the day to avoid predators. The burrows likely started, as in modern auks, as hollows or small chambers to lay eggs and grew larger and more elaborate as more of the animal’s time was spent in them.

The wings of Rhamphosfinus milagresensis were still fairly large and some models propose very limited flight would still be possible. It is likely the species represents a dual transition to both flightlessness and fossoriality which would become codified in later descendants.

[...]