Tag: accipitridae

Spectember/Spectober 2023 #07: Terror Eagle

September might have ended, but guess what? I am not remotely done with this yet so we’re continuing on.

Now it’s Spectober.

(Also just a reminder: I am not currently taking new requests. I’ve got far too many existing ones that I’m still working through!)

Someone who identified themself only as “Adam” asked for “eagles evolving into terrestrial predators to pursue larger prey”:

A shaded sketch of a speculative flightless eagle. It has a large head with a big hooked beak, with the skin on its head and neck only sparsely feathered like a vulture. It's body is chunky and covered in shaggy feathers, and it has thick legs and stumpy tail. Its small vestigial-looking wings have a large bony knob growing from just below the wrist joint.

A flightless eagle occupying an apex predator niche in the same island chain as the giant herbivorous tegu, Terraetus adamii is descended from a species similar to the modern harpy eagle. It isn’t substantially larger than the biggest modern eagles – standing about 1.2m tall (~4′) – but it’s certainly much more massive, weighing around 25kg (~55lbs).

In the absence of other large terrestrial predators its ancestors originally took up a caracara-like lifestyle, preferring hunting on foot over flying, before gradually becoming totally flightless and converging on terror birds with large heavy skulls, reduced wings, and powerful legs.

Terraetus’ head is only very sparsely feathered, an adaptation for feeding inside the carcasses of large prey, which it dispatches using a combination of kicking and blows from its large hooked beak. It’s usually a solitary hunter that can tackle prey up to two or three times its own weight – preferencing the juveniles of the herbivorous tegu – but during the breeding season pairs will occasionally hunt cooperatively to take down larger targets.

Despite possessing sharp beaks and talons, these weapons aren’t actually used in fights between individuals of this species. Instead they bodily shove each other back and forth, battering at each other with large bony knobs that grow from the hand bones of each wing.

Island Weirdness #36 — Haast’s Eagle

Before the arrival of humans in New Zealand, the only predator of the native giant flightless birds was Haast’s eagle (Hieraaetus moorei), an enormous species of raptor from the South Island.

With the slightly larger females reaching lengths of around 1.4m (4’7″) and hefty body weights of 15kg (33lbs), it was the largest eagle known to have ever lived, and one of the largest of all birds of prey. Yet its wings were actually proportionally short for its size, spanning about 2.5m (8’2″), giving it less ability to soar but much better maneuverability when flying in the thick vegetation of scrubland and forests.

Its closest living relatives are the much smaller Australian little eagle and the Palearctic booted eagle, which its lineage split from sometime in the Pleistocene — meaning that it must have evolved to be so huge incredibly quickly.

At its size and weight when striking it would have hit its prey with a force equivalent to a falling cinder block, and with no major competitors it could have fed from a single large kill for many days.

As apex predators Haast’s eagles would also have never been particularly numerous, and their population was very sensitive to the availability of their prey species.

Unfortunately the early Māori people also found the large flightless birds like the moa to be delicious easy meals, and by the early 1400s the eagle had disappeared alongside its main food sources. Legends about a giant human-eating bird called the pouakai are thought to be based on folk memory of Haast’s eagle, and it may well have occasionally preyed on the settlers during the short time it coexisted with them.

Buteogallus daggetti

This leggy bird is known as Daggett’s eagle (Buteogallus daggetti), a bird of prey from the Late Pleistocene of southwest North America.

Living between about 2.5 million and 13,000 years ago, it would have stood around 80cm tall (2′7″), with a wingspan of almost 2m (6′6″) and a  fairly hefty body weight of 3kg (6.6lbs). Its feet had less grasping ability than other eagles but it also had particularly strong leg muscles, suggesting it was much better adapted for walking around on the ground.

With its large size, long legs, and terrestrial habits it seems to have convergently evolved to fill the same ecological niche as the modern secretarybird – a grassland-dwelling walking predator that hunted on foot, kicking and stomping small prey animals like snakes, lizards, and rodents.