The hawk whistler is deterred by the terratail's snakelike tail.
The Pacauan whistlers, Insulornis spp., are a group of pachycephaline pachycephalids, descended from the Australian golden whistlers that were blown across the ocean from Australia and colonized the islands of Pacaus (being the first birds to arrive there).
Originally a fairly unspecialized bird it had, during the rule of humanity, begun to show some differentiation, with distinct beak shapes developing on the islands around the Australian coast. However, it was only on the Pacaus Archipelago, where all the ecological niches were thrown open to them, that the whistlers really developed spectacularly, producing both insectivorous and seed-eating as well as predatory forms.
The descendants of the particular group of golden whistlers that colonized these islands are now regarded as belonging to a single genus, Insulornis. All the species within this genus are now highly specialized and quite different from one another excepting I. harti, which is similar in form to the original ancestral bird.
The insect-eater, I. piciforma, has developed a strong, chisel-like bill with which it tears into the bark of trees to get at burrowing insects. Its feet are modified to allow it to cling to the vertical trunks and the bird closely resembles the extinct woodpeckers of the Northern Continent whose mode of life it closely follows in almost every respect.
The insect-eater has a strong, pointed bill for penetrating the bark of trees.
Nuts and tough seeds are eaten by the nut-eater, I. macrorhyncha, a parrot-like species which has developed a massive bill and the powerful musculature to operate it. This bird has retained the perching feet of its ancestor and has grown a long tail to balance the weight of its large head.
The nut-eater has a heavy bill for cracking shells.
Most Pacauan whistlers are preyed upon by their hawk-like relative the hawk whistler, I. aviphaga, which shows the same adaptations that are found in birds of prey throughout the world, irrespective of their ancestry (a hooked beak, binocular vision through forward-facing eyes, and a high degree of maneuverability in pursuit).
The hawk whistler has a powerful hooked bill for tearing flesh.
Apart from the hawk whistler the only natural enemies faced by all the Pacauan whistlers, including the hawk whistler itself, are the snakes (the Pacauan bird snake specifically being the Pacauan whistlers' most important predator), which have been rafted to Pacaus from Australia or the other islands in that corner of the Pacific at one time or another. The Pacauan whistlers' wariness of snakes is exploited by the terratail.