Bonin Petrel

The Bonin petrel, or nunulu, is a diminutive member of the gadfly petrels, with a length of 30 cm and a wingspan of approximately 67 cm. It is adorned with a white head, contrasted by a black cap and facial markings, giving it a scaled appearance. The upperparts are a pale grey, with darker primaries and wing coverts that form a distinctive "M" shape across the back. The underwing is predominantly white, edged with darkness and marked with a patch at the carpal joint and across the underwing coverts. The tail is a dark grey, while the rest of the plumage is white, save for a dark half collar on the breast. Its bill is black, short, and hooked, typical of Pterodroma petrels, and its legs and feet are pink with dark patches.

Identification Tips

When observing the Bonin petrel, look for the scaled appearance of the head, the "M" mark across the back, and the dark half collar on the breast. The underwing pattern is also distinctive, with its white base, dark edging, and specific dark patches. The pink legs with dark patches can also aid in identification.

Habitat

The Bonin petrel is a seabird of the open ocean, only coming to land to breed.

Distribution

This species is predominantly found in the northwest Pacific Ocean. A staggering 99% of the population breeds on the Northwestern Hawaiian Islands, while a mere 1% breeds off the coast of Japan on the Ogasawara and Volcano Islands.

Behaviour

The Bonin petrel is typically a solitary creature when at sea, though it may occasionally join large multi-species flocks. It exhibits nocturnal feeding habits and is known to produce stomach oil, a nutrient-rich substance, from its prey.

Diet and Feeding

The diet of the Bonin petrel is primarily composed of fish from the Myctophidae (lantern fish) and Sternoptychidae (hatchetfish) families, as well as squid from the Ommastrephidae family. These midwater creatures migrate to the surface at night, where the petrels feed on them, either while resting on the sea or in flight. The petrels' eyes contain high levels of rhodopsin, which enhances their vision in low light conditions.

Breeding

Breeding takes place in dense colonies on remote islands. The Bonin petrel shows philopatry, often returning to the same burrow each year. Mating for life is common, and the species engages in mating flights over the colony. A single egg is laid per season, with both parents sharing incubation duties. Chicks are semi-altricial at hatching and are fed by both parents at night. Fledging occurs approximately 80 days after hatching.

Migration

Post-breeding, the Bonin petrel migrates northwards to the seas off Honshū and Sanriku in Japan. The migration is influenced by sea surface temperatures, with birds dispersing south and east in August and returning to breeding islands in September.

Conservation status

The Bonin petrel is currently classified as Least Concern by the IUCN. However, it faces threats from introduced species such as rats, which have had devastating effects on populations by preying on eggs. Conservation measures, including island restoration and the removal of invasive species, are in place to protect this and other seabirds on their breeding islands.