Hornby's storm petrel

Oceanodroma hornbyi (G. R. Gray, 1864) is a storm petrel, one of the smallest seabirds of the order Procellariiformes, belonging to the Humboldt Current ecosystem ^[1], with a range from Ecuador to Chile ^[2]. Its accepted name in WoRMS is Oceanodroma hornbyi ^[3].

In Peruvian waters it is one of the two most common storm petrels. In 2019, observers on IMARPE cruises counted 556 birds in summer, 331 in autumn and 496 in spring ^[1]. No breeding colony is yet known in Peru, and what is best known about it in Peru comes from birds that fall in coastal cities, disoriented by lights ^[1],[4].

It is a small seabird of the family Hydrobatidae, among the tiniest members of the order that also holds albatrosses, petrels and shearwaters ^[1]. SERFOR asks that it not be mistaken for pigeons, doves or ground-doves when one turns up in a city, and names tube-shaped nostrils and webbed feet as features ^[4].

At sea off Peru it shares its range with Oceanodroma markhami, the other most common storm petrel there ^[1]. In the 2019 shipboard counts they were logged separately: 556, 331 and 496 individuals of Oceanodroma hornbyi in summer, autumn and spring, against 231, 154 and 333 of Oceanodroma markhami ^[1].

The species is restricted to the cold waters of the Humboldt Current in the Pacific, from northern Ecuador to central Chile ^[2]. A bird that looked exhausted was found on 25 April 2020 in San Martín de los Andes, Neuquén (Argentina), more than 1000 km south of its known range; the day before there had been a brief, intense thunderstorm ^[2].

In summer 2019 it was seen off the Peruvian coast between Paita (5° S) and the Chilean border (18° S), with moderate concentrations south of Huarmey (10° S) and the highest one between 14° S and 16° S and at 18° S ^[1]. In autumn the pattern repeated, with more presence south of 14° S and the largest aggregation south of 13° S, off Ica, Arequipa, Moquegua and Tacna; in spring there were large (100 to 500 birds) and medium (50 to 100) concentrations throughout its range ^[1].

The mean distance of sightings from the coast was 42.8 nautical miles in summer, 38.5 in autumn and 63.4 in spring, with a minimum of 2.8 and a maximum of 112.9 miles across the year ^[1]. It was found in cold coastal waters, in subtropical surface waters and in the mixed waters between the two ^[1].

No breeding colony that can be monitored has been found in Peru. The first colony of the species on the South American coast was discovered in Chile, in the Atacama Desert, where the nests were in crevices and crusts ^[1]. It has been suggested that in Peru it might breed in high Andean areas, but its nesting place is still not known with precision ^[1],[4].

Romero, drawing on other authors' studies, reports that the breeding cycle would run from December to June and that juveniles still downy land on the coast from April to July, when they would be leaving the nest for the sea ^[1]. SERFOR notes that in Arequipa these birds are frequently found between June and September, and in a note of 3 April 2025 it reported the rescue of a juvenile in Tacna ^[4],[5]. In 2019 the southward pull-back in summer and autumn matched that season, which the author takes as a clue to where the nests are ^[1].

Because it was seen in three water masses, Romero infers a flexible diet, and proposes that during breeding (summer and autumn), when it is mostly in cold coastal waters, it could be feeding on a very abundant resource such as anchoveta; this is a hypothesis based on where it was seen, not a verified diet ^[1].

According to that author, an earlier study found the highest autumn concentration off Paracas and higher density where salinity is higher; she found a similar relation in her own data between the number of birds (both species) and salinity ^[1]. The breeding periods of this species and of Oceanodroma markhami would overlap for only a few months, which could ease competition for prey and space ^[1].

For birds of the Humboldt Current, El Niño is one of the main causes of dispersal, because it reduces food in the eastern Pacific and forces many to move ^[2].

The Peruvian State protects the species, as SERFOR states on the basis of the first complementary final provision of Supreme Decree 019-2015-MINAGRI ^[5].

The best-documented threat in Peru is artificial light. SERFOR explains that intense lighting disrupts the birds' navigation and can cause disorientation, momentary blindness and collisions with buildings, poles and vehicles ^[5]. Romero recommends identifying and protecting breeding areas, avoiding human settlements that cast light pollution on juveniles heading out to sea ^[1].

By 1 July 2025, SERFOR had received 29 birds from urban Arequipa that season, and since 2020 more than 400 have been reported in the region and returned to their habitat ^[4].

Most of what is known about its biology in Peru comes from birds that fell on the coast: reports from local people and rehabilitation projects, promoted by scientists and by the State, have supplied data that the open sea does not give ^[1].

SERFOR treats and rehabilitates the birds and releases them on the beaches of Islay province, in Arequipa. For anyone who finds a bird, it advises using gloves or a cloth, placing it in a cardboard box with ventilation holes and a soft base, and not feeding it, because it has a special diet ^[4].

It is an open-sea bird: on the 2019 IMARPE cruises it was seen between 2.8 and 112.9 nautical miles from the coast, with a mean of 38.5 to 63.4 depending on the season, so seeing it depends on going far out by boat ^[1]. The three 2019 cruises covered, depending on the season, from Tumbes or Lobos de Tierra island to Tacna (and in spring to 18°20' S) ^[1].

Juveniles that turn up in yards and streets of coastal cities between June and September in Arequipa are disoriented birds that fell by accident, not a way of watching the species; SERFOR asks that they be handed in to its offices ^[4]. No breeding site is known in Peru ^[1].