Peruvian diving petrel

Pelecanoides garnotii is a diving petrel of the family Pelecanoididae, described by Lesson in 1828 ^[1]. In Peru it is called «potoyunco peruano» ^[2] or simply «potoyunco» ^[3]. It is endemic to the Humboldt Current ^[3], and its breeding grounds now form two blocks some 1300 km apart: one on the central coast of Peru and another in northern Chile ^[4]. The IUCN Red List classifies it as Near Threatened in its 2020 assessment ^[5], and Peru's official list of threatened wild fauna approved in 2014 places it in the Vulnerable category ^[2].

It is a small, compact seabird: 46 birds weighed on Choros Island, Chile, gave a mean mass of 220 ± 27 g ^[6], and a pair caught in their nest on Corcovado Island weighed 201 g and 213 g ^[7]. Like the other diving petrels, it has wings highly specialised for diving, a trait that goes together with limited flying ability ^[6].

Underwater it propels itself with its wings, as penguins do, and not with its feet, as cormorants do ^[8]. Its skeleton has features that tell it apart from other diving petrels: the palatine bones of the skull almost meet along the midline, whereas in Pelecanoides urinatrix and Pelecanoides georgicus they stay apart, and its ulna is shorter than that of Pelecanoides urinatrix even though its other wing bones are larger ^[8]. No significant differences between the two species were found in the legs, girdles or vertebrae ^[8].

In the early twentieth century it was documented from Lobos de Tierra Island, at 6° S in Peru, to Chiloé, at 42° S in Chile ^[4]. A 2019 study counted only 7 active breeding sites: in Peru, between 8° S and 14° S, Corcovado Island, with few pairs, and San Gallán and La Vieja islands, which together hold 12 000 to 13 000 pairs; in Chile, between 26° S and 29° S, the islands of Pan de Azúcar, Choros, Grande and Pájaros II ^[4].

At sea off Peru, it was recorded 57 times between 2005 and 2010 from Sechura, in Piura (05°29′ S), to Cañete, in Lima (13°16′ S), in groups of 1 to 10 birds feeding or resting on the water, 6.1 km to 51.02 km from shore; 52.6 % of sightings fell between 10 km and 20 km out, and surface waters where it was seen ranged from 16 °C to 23 °C ^[3]. Adding records from other observers, its presence in the country spans about 1850 km, from Sechura to Boca del Río, in Tacna (18°19′ S) ^[3].

On 21 March 2005 two small colonies, with 3 and 7 active nests, were found on Corcovado Island, 0.038 km² in area and lying 20 km northwest of Chimbote; the birds nested in crevices among fallen rocks, and the find moved the northern limit of breeding some 700 km beyond La Vieja Island ^[7]. On the Lobos de Afuera Islands 2 individuals were seen, one in September 2003 and one in November 2004, the first report of the species there in about 40 years, although no nests were found ^[9].

It nests in burrows, and in Peru it breeds at least twice a year ^[6]. According to a review of its records in Peru, breeding takes place in any month, with more activity from December to March and from May to September; the first peak is the more synchronised, with laying in January and chicks in March ^[3]. On Corcovado an adult was found incubating an egg of 32 g ^[7].

A 2023 study reports from earlier work that incubation lasts 50 to 60 days and chick rearing 70 to 80 days ^[6]. In that same study, made on Choros Island, Chile, adults fitted with geolocators spent on average 212 ± 87 days away from the nest outside the breeding season ^[6].

A genetic study across the whole breeding range found the colonies of different islands to be strongly isolated from one another, a sign that birds return to breed on their own island; the two Peruvian colonies, La Vieja and San Gallán, did not differ from each other ^[4]. Most Peruvian samples descend from a single ancestral haplotype, a pattern typical of a founder effect after colonisation from Chile ^[4].

It is a remarkable diver: in 22 adults the mean maximum depth was 31.6 ± 3.6 m and the deepest dive reached 83.1 m, 81 % more than would be expected from its body size (46 m) ^[10].

On La Vieja Island, 118 stomach samples taken between 1997 and 2000 held fish 67.8 % of the time, while 65.3 % had crustaceans and 7.6 % had squid ^[11]. By mass, the main prey were the anchoveta Engraulis ringens (33.9 %), the krill Euphausia mucronata (26.8 %) and the squat lobster Pleuroncodes monodon (24.3 %), followed by Normanichthys crockeri and fish larvae; the dominant prey shifted strongly from month to month and year to year, pointing to opportunistic feeding ^[11].

The anchovetas it ate averaged 45.9 mm, far below the minimum commercial size of 120 mm and the 149.9 mm mean caught by the fishery in the area, with no size overlap between the two ^[11]. Its diet became less diverse in months with warmer sea temperature anomalies, and in November 2000 some 5000 of these birds were seen feeding alongside about 100 dusky dolphins off Punta Carreta ^[11].

In Chile, 12 birds carrying geolocators ranged 60 km to 144 km north of their colony while breeding and moved some 1200 km south outside the breeding season; overall they spent 37 % of their time flying or on land and 63 % on or under the water ^[6].

Its global category is Near Threatened ^[5]. In Peru it is classified as Vulnerable ^[2], and since 2002 it has been listed in Appendix I of the Convention on the Conservation of Migratory Species of Wild Animals ^[12].

Guano extraction removed the substrate in which it dug its nests, and guano workers and fishers took birds and eggs ^[9],[3]. Its breeding area in Peru shrank from about 1000 km, between Lobos de Tierra and La Vieja, to a 55.5 km stretch between La Vieja and San Gallán that holds 99.9 % of the colonies, plus the small Corcovado colony ^[3]. In the 1995 and 1996 census, La Vieja held 11 748 pairs (91.3 %) and San Gallán 1109 (8.6 %) ^[3].

Its habit of feeding in large flocks near the coast exposes it to bycatch, and it has been seen dying in the artisanal fishery of Independencia Bay ^[3]. El Niño hits it too: in 1965, 72 dead birds turned up on Peruvian beaches, and on La Vieja the proportion of chicks fell from 15 % to less than 5 % during the 1997-1998 El Niño ^[3]. The summer breeding season of 1998 failed completely on the island ^[11].

Because each island's colonies are demographically independent, immigration is unlikely to rescue a declining colony, and each island with its nearby sea should be treated as a conservation unit; reported population increases on the Peruvian islands should be read with caution ^[4]. To give it more places to nest, it has been proposed to declare islands and headlands off-limits, with no guano extraction ^[3].

Between the middle and the end of the nineteenth century it was very abundant along the coasts of Peru and Chile, and was counted among the most important guano birds of the islands ^[3]. Antonio Raimondi even considered it, in 1874, the species that produced the most guano on the Chincha Islands ^[9].

The oldest known record in Peru dates from 1684, when William Dampier was told on the Lobos de Afuera Islands of a small black bird that dug holes in the sand to spend the night and whose meat tasted good ^[3]. Prosper Garnot collected it and René Lesson described it between 1822 and 1825, and Charles Darwin observed or collected it in the country between 1831 and 1836 ^[3].

In a study with data from 1997 to 2000, its main breeding colonies were those of La Vieja and San Gallán islands; La Vieja lies in Independencia Bay, within the Paracas National Reserve ^[11]. The small colony found in 2005 on Corcovado Island sits on a rugged island whose terrain limits access ^[7].

At sea, 41 of the 57 sightings made between 2005 and 2010 were off Lima, mostly between Pachacamac and Cañete, and off Sechura and Huarmey it was seen in different years, suggesting that it visits those areas regularly ^[3]. Other observers have recorded it around Palomino and San Lorenzo islands, in Callao, and off Pucusana ^[3].