Peruvian booby

Sula variegata is a seabird of the family Sulidae described by Tschudi in 1843 ^[1]. In Peru it is called «piquero peruano» ^[2]. It is endemic to the Humboldt Current upwelling, nests on islands and headlands from northern Peru to south-central Chile, and is one of the three guano birds whose guano has been produced in commercial quantities ^[3]. The IUCN Red List classifies it as Least Concern in its 2018 assessment ^[4], and Peru's official list of threatened wildlife, approved in 2014, places it in the Endangered category ^[5].

It is a fairly small sulid of 1200 to 1500 g, and females are 19 % heavier than males ^[6]. On Isla Pescadores, 51 adults caught while raising chicks averaged 1520 g ^[7]. The secondary wing feathers carry a variegated pattern and the legs are grey ^[8].

Voice helps to sex birds: when handled, males give a higher-pitched call than females ^[7], and the nasal honk belongs to the female ^[9].

Similar species

• Sula nebouxii: It shares Lobos de Tierra and Lobos de Afuera with the Peruvian booby, and birds looking intermediate between the two species have been seen there; its secondaries are brown, lacking the variegated pattern of the Peruvian booby ^[8]. Its eyes are yellow, whereas the Peruvian booby has red eyes and grey legs ^[9].

It breeds along the Peruvian coast from Lobos de Tierra Island to Morro Sama and continues south to the central coast of Chile ^[8]. About 95 % of the population lies between Lobos de Tierra and Isla Pájaros in Chile, according to Nelson's 1978 estimate as cited by a genetic study; during El Niño events adults may disperse north as far as the Gulf of Panama ^[3].

In Peru it nests on coastal islands and headlands ^[2]. On Lobos de Tierra and Lobos de Afuera, during the 2006 and 2007 studies, it gathered in dense groups of 100 to 3000 nests, at about 2 nests per square metre ^[6]. In central Chile the Quirilluca cliffs, in Valparaíso, are its only mainland breeding site; there, between 2013 and 2015, counts reached 4638 adults, mostly between December and February ^[10].

For its family it lays large clutches of up to 5 eggs and can raise up to 4 chicks, whereas other booby species rear 1 single chick and some, in favourable conditions, 2 ^[7]. In the nests studied in December 2006 on Lobos de Tierra and in December 2007 on Lobos de Afuera, the commonest number of chicks was 2 and 3 respectively ^[6]. In central Chile breeding is not synchronised and runs from September to April ^[10].

A genetic analysis of 153 birds from five colonies, four Peruvian and one Chilean, found no significant differences between them, a case so far unique among sulids, although nuclear markers suggest that colonies farther apart differ somewhat more; it also found high genetic diversity ^[3]. The authors explain this by frequent movement of birds between colonies, driven by El Niño and by tick infestations, and by the species keeping at least 1 million individuals even at its lowest points ^[3].

As in the rest of its family, the skull is built for the impact of the plunge: the bony nasal opening that chicks have closes completely as they grow and breathing moves to a space below the preorbital fenestra, covered by the horny sheath of the bill; and the inside of the bill, hollow in the chick, fills up with sponge-like trabecular bone ^[2].

Boobies tracked with GPS units and depth recorders on Lobos de Tierra and Lobos de Afuera fed on nothing but anchoveta (Engraulis ringens), caught in shallow plunges: the median was 2.5 m and the deepest reached 8.8 m ^[6]. They went out to fish in daylight only, between 1 and 3 times a day; each trip lasted a median of 1.8 hours, nearly all of it on the wing (92 % of the time), and took them a median of 25 km from the colony, at most 68 km ^[6].

On Isla Pescadores, between November and December 2008, boobies and guanay cormorants fished in the same waters, some 19 to 20 km from the colony on average, and at similar hours, though the boobies tended to set out earlier and come back later ^[7]. The difference lay in depth: the cormorant, which dives from the surface, reached 32 m, whereas the booby, which plunges from the air, stayed at a median of 2 m and only now and then got to 10 m, so only the shallowest schools are within its reach; it also leaves the colony on its own, without forming the long columns of the cormorant ^[7].

In northern Peru, at the opening of the industrial fishing season, the anchoveta carried by boobies were largely juveniles under 12 cm, a size poorly represented in what the fleet landed; prey size was the same whether or not vessels were close by, which indicates that the booby mostly fishes on its own ^[11].

Between August 2016 and November 2018, interval cameras set up on several islands and one guano headland showed that the tick Ornithodoros amblus, the only one recorded on guano birds, can empty a colony: in five of the six colonies followed the adults left every nest within two to three weeks, and the more they preened or stood up on the nest, the greater the desertion; the guano of recently deserted colonies held more than twice as many ticks as the ground outside them ^[12].

Its global category is Least Concern ^[4]. In Peru it is classified as Endangered, and the norm that lists it prohibits hunting, capture, possession, trade, transport and export for commercial purposes of the listed species ^[5].

Unlike the guanay cormorant, its numbers have held steady over the long term, though they have dipped for a while after El Niño events ^[13]. Over 50 years the cormorant went from about 21 million to 2 million birds, while the Peruvian booby stayed near 2 million ^[7]. Over the last 150 years its population has swung widely because of habitat degradation, introduced cats and rats, human disturbance, competition with fisheries, El Niño and ectoparasites ^[3].

H5N1 avian influenza caused heavy mortality from late 2022: on 29 November that year SERFOR counted 2919 dead Peruvian boobies among more than 13869 seabirds ^[14]. By 6 February 2023 SERNANP had recorded about 55 thousand dead birds in eight coastal protected areas, mostly boobies, pelicans and guanay cormorants, and the booby samples analysed tested positive for the virus ^[15].

In Chile, where it is regarded as threatened, eggs and chicks at Quirilluca suffer from predators, falls from the cliff, visitors, eggs taken to be eaten and paragliders flying over the colony ^[10].

Not medical advice. For a sting, a poisoning or an allergy, seek medical care.

During the H5N1 avian influenza epizootic, SERNANP asked people not to handle sick or dead animals inside protected areas and to alert the park rangers ^[15], and SERFOR advised not to approach seabirds or take them home, so as not to carry the virus to poultry ^[14]. A study of the outbreak in Peru warns that anyone in contact with animals sick or dead from the virus is at risk of infection ^[16].

It is one of the three guano birds whose guano has been exploited in commercial quantities ^[3]. In the nineteenth century the guano of these birds was a main source of fertilizer, and it is still harvested industrially today, though in far smaller amounts ^[7].

Because its diet holds juvenile anchoveta that the fishery barely catches, it has been proposed as a sentinel for the arrival of new anchoveta generations, information that complements landing records ^[11].

In the north, in December 2006 some 2000 to 2500 pairs bred on Lobos de Tierra Island, and in December 2007 some 5500 to 6000 pairs on Isla Independencia, in the Lobos de Afuera group, 61 km from the mainland ^[6]. On the central coast, in late 2008 about 41 thousand boobies were nesting on Isla Pescadores, 7.5 km offshore ^[7]. Lobos de Afuera is not open to all: in 2004 and 2005, researchers needed an entry permit from Proabonos, the guano extraction company ^[8].