Royal tern

Thalasseus maximus (Boddaert, 1783) is a tern of the family Laridae ^[1]. In Peru it is called «gaviotín real» ^[2]. It ranks among the largest terns in the world and breeds in very tightly packed colonies; birds from the mid-Atlantic colonies of the United States winter along the coast as far as Peru ^[3]. It does not breed in Peru: the master plan of the Illescas National Reserve counts it among the migratory birds that visit its beaches seasonally ^[2], and a bird inventory of the Pisco coast classifies it as a Nearctic migrant ^[4]. Its global category on the IUCN Red List is Least Concern ^[5].

Birds from the mid-Atlantic colonies of the United States first move north when breeding ends and then south, to coastal wintering areas that reach Peru; in winter, birds banded at the Virginia colony have been resighted or recovered from New England to Peru ^[3]. In Argentina it breeds in mixed colonies with another tern, Thalasseus sandvicensis eurygnathus ^[6], for example in the Punta León Protected Area, in Chubut, at 43° S ^[7].

In Peru it is a non-breeding visitor. In the inventory of the beaches of San Andrés, in Pisco, it was recorded as a Nearctic migrant and as a bird of stony and rocky beaches; the author explains that species from other latitudes do not breed on those beaches or in the Paracas National Reserve, but thousands of kilometres away ^[4]. Further north, the master plan of the Illescas National Reserve says its beaches are a seasonal stopover for at least 23 migratory species, among them the gaviotín real ^[2].

It breeds in dense colonies, usually lays a single egg, and its chicks gather in crèches soon after hatching; it starts breeding late in life ^[3]. At the Virginia colony, most chicks had already fledged by late June or early July, which marks the end of breeding and the start of migration ^[3].

In a barrier-island refuge in Louisiana 283 adults were banded between 2012 and 2016, and 36% reappeared at the colony in a later season; apparent annual adult survival was 0.68 (95% confidence interval: 0.54 to 0.80), a figure the authors put down more to emigration than to mortality, because the refuge's population was growing ^[8]. One bird captured there in 2012 had been banded as a chick in North Carolina on 24 June 1986 ^[8].

With GPS transmitters on 9 adults from Virginia, the birds moved north as far as Long Island after breeding and began to head south in October and November; one of them spent the non-breeding season near Honduras ^[3]. In Patagonia, during late incubation (21 November to 3 December 2006), the birds hunted mostly in waters less than 15 km from the coast, and their departures from the nest clustered around midday ^[7].

At Punta León, in 2004 and 2005, the food that adults carried to their mates and chicks included 10 prey species, and more than 99% were fish; the Argentine anchovy (Engraulis anchoita) was the main prey ^[6]. In a barrier-island refuge in Louisiana the most frequent prey were Micropogonias undulatus, of the family Sciaenidae, and the bay anchovy Anchoa mitchilli; adults ate more sciaenids than the other tern groups studied, and the presence of bottom-dwelling fish points to use of discards from local fishing ^[9].

In Virginia the fish Brevoortia tyrannus is proposed as its main food, and the northward movement after breeding might follow that fish, although the authors present this as an idea still to be tested ^[3]. The remains of one of the tagged birds, with its transmitter, were found beneath the perch of a predatory bird, and the cause could not be determined ^[3].

Its global category on the IUCN Red List is Least Concern ^[5]. Even so, in the mid-Atlantic of the United States its numbers fell in the 1990s and 2000s, and survival of birds aged 0 to 2 years has dropped by 55% since the 1960s, although all age classes appear to be rebounding ^[3].

An analysis of 60 years of banding data concluded that rising sea temperature raised the risk of dying by an amount comparable to what entanglement in fishing gear and the depletion of forage fish by commercial fisheries once contributed, which have declined; the youngest birds are the most affected, and in the mid-Atlantic, the main future threat would be too few juveniles reaching breeding ^[10]. A sudden shift in the availability of the fish they depend on could be catastrophic, as it was for royal terns in California that fed on the Pacific anchovy Engraulis mordax ^[3].

In a barrier-island refuge in Louisiana, where land is being lost at a high rate, the species breeds on islands where restoration projects have been carried out to sustain the nesting birds ^[8].

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In October and November 2023, during an outbreak of highly pathogenic avian influenza H5N1 (clade 2.3.4.4b) in elephant seals at Punta Delgada, on the Península Valdés (Argentina), the number of dead terns reached almost 400, mostly South American terns (Sterna hirundinacea); most died between 3 and 13 November ^[11]. Among the dead animals sampled were 2 royal terns, and their pooled samples tested positive for influenza A virus (matrix gene); the virus genome of one of them was sequenced ^[11].

The tern virus was closely related to the marine mammal virus of the outbreak, and the authors read it as independent spillovers from mammal to bird, with no evidence of transmission between terns; the viruses from Argentina cluster with those isolated from marine mammals in Peru, Chile, Brazil and Uruguay ^[11].

In counts on the beaches of San Andrés, in Pisco, in the zone of influence of the northern sector of the Paracas National Reserve, the sum of the daily counts of royal terns was 108 in the January 2023 survey and 1031 in the December 2024 one, an average of 257 birds per day in the second ^[4]. The author describes that coast, about 4 km long, as a shoreline under strong human influence ^[4].

Because the royal tern does not breed in Peru, it is seen during the season of migratory birds: the 38 Nearctic species of that inventory reach Peruvian beaches between September and March ^[4]. It is also named among the migratory birds of the beaches of the Illescas National Reserve ^[2].