Common tern

Sterna hirundo is a tern of the family Laridae described by Linnaeus in 1758 ^[1]. Adults from lake colonies in North America, followed with geolocators between 2013 and 2015, spent the boreal winter mostly on the coast of Peru: 70% of the birds whose wintering site could be identified ^[2]. The IUCN classifies it as Least Concern globally ^[3].

It is a small tern. In birds caught at five lake colonies in North America, the mean weight before the geolocator was fitted was 128 g in females and 125 g in males ^[2].

The Arctic tern, very closely related to this species, mostly breeds farther north, although on the northwest Atlantic coast of the United States the two ranges overlap between 41° N and 52° N ^[4]. In a mixed pair studied on an island in Massachusetts, the first-generation hybrid young had secondaries darker than the wing coverts, as in the common tern, and dark feathers below the eye that gave them an Arctic tern look ^[4].

Similar species

• Sterna paradisaea: Its breeding range lies largely south of the Arctic tern's, and in hybrid young the dark secondaries are the common tern trait, while dark feathers below the eye recall the Arctic tern ^[4].

The geolocators were fitted between 2013 and 2015 at 5 colonies on 4 lakes: Winnipeg in Manitoba, Superior in Minnesota and Wisconsin, Huron in Ontario and Oneida in New York ^[2].

Of the birds with an identified wintering site, 70% wintered in Peru; the rest did so in the Gulf of Mexico, Central America, Venezuela and Ecuador ^[2]. The nonbreeding stay lasted on average 151 days, between 130 and 183 ^[2].

The authors link the Peruvian coast to the Humboldt Current system, and recall that rings and resightings of marked birds had already pointed to Peru as a wintering area for Great Lakes terns ^[2].

Among the 46 usable geolocators, autumn migration lasted on average 88 days, between 19 and 170, and spring migration 32 days, between 19 and 86 ^[2]. The 19 birds with a full year of record travelled on average 15141 km, between 9511 and 19639 km ^[2]. In autumn the females left the colonies on average around 9 August and the males around 24 August ^[2].

In another geolocator study, seven females set off between 1 and 22 August with an average date of 9 August, while four males set off from 12 August to 5 October, on average 6 September ^[5]. The authors see this as consistent with males giving more care to fledglings, which would cost them more ^[5].

On an island in Massachusetts, between 2007 and 2015, a male Arctic tern and a female common tern kept a pair bond for 8 years and left at least 9 young (at least 5 males and 4 females) ^[4]. A hybrid male returned to breed and had 3 young with a common tern ^[4].

In the boreal winter it follows the distribution and abundance of the fish it eats. The Peruvian upwelling ecosystem, part of the Humboldt Current system, is dominated by the anchoveta, Engraulis ringens, the most abundant pelagic fish in the system ^[2].

During migration the birds spent on average about 21 days per stop in autumn and about 12 in spring, with stops in the Gulf of Mexico and on the Atlantic coast of the United States, and some farther south, as far as Florida, Cuba and the Caribbean ^[2].

On Lake Superior, adults got 42% of their food from the river during incubation, and chicks 68% during chick-rearing; outside breeding the birds feed mainly in coastal marine environments ^[6].

The IUCN classifies it as Least Concern globally ^[3]. Even so, the populations that nest on inland lakes of North America have declined: Great Lakes pairs fell about 18% between 1976 and 2009, and in southern Manitoba nests dropped between 57% and 67% from the early 1990s to 2012; 6 US states list it as threatened or endangered, while Atlantic coast colonies are considered stable ^[2].

Because a large share of the inland birds winter in a small region, what happens there could affect several colonies at once ^[2]. The El Niño events of 1965 and 1972 and heavy fishing collapsed the anchoveta and the seabirds in the late 1960s and early 1970s, and in 1982 and 1983 common terns wintering in southern Peru were well below normal ^[2].

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

Total mercury was measured in common terns on Lake Superior: winter-grown feathers had the lowest geometric mean, 1.32 µg/g dry weight (n = 60), and chick feathers from the more industrially influenced colony 4.95 µg/g (n = 20) ^[6]. During breeding males had more mercury in the blood than females, and exposure rose with a river-based diet ^[6].

The literature on Peru is that of the geolocator tracking from 2013 to 2015: 70% of the birds with an identified wintering site spent the boreal winter on the Peruvian coast ^[2]. Positions between 15 December and 31 March were used to locate those stays; females reached the wintering areas around 2 November and males around 20 November ^[2]. The source does not say which stretch of coast or whether access is restricted ^[2].