Least tern

Subspecies

The differences among the three subspecies may not be as much as had been thought.

• S. a. athalassos – (Burleigh & Lowery, 1942): Breeds on the rivers of the Arkansas River, Mississippi River, Brazos River, Trinity River, and Rio Grande basins; winters south to northern Brazil.
• S. a. antillarum – (Lesson, 1847): nominate, Breeds on the Atlantic coast of North America, from Maine south along the east and south coasts of the United States, Bermuda, the Caribbean, and Venezuela; winters south to northern Brazil.
• S. a. browni – (Mearns, 1916): California least tern. Breeds on the Pacific coast of North America, from central California south to western Mexico; winters mainly in Central America. Additionally, least terns of an unknown subspecies were found in 2012 nesting on the Big Island of Hawaii.

Conservation and status

''S. a. antillarum''

The population is about 21,500 pairs; it is not currently considered federally threatened, though it is considered threatened in many of the states in which it breeds. Threats include egg and fledgling predators, high tides and recreational use of nesting beaches.

''S. a. athalassos''

The interior subspecies, with a current population of about 7000 pairs, was listed as an endangered subspecies in 1985 (estimated 1000 breeding pairs), due to loss of habitat caused by dams, reservoirs, channelization, and other changes to river systems. It was delisted on 13 January 2021.

''S. a. browni''

The western population, the California least tern, was listed as an endangered species in 1972 with a population of about 600 pairs. With aggressive management, mainly by the exclusion of humans via fencing, the Californian population has rebounded in recent years to about 4500 pairs, a marked increase from 582 pairs in 1974 when census work began, though it is still listed as an endangered subspecies. The California subspecies breeds on beaches and bays of the Pacific Ocean within a very limited range of southern California, in San Francisco Bay and in northwestern Mexico. While numbers have gradually increased with its protected status, it is still vulnerable to predators, natural disasters or further disturbance by humans. Recent threats include the gull-billed tern (Sterna nilotica), which can decrease reproductive success in a colony to less than 10%.

Nesting and breeding behavior

The least tern arrives at its breeding grounds in late April. The breeding colonies are not dense and may appear along either marine or estuarine shores, or on sandbar islands in large rivers, or in areas free from humans or predators. Courtship typically takes place removed from the nesting colony site, usually on an exposed tidal flat or beach. Only after courtship has confirmed mate selection does nesting begin by mid-May and is usually complete by mid-June. Courtship takes the form of either an aerial display in which the female follows the flight of the male or through courtship feeding. Nests are situated on barren to sparsely vegetated places near water, normally on sandy or gravelly substrates. In the southeastern United States, many breeding sites are on white gravel rooftops. In the San Francisco Bay region, breeding typically takes place on abandoned salt flats. Where the surface is hard, this species may use an artificial indentation (such as a deep dried footprint) to form the nest basin.

The nest density may be as low as several per acre, but in San Diego County, densities of 200 nests per acre have been observed. Most commonly the clutch size is two or three, but it is not rare to consist of either one or four eggs. Adults are known to wet themselves and shake off the water over the eggs when arriving at the nest. Both females and males incubate the eggs for a period of about three weeks, and both parents tend the semiprecocial young. Young birds can fly at age four weeks. After the formation of the new families, groupings of birds may appear at lacustrine settings in proximity to the coast. Late-season nesting may be renests or the result of late arrivals. In any case, the bulk of the population has left the breeding grounds by the end of August.

File:lagoonb.jpg|Batiquitos Lagoon, a breeding site in San Diego County, California File:Nesting least terns Missouri River.jpg|Nesting pair on the Missouri River in South Dakota File:Core Banks - Black Skimmer chased by Least Tern - 02.JPG|A least tern attacking a much larger black skimmer near Drum Inlet in North Carolina, United States File:Least Tern (Sternula antillarum) RWD3.jpg|Mating pair at Sunset Beach, North Carolina File:Least Tern Mating Sequence.jpg|The mating sequence of a pair of least terns processed into one image (St. Augustine, Florida) File:Least Tern on Nest with Two Eggs Visible.jpg|This least tern is on the nest. There are two eggs which can be seen if one looks closely. File:Two Least Tern Eggs in Typical Shallow Nest.jpg|The eggs are speckled and generally blend well with the sandy surface. File:Least Tern on Nest.jpg|A least tern lying just below the surface of the sand is probably on its nest. The nests are very shallow and minimally scooped out. File:First Fish Feeding1.jpg|First fish feeding at Quintana, Texas File:Least Tern Chicks Day 2.jpg|Two-day-old chicks File:Least Tern Day Seven.jpg|Seven-day-old chicks File:LkJacksonFLTernMay2004.JPG|Least tern (S. a. antillarum) at Lake Jackson, Florida

Feeding and roosting characteristics

The least tern hunts primarily in shallow estuaries and lagoons, where smaller fishes are abundant. It hovers until spotting prey, and then plunges into the water without full submersion to extract meal. The most common prey recently for both chicks and adults are silversides smelt (Atherinops spp.) and anchovy (Anchoa spp.) in southern California, as well as shiner perch, and small crustaceans elsewhere. Adults in southern California eat kelpfish (most likely giant kelpfish, Heterostichus rostratus). Insects are known to be eaten during El Niño events. In southern California, least terns feed in bays and lagoons, near shore, and more than 24 km from shore in the open ocean. Elsewhere, they feed in proximity to lagoons or bay mouths.

Adults do not require cover, so that they commonly roost and nest on the open ground. After young chicks are three days old, they are brooded less frequently by parents and require wind blocks and shade, and protection from predators. In some colonies in southern California, Spanish roof tiles are placed in colonies so chicks can hide there. Notable disruption of colonies can occur from predation by burrowing owls, gull-billed terns and American kestrels. Depredation by domestic cats has been observed in at least one colony. Predation on inland breeding terns by coyotes, bobcats, feral dogs and cats, great blue herons, Mississippi kites, and owls has also been documented.

References

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• Gary Deghi, C. Michael Hogan, et al., Biological Assessment for the Proposed Tijuana/San Diego Joint International Wastewater Treatment Plant, Publication of the U.S. Environmental Protection Agency Region IX, Earth Metrics Incorporated, Burlingame, CA with Harvey and Stanley, Alviso, CA

References

1. BirdLife International. (2018). "''Sternula antillarum''".
2. "Endangered and Threatened Wildlife and Plants; Removal of the Interior Least Tern From the Federal List of Endangered and Threatened Wildlife".
3. (13 November 2014). "Least Tern".
4. Tomkins, Ivan R.. (1959). "Life history notes on the least tern". Wilson Bulletin.
5. (1997). "Least Tern (''Sterna antillarum'')". Cornell Lab of Ornithology.
6. (2012). "Temporal Analysis of mtDNA Variation Reveals Decreased Genetic Diversity in Least Terns". The Condor.
7. (2010). "Subspecific Status and Population Genetic Structure of Least Terns (Sternula antillarum) Inferred by Mitochondrial DNA Control-Region Sequences and Microsatellite DNA". The Auk.
8. Marschalek, D.. (2009). "California Least Tern Breeding Survey 2009 Season". California Department of Fish and Game.
9. (2006). "Roof-top Selection by Least Terns in Pinellas County, Florida". The Waterbird Society.
10. (1980). "California least tern nesting season at [[Naval Air Station Alameda".
11. (November 1988). "California Wildlife". [[California Department of Fish and Game]].
12. Jones, Ken. (1997–2006). "Least tern data archives".
13. (2014). "Least Terns Nesting on the Big Island of Hawaii". Proceedings of the 38th Annual Meeting of the Waterbird Society.