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Federal Register / Vol. 86, No. 8 / Wednesday, January 13, 2021 / Rules and Regulations Lowery 1942, pp. 173177. In 2006, the American Ornithologists Union recognized least terns under a previously published genus Sternula based on mitochondrial DNA phylogeny Bridge et al. 2005, p. 461. Interior least tern was one of three subspecies of New World North and South America least terns previously recognized by the American Ornithologists Union 1957, p. 239, including the eastern least tern and the California least tern S.
antillarum browni. However, due to taxonomic uncertainty surrounding least tern subspecies at the time of listing 50 FR 21784; May 28, 1985, we treated the Interior least tern as a population of eastern least tern.
Since that time, genetic analyses of North American populations of least tern found no evidence of differentiation warranting subspecies recognition e.g., Whittier 2001, p. 10;
Draheim et al. 2010, pp. 813815;
Draheim et al. 2012, p. 146. Genetic exchange between eastern least terns and Interior least terns is occurring at a rate greater than three migrants per generation between populations Whittier et al. 2006, p. 179. After reviewing the best available scientific information regarding the taxonomy of the Interior least tern, we continue to conclude that it is a distinct population segment of the eastern least tern Sternula antillarum.
Species Description Least terns are the smallest members of the family Laridae, measuring 21 to 23 centimeters cm 8 to 9 inches in long with a 56-cm 22-in wingspan Thompson et al. 1997, pp. 12. Sexes look alike, characterized in the breeding plumage by a black crown, white forehead, grayish back and dorsal wing surfaces, snowy white undersurfaces, orange legs, and a black tipped yellow bill. Immature birds have darker plumage, a dark bill, and dark eye stripes on their white heads. Least terns are distinguished from all other North American terns by their small size.
Interior least terns can only be separated from eastern and California least terns by the geographic area used for nesting.
Life Span Interior least terns are potentially long-lived, with records of recapture more than 20 years following banding Thompson et al. 1997, p. 15; however, the average life span is probably less.
Nesting Habitat and Behavior Least terns begin breeding and nesting in their second or third year and breed annually throughout their lives Thompson et al. 1997, p. 15. Prior to
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nesting, young birds exhibit some level of prospecting behavior exploratory dispersal across the landscape e.g., Boyd and Thompson 1985, p. 405; Lott 2012, p. 12; Shigeta in litt. 2014, entire.
Interior least terns generally nest on the ground, in open areas, and near appropriate feeding habitat Lott and Wiley 2012, pp. 911. Nests are simple scrapes in the sand, and nesting sites are characterized by coarser and larger substrate materials, more debris, and shorter and less vegetation compared to surrounding areas Smith and Renken 1993, p. 501; Stucker 2012, p. 49.
Typical least tern clutch size is reported as two to three eggs Thompson et al.
1997, p. 15; however, clutch size may vary by location and year e.g., Szell and Woodrey 2003, p. 37; Jones 2012, p. 3.
Natural nesting habitat features are maintained and influenced by magnitude and timing of riverine flood events Sidle et al. 1992, p. 134; Renken and Smith 1995, pp. 194195; Pavelka in litt. 2012, p. 2. The Interior least tern prefers vegetation-free sand or gravel islands for nesting, although sand banks, point bars, salt flats or plains, and beaches may also be used. Interior least terns prefer areas remote from trees or other vegetation that may hide or support predators Lott and Wiley 2012, pp. 911. Least terns also nest on anthropogenic sites originating from human activity Jackson and Jackson 1985, p. 57; Lott 2006, p. 10 near water bodies that contain appropriate and abundant prey fishes. Anthropogenic sites used by the tern include industrial sites Ciuzio et al. 2005, p. 102; Mills 2012, p. 2, dredge spoil Ciuzio et al.
2005, p. 102, sand pits Smith 2008, p.
2, constructed habitats Stucker 2012, pp. 5966, and rooftops Boland 2008, entire; Watterson 2009, entire.
Lott and Wiley 2012, pp. 911
described five physical and biological conditions that are necessary for Interior least tern nest initiation and successful reproduction:
1 Nest sites that are not inundated flooded during egg laying and incubation;
2 Nesting sites that are not inundated until chicks can fly;
3 Nesting sites with less than 30
percent ground vegetation;
4 Nesting sites that are more than 76
meters m 250 feet ft from large trees; and 5 Availability of prey fishes to support chick growth until fledging.
Interior least terns are colonial nesters. Colony size may vary from a few breeding birds to more than 1,200
Jones 2012, p. 3. Populations in some river drainages may be limited by annual availability of nesting habitat
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e.g., Missouri River; Stucker 2012, p.
104, while potential nesting habitat is generally abundant and underused in other drainages e.g., Mississippi River;
U.S. Army Corps of Engineers USACE
2008, pp. 1013. Nesting site conditions e.g., habitat suitability, flood cycles, prey fish abundance, predation pressure can vary significantly from year to year in all drainages, resulting in wide fluctuations in bird numbers Jones 2012, p. 14 and/or nesting success Smith and Renken 1993, p. 41;
Lott and Wiley 2012, p. 15. However, Interior least terns may re-nest, or relocate and re-nest, if nests or chicks are destroyed early in the season Massey and Fancher 1989, pp. 353
354; Thompson et al. 1997, p. 15.
Interior least tern chicks leave their nests within a few days of hatching semiprecocial, but remain near the nests and are fed by their parents until fledging Thompson et al. 1997, pp. 14
15.
Food and Foraging Habitat Interior least terns are primarily piscivores fish-eaters, and feed opportunistically on small fish species or the young of larger fish species. Prey species include native species such as shad Dorosoma spp., carps and minnows Cyprinidae, freshwater drum Aplodinotus grunniens, largemouth bass Micropterus salmoides, white bass Morone chrysops, sunfishes Lepomis spp., and top minnows Fundulus spp., as well as invasive species such as silver and bighead carp Hypophthalmichthys spp. USACE
2008, pp. 16, 26. On the Missouri River, prey species include emerald shiner Notropis atherinoides, sand shiner Notropis stramineus, spotfin shiner Cyprinella spiloptera, and bigmouth buffalo Ictiobus cyprinellus Stucker 2012, p. 6. Least terns will also occasionally feed on aquatic or marine invertebrates Thompson et al. 1997, pp.
67. Riverine foraging habitats and fish abundance may be influenced by stochastic random hydrological conditions and events i.e., flow, and flood timing and magnitude, and channel engineering Schramm 2004, pp. 307, 321323.
In the Missouri River drainage, Interior least terns forage for fish in shallow water habitats and within 12
kilometers km 7 miles mi from colony sites Stucker 2012, p. 24. In the Lower Mississippi River, foraging terns have been observed feeding in a variety of habitats within 3 km 2 mi of colony sites Jones 2012, pp. 56.

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