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Federal Register / Vol. 86, No. 8 / Wednesday, January 13, 2021 / Rules and Regulations
Migration and Winter Habitat Interior least tern fall migrations generally follow major river basins to their confluence with the Mississippi River and then south to the Gulf of Mexico; however, late summer observations of least terns more than 150 km 93 mi from major river drainages indicate that some birds migrate over land Thompson et al.
1997, p. 16. Interior least terns gather in flocks in August prior to migration.
Once they reach the Gulf Coast, they cannot be distinguished from other least tern populations en route to, or within, their winter habitats i.e., Gulf of Mexico, Caribbean islands, Central and South America; therefore, the limited information on migration and winter habitat is inclusive of other populations i.e., Caribbean, Gulf Coast, East Coast.
Least terns appear to migrate in small, loose groups along or near shore, feeding in shallows and resting onshore Thompson et al. 1997, pp. 46. Very little is known of least tern winter habitats, other than that the birds are primarily observed along marine coasts, in bays and estuaries, and at the mouths of rivers Thompson et al. 1997, p. 6.
Breeding/Natal Site Fidelity and Dispersal Breeding-site fidelity for least terns varies in different populations and breeding areas. Return rates of banded adults to the sites where they were banded was 36 to 86 percent in California colonies; 42 percent on the Mississippi River; 28 percent on the central Platte River, Nebraska; and 81
percent at Quivira National Wildlife Refuge in Kansas and on the Cimarron River in Oklahoma Thompson et al.
1997, p. 16. Fidelity to natal site is also variable and difficult to estimate because re-sightings or recaptures of terns banded as chicks have been limited. Estimates of natal site fidelity have varied from 5 percent on the Mississippi River, to 82 percent in Kansas and Oklahoma Thompson et al.
1997, p. 16.
Site fidelity in least terns may be affected by physical habitat variables or the extent and type of predation Atwood and Massey 1988, p. 394. As noted above, least terns are strong fliers and can relocate if conditions on natal or previous-year nesting grounds become unfavorable. A study of eastern least terns found an average 22 percent turnover rate in nesting colony sites, primarily due to changes in habitat condition or disturbance Burger 1984, p. 66.
Lott et al. 2013, pp. 36173618
found that 50 to 90 percent of reported
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recaptures occurred less than 26 km 16
mi from the original banding sites, while more than 90 percent dispersed less than 96 km 59 mi, indicating a high degree of adult site fidelity and natal site philopatry remaining near their point of origin. However, long distance dispersal up to 1,000 km; 621
mi has been documented e.g., Renken and Smith 1995, pp. 196198; Boyd and Sexson 2004, p. 88; Lott et al. 2013, pp.
36173618, and may not be uncommon Boyd and Thompson 1985, p. 405.
Least tern nesting has also been documented in Brazil Rodrigues et al.
2010, entire and Hawaii Conant et al.
1991, entire; Pyle et al. 2001, entire.
During 2014, an Interior least tern banded in the Missouri River drainage was captured in Japan, along with another unbanded tern Shigeta in litt.
2014.
Predation Interior least tern eggs, chicks, and adults are prey for a variety of mammal and bird predators. Reported predators include birds e.g., crows, herons, owls, and hawks, mammals e.g., fox, coyote, racoon, and skunk, and catfish, as well as domesticated and feral dogs and cats Thompson et al. 1997, pp. 1011. The cryptic coloration of eggs and chicks, the secretive behavior of chicks, and the mobbing behavior attack flights on potential predators of adults, all serve to protect eggs and chicks from predators Thompson et al. 1997, p. 11.
Location and size of nesting colonies also has a significant influence on degree of predation. Interior least tern reproductive success is higher on island colonies as compared to connected sandbar colonies, and when water levels maintain isolation of islands and nesting bars from mammalian predators Smith and Renken 1993, p. 42; Szell and Woodrey 2003, p. 41. Additionally, significantly higher rates of predation were documented in larger colonies compared to smaller colonies Burger 1984, p. 65.
Historical Distribution and Abundance The Service defined the historical breeding range of the Interior least tern to include the Colorado in Texas, Red, Rio Grande, Arkansas, Missouri, Ohio, and Mississippi Rivers systems from Montana south to Texas, and from New Mexico east to Indiana 50 FR 21784;
May 28, 1985. However, in order to avoid confusion with eastern least tern, the Service excluded the Mississippi River south of Baton Rouge, Louisiana, the Texas Coast, and a 50-mile zone inland from the coast of Texas from the protected range of Interior least tern 50
FR 21784, May 28, 1985, p. 21789.

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The historical distribution and abundance of the Interior least tern within this range is poorly documented.
Hardy 1957, entire provided the first information on least tern distribution on large interior rivers, documenting records of occurrence and nesting in the Mississippi, Ohio, Missouri, Arkansas, and Red River drainages. Downing 1980, entire published results from a rapid aerial/ground survey of a subset of these rivers, identifying additional nesting populations within the range noted above, and estimated the Interior least tern population at approximately 1,250 adult birds. Ducey 1981, pp. 10
50 doubled the number of known nesting sites, including areas between the scattered observations reported in Hardy 1957, entire. Ducey also extended the northern distribution of the Interior least tern to include the Missouri River below Garrison Dam in North Dakota and Fort Peck Dam in Montana. These three publications Hardy 1957, entire; Downing 1980, entire; Ducey 1981, entire provide the primary historical sources of information about the Interior least terns geographic range, and were used to reach the estimate of 1,400 to 1,800
adults rangewide in the listing rule 50
FR 21784; May 28, 1985.
Current Distribution and Abundance The current east to west distribution of summer nesting Interior least terns encompasses more than 18 degrees of longitude, or 1,440 km 900 mi, from the Ohio River, Indiana and Kentucky, west to the Upper Missouri River, Montana. The north to south distribution encompasses over 21
degrees of latitude more than 2,300 km 1,450 mi from Montana to southern Texas. Interior least terns currently nest along more than 4,600 km 2,858 mi of river channels across the Great Plains and the Lower Mississippi Valley Lott et al. 2013, p. 3623, with nesting colonies found in 18 States, including:
Montana, North Dakota, South Dakota, Nebraska, Colorado, Iowa, Kansas, Missouri, Illinois, Indiana, Kentucky, New Mexico, Oklahoma, Arkansas, Tennessee, Texas, Louisiana, and Mississippi. As noted above, this does not include least tern colonies nesting along the coasts of Texas, Louisiana, and Mississippi.
Rangewide surveys in 2005 estimated an approximate minimum adult population size of 17,500, with nesting occurring in more than 480 colonies spread across 18 States, which is likely an underestimate given imperfect detection of adults and survey coverage of potential habitat Lott 2006, pp. 10
21, 50. Lott 2006, pp. 1315 also
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