Northern Prairie Wildlife Research Center
The interior population breeds in the Mississippi, Missouri, and Rio Grande River systems where a total of about 2,500 pairs exist. In North Dakota, about 100 breeding pairs are found annually along the Missouri and Yellowstone River systems. Here, they utilize sparsely vegetated sandbars to nest, raise young, relax, and feed. Unlike gulls, they dive into the water to capture small fish.
Breeding season lasts from May through August with peak nesting occurring from mid-June to mid-July. Nests are scratched-out bowl shaped depressions in barren sand measuring about 4" across.
A typical clutch contains 2-3 eggs and takes about 24 days to hatch. Both parents incubate the eggs and feed the young which are able to fly in about 21 days. Least terns typically live 1 to 5 years.
Least terns have declined in number due to construction of dams along the river systems. Dams have created lakes that cover natural river corridors which contained bare sandbar habitat required by the birds. These lakes, which created deep and cold water, may have also reduced the number and type of forage fish available for feeding.