health of the endangered least tern population is tied to the environmental health of the big river basins they inhabit. As we witnessed during the floods of 1989, 1992, 1993 and 1995 in the Mississippi River valley, the ability of terns to successfully hatch eggs and raise chicks can be reduced to zero when we experience dramatic rises of the river in late June and July.
However, flooding in May can be beneficial to least terns for a couple of reasons. Spring flooding can deposit new sand on the islands and reduce chances that islands will be overtaken by young trees and other vegetation. Spring flooding also allows fish to spawn in the shallow, warmer water covering the floodplain. This results in lots of young, small fish, such as shad, minnows, carp and crappie, for least terns to eat.
Adult terns eat fish that are less than 4 inches long. Tern chicks are fed even smaller fish, and hungry chicks need to be fed about one fish every half hour. When rivers are leveed, channelized and restricted with dikes and dams, native river fish have fewer opportunities to spawn on the floodplain, which translates into fewer small fish for terns to eat.
Terns historically nested on sand islands on both the Missouri and Mississippi rivers. In his 1932 checklist of birds in Missouri, Rudolf Bennitt noted that least terns nested on the Missouri River in Lafayette County. Otto Widmann, in his 1907 book, stated that least terns were common summer residents of both rivers. Modifications that have eliminated sand islands on both the Missouri and Mississippi rivers have caused the terns' nesting range to dwindle. They now nest only in the southeastern tip of the state. Since 1985, researchers have found tern colonies only in or adjacent to Scott, Mississippi, New Madrid and Pemiscot counties.
Terns are expert colonizers when they can find suitable new habitat. Several terns that Missouri researchers banded between 1987 and 1991 have nested at a new colony next to a cooling pond in Indiana-190 miles from their original home! A few of our banded birds also showed up on the Upper Missouri River in South Dakota. Similarly, Missouri sites attract birds from other nesting areas that are undergoing flooding or other habitat disruption.
Today the number of least terns in Missouri ranges between 500 and 900 nesting pairs. We are concerned, however, that these sand island nesters are not reproducing