Record Floods Shore up Interior Least Tern Habitat

by Jane Ledwin

Water, water, everywhere…That certainly was true this year in both the Missouri and Mississippi River basins, where record flooding washed away trees, soil, crops, roads, bridges and other infrastructure. Many communities are still dealing with the aftermath; and so too are the fish and wildlife that live in and along these rivers.

The interior least tern (Sterna antillarum) is a federally endangered shorebird whose breeding population is found primarily along major inland rivers. These small birds nest on barren sandbars along rivers, oxbows, and man-made impoundments adjacent to rivers. This year, in many places, flooding forced terns to head towards higher ground. During high water years, these birds are often found nesting on floodplain roads and farm fields—areas where they are extremely vulnerable to human disturbance and increased predation from dogs, cats, coyotes, raccoons, and other small predators. Unfortunately, few nests in these areas are able to successfully fledge young birds.

And yet, least terns live in an ecosystem that, to a large extent, requires seasonal flooding and the occasional “great flood” to ensure continued foraging and nesting habitat. Studies on the Mississippi River show a fascinating relationship between these birds, fish, and river levels. Most years, terns arrive in the spring when the river is often at its highest. The birds may wait days, or even weeks, for the water level to come down enough to expose clean, bare sandbars. These sandbars provide the perfect nesting habitat for the species, with good visibility to see approaching predators, few suitable perches for avian hunters, and protection by water that is deep enough to dissuade all but the most determined swimmers.

At the same time, the high water is the cue for many species of river fish to move into the floodplain to spawn and rear their young. As the water recedes, millions of young fish become available to terns feeding in and along the rivers, creeks, and ponds. As the water level continues to drop, these fish become concentrated in smaller waterbodies, making easy meals for terns and other water birds. By summer’s end, young terns are ready to begin their journey south to their wintering grounds.

The annual river patterns are important for these species. Without periodic flooding, sandbars can become overgrown with vegetation, increasing predation and eventually covering the sandbar. Because so many of our inland rivers have been modified to provide navigation, hydropower, flood control, and irrigation, we need to find other ways to mimic some semblance of the natural processes if we hope to conserve the tern.

The U.S. Fish and Wildlife Service, with state, federal, and local partners, is focusing recovery efforts on protecting and restoring interior least tern nesting habitat through acquisition/easements and habitat restoration. Looking at the Platte River in Nebraska, projects are designed to provide high quality nesting habitats (i.e., bare sandbars near forage) by using bulldozers to clear vegetation that has overtaken many riverine sandbars as a result of water withdrawals. Along the Missouri River above Sioux City, dredging provides clean sandbars that will not flood during the nesting season, when dam releases are often increased to help support navigation. While these areas can be highly productive for least terns, in the long-term, they are simply too expensive and limited to support the species range-wide.

Fortunately, least terns are perfectly adapted to our big river systems. They are a long-lived species, with some making the annual nesting migration for 15 years. While in any given year they may not produce chicks that fledge, over their lifetimes, they have multiple opportunities to produce young. Additionnally, in high water years, least terns can move between rivers to find suitable nesting areas. Band returns have shown these birds commonly move among rivers over several nesting seasons depending on river conditions.

While least terns may adapt easily to new conditions, their life history relies on a large inland river network that supports a critical mass of suitable habitat. Large flood years like 2011 essentially reset these big river systems. After the 1997 Missouri River flooding, least terns and other shorebirds, including the federally listed piping plover (Charadrius melodus), returned to a river with a habitat base (i.e., sandbars) not seen since the construction of the dams. The birds responded with record reproductive success, not only as a result of new habitat, but also supported by high fish production and low predator success over much of the nesting habitat. The latter is thought to be because the dens of mammalian predators were flooded along the river. This reproductive boost lasted several years.

Biologists anticipate the 2011 flooding along the Missouri River will result in a similar pattern next year. However, in highly regulated rivers, like the Missouri, this process is limited. Dams that interrupt the supply of sediments constrain the basic building blocks of sandbars. With each flood, some of this finite resource is washed into the navigation channel at Sioux City, where it slowly works its way down the channel to the Mississippi. Only beyond the mouth of the Ohio River does this sand enter a more natural system that supports large sandbars, backwaters, and seasonal ebb and flows of the river. The Lower Mississippi River hosts the largest numbers of interior least terns, due in a large part to a more naturally functioning system. The future of the tern looks bright provided we can sustain these processes long-term, not only along the Lower Mississippi River, but throughout other big river systems.

 

Jane Ledwin, a fish and wildlife biologist in the Service’s Columbia Ecological Services Field Office, can be reached at jane_ledwin@fws.gov or 573-234-2132, ext. 109.