Piping Plover and Least Tern Success Below Gavin's Point Dam

Conservation means to use, and the use of all methods and procedures which are necessary to bring any endangered or threatened species to the point at which the measures provided pursuant to this Act are no longer necessary. Further, the Act states that federal agencies must "utilize authorities in furtherance of the purposes of this act by carrying out programs for the conservation of endangered and threatened species".

Two of the species on the threatened and endangered species list are the Piping Plover (Charadrius melodus) and the Least Tern (Sterna antillarum). The Piping Plover is a migratory shore bird that breeds along prairie rivers, alkali lakes and ponds of the northern Great Plains, on sandy beaches along the Great Lakes and on the vast beaches of the Atlantic Coast "Since 1900, Piping Plover populations have fluctuated drastically” (Smith 1987).

Uncontrolled market hunting in the early 1900's brought the species close to extinction. Protective legislation helped plover populations recover by 1925. Since then, human encroachment, and an increase in the recreational use of sandbars and beaches, the channelization and impoundment of rivers and the resultant modification and destruction of habitat have caused numbers to decline again. The Piping Plover's historic breeding range included the Missouri and Platte Rivers, parts of the Loup River system and approximately 75 miles of the Niobrara River (Nebraska Game & Parks Commission 1978-86).

According to the U.S. Fish and Wildlife Service (1990) the Piping Plover is a sandy-gray, robin sized bird with one distinctive dark band on its white breast. The Piping Plover spends much of its time along shorelines searching for insects to eat. Nesting Piping Plovers also are very visible, performing a "broken-wing" act to lure intruders away from the nesting area (Smith 1991).

The Least Tern is the smallest member of the tern family. The Interior Least Tern breeds locally along the major tributaries of the Mississippi River drainage basin from eastern Montana south to Texas and east to western Illinois, Missouri, Arkansas, and Louisiana. The Least Tern is a small swallow-like bird. The black head, white forehead, and yellow bill contrast with its pale gray body and white belly. Least Terns catch fish by diving into the water (U. S. Fish and Wildlife Service 1990). The Least Terns apparently were abundant through the late 1800's, but were nearly extirpated as changing fashions popularized bird feather and skins on hats. In the late 1800's it was reported that as many as 1,200 adult Least Terns were killed for their delicate plumage in one day, and as many as 100,000 were killed in one season (Sidle and Harrison 1990).

Least Tern reproductive success also can be limited by human related disturbances such as foot traffic, unleashed pets, swimmers, canoeists, and off-road vehicles. Predation by avian and mammalian predators, such as American Crows, American Kestrels, Great Horned Owls, raccoons, coyotes, and mink have been documented at nesting sites of these two birds.

Studies along the Platte River Valley have shown that selenium and mercury concentrations were elevated above background levels, and selenium in particular might be affecting the reproductive success of least terns in the Platte Valley study area (Kirsch 1992). Contaminants and physical degradation of habitat and the increase in human disturbance could further accelerate population declines.

Ultimately water fluctuations from May 1 to September 1 are a major cause of nest and chick losses of Interior Least Terns and Piping Plovers along the Missouri River in South Dakota, North Dakota, Montana, and Nebraska (Nebraska Game & Parks Commission 1978-1986, Schwalbach 1988, Dirks 1990, Kruse 1990,1991). Releases from Gavins Point Dam have been above normal for the last 3 years. Gavins Point releases were 60,000 cfs (cubic feet per second) in August of 1995 which was 26,000 cfs above the annual average from 1967 through 1994. The last two years have been extremely high with record breaking releases of 61,500 cfs in 1996, and 64,500 cfs being released in August, 1997 (Fig 1 ).

The US Army Corps of Engineers received a Jeopardy Biological Opinion on the operations of the Missouri River Main-stem System from the U. S. Fish and Wildlife Service (USFWS) on November 14, 1990. This Biological Opinion concluded that if the operations of the Missouri River remained unchanged, the continued existence of the interior population of the Least Tern and the Great Plains population of the Piping Plover would likely be jeopardized. The Least Tern was listed on the federal endangered species list in 1985. The Piping Plover was put on the federal threatened species list in 1985 (Ducey 1985).

Piping Plovers usually lay 3-4 eggs, while Least Terns typically lay 2-3 eggs. Both parents care for the young throughout the summer until the chicks are able to fly. The tern chicks are much more dependent on parental feeding than the plover chicks are. The plover chicks are usually up and running the sandbar at one day of age. The adult birds and their young leave the nesting areas by late August and begin their migration south to the Gulf of Mexico and the Caribbean. The parents regularly return to the same nesting areas year after year to raise families (Lingle 1988).

Normal flows from Gavins Point Dam are 33,000 cfs during the summer months, however, the increased runoff from the mountains each year has caused the Corps of Engineers to increase releases from the dam dramatically. In 1996, the releases during the summer months reached a high of 62,000 cfs. In 1997, the high reached 65,000 cfs. With these high volumes of water coming through the dam, the habitat situation for the birds changes drastically each year, even weekly. The habitat that is available to the birds is in very good condition though. The sandbars are mostly cleared of vegetation because they are continuously moving throughout the year with the swift current. Artificial islands may be needed for recovery of the interior least tern (U. S. Fish and Wildlife Service 1990). This habitat restoration and building has helped the species along with the Piping Plover in giving them more suitable habitat to nest and raise their young.

Intensive efforts began in 1990 to identify specific predators affecting nest and chick survival and to develop and implement aversive deterrents to reduce or eliminate their impact on Least Tern and Piping Plover productivity (Kruse 1991). Predators have a great impact on fledgling success rates in these two species of birds. Intensive research on habitat selection and availability, reproduction, foraging habits and limiting factors has been conducted in many states, and comprehensive river management is underway, particularly along the Missouri and Platte Rivers, where development pressure is most threatening (Ziewitz et al. 1992).

This study was intended to show how high water level and predators play a dramatic role in the survival of the two species. From this information the following questions were derived: 1) how does the water level affect bird nesting and fledgling rates? 2) does the continual fluctuation of the water levels in the river have a significant impact on the number of nests layed? 3) how does the amount of water passing through the gates at the dam impact the habitat below the dam, and can this be detrimental for the success rates of the two species?

The following hypotheses were proposed. The first hypothesis states: there are fewer Least Tern nests on this reach of the river in 1997 than 1996. The second is: there are fewer Piping Plover nests on this reach of the river in 1997 than 1996. And the third is: the nest and fledging success is lower for both species of birds in 1997 than in 1996. Hypotheses were based on the greater volumes of water released from Gavins Point Dam in 1997 than in the previous years of 1995, 1996, and the average from1967-1994.

MATERIALS AND METHODS

This census and survey was done according to the U.S. Fish and Wildlife Service specifications and regulations. Data collected were then analyzed to test the hypotheses. Data were collected on the Missouri River below Gavins Point Dam in 1997, to determine the number of eggs, eggs hatched, fledging ratio, and overall nesting success. This was done at 11 different sites, all of which consisted of sandbars on the river system at various locations. The sandbars varied in size, shape, vegetative cover, and elevation above the water.

Sampling was conducted in May, June, July, and August of 1997. The productivity was monitored weekly. Sites were accessed by boat with nest searching and monitoring conducted on foot. In an effort to preserve embryo viability, surveys were conducted when ambient air temperature was below 90 degrees, and nest site disturbance was limited to 20 minutes or less. Each nest was marked with a wooden tongue depressor 2 meters from the nest. Nest data (Fig. 2) were collected and recorded on Corps of Engineers standardized nest cards which included species, habitat type, nest location, nest initiation date, clutch size, number of eggs hatched, determination of causative factor or factors for nest termination, and nest hatching date. Egg floatation to determine stage of incubation was only used in cases where nests were located with complete clutches. Excessive handling of early term embryos had been observed in a captive situation, to result in embryo death.

Least Tern and Piping Plover nests or chicks threatened by unforeseen natural events, including, but not limited to: sand bar erosion, abandonment, excessive water levels, and/or instability of selected nesting sites, were evaluated according to pre-approved contingency plans by the U.S. Fish and Wildlife Service. The measures were taken to ensure possible protection of the two species in the future.

Weekly status reports were composed during the nesting season. These reports were divided by reach and included by site the number of active nests of each species, number of chicks present of each species, adult census, total number of fledged chicks of each species, and other pertinent data such as status of nests in relation to water elevation. Reports were discontinued when all activity was terminated in each respective reach.

The water levels affecting bird nesting were at 61,000 cfs in May, 61,500 cfs in June, 62,500 cfs in July, and 64,400 cfs in August, nearly doubling the average amount of water flow from Gavins Point Dam (Fig. 1). The continual fluctuation of the river level does make a change in the nesting habitat for both species of birds. The constant change of water fluctuation creates changes in the main channel of the Missouri River. These changes allow the river to eliminate the sandbars and create new sandbars along different areas of the river. The old sandbars that once contained proper habitat and provided nests, shelter, and protection for the birds is no longer available. The birds that had already produced eggs and were caring for them were washed away having no chance for their young to survive. The new sandbars however, do create applicable cover and adequate areas for nesting in a very short amount of time, once the bar is dry. The only problem with this is that there is no telling how long the sandbars will remain there before the nests are washed away. The water level below the dam is determined by the amount of water passing through the gates above. The water level was higher in 1997 than in the past, affecting the habitat avialable below the dam. This can be detrimental to the success rates if a majority of the habitat is destroyed.

RESULTS

The study in the summer of 1997 also yielded the following results for the Piping Plover success below Gavin’s Point Dam. The number of nests = 14, number of nests and eggs collected = 0, number of nests hatched = 7 number of eggs = 50, number of eggs hatched = 23, number of collected eggs that were hatched and released = 0, and number of chicks fledged = 0. These numbers are all less than 1995. The number of eggs, number of eggs hatched, and number of nests hatched were greater in 1997 than 1996, with the number of nests being the same (Fig. 3).

DISCUSSION

The Least Tern has not had a problem adapting to the high water and changing sandbar structure, according to the results. In 1997, the numbers were clearly higher than in 1996, with 52 chicks fledging off in this reach of river. The Least Tern has an incubation period of only 18 days, compared to the plover’s 28 days. This may be one of the factors that help the tern to hatch and be old enough to fly before the sandbar is destroyed by high water or overtaken by predators. On the other hand, the plover takes an additional 10 days, during which the water levels may change drastically and wipe out the nesting area or give the predators more time to find nests or young chicks. There are many factors that come into play with the changing water levels, but the Least Tern has clearly adapted well to the high water in the 1997 nesting season, while the Piping Plover had poor success on this reach of the river as in the previous two years.

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