Conservation and Management

Shooting And Trapping

Shooting and trapping led to population declines and range retractions (particularly on Atlantic Coast) before 1840s (see Distribution: historical changes, above). Historically a minor game bird in California with small numbers sold in markets (Grinnell et al. 1918). Hawaiian Stilts were considered a game species until 1941, and this pressure severely impacted population sizes (Schwartz and Schwarz 1949, Shallenberger 1977). Probable hunting to extirpation on some islands, but now all islands of historic distribution recolonized (Bachman et al. 1982). All subspecies protected under Migratory Bird Treaty Act of 1918 (16 U.S.C. 703–712); illegal shooting and trapping now probably minor source of mortality.

Pesticides And Other Contaminants/Toxics

DDT and Its Metabolites. Elevated egg residues of DDT metabolites (particularly DDE) detected in California and New Mexico 1989–1990 (Ong et al. 1991, Setmire et al. 1993). At Salton Sea, CA, eggs contain up to 12 ppm DDE (wet wt.) compared to presumptive adverse effects threshold of 3–8 ppm; 10% of eggs ≥8 ppm. Regression of eggshell thickness against DDE concentration for Salton Sea eggs not statistically significant (p = 0.25), but 7% eggshell thinning suggested at 12 ppm DDE. In San Joaquin Valley, CA, eggs collected near cotton fields treated with Dicofol (an organochlorine pesticide containing DDT as an impurity) contained up to 9.6 ppm DDE wet weight (mean = 2.79, n = 7) which exceeds toxic thresholds for eggs of other bird species (Clark et al. 1995). In Tulare Basin, CA, a maximum of 7 ppm DDE wet weight detected in eggs (JPS). DDE in Black-necked Stilt carcass wintering in lower Rio Grande Valley, TA, not of concern (3.30 ppm wet wt.; Gamble et al. 1988).

Selenium. Many wetlands used by Black-necked Stilts in w. U.S. have been contaminated as a result of irrigation and other human activities (USFWS 1992). Black-necked Stilt an important indicator species in identifying effects of contaminants in irrigation drain water on wildlife. At Kesterson NWR, San Joaquin Valley, CA (Ohlendorf et al. 1986b, 1989, 1990), irrigation drain water was diverted to the refuge for disposal; the project was viewed initially as a habitat replacement success. Like other waterbirds, Black-necked Stilts breeding at Kesterson failed to reproduce successfully (Williams et al. 1989) because drain water supplying the wetland was contaminated with selenium (Se) concentrated through soil-leaching and water recycling (Ohlendorf et al. 1986a). Bioaccumulation of these toxicants resulted in reduced egg viability, dramatic developmental defects in embryos that survived to later stages of development (teratogenesis; Hoffman et al. 1988), and subsequent death of chicks that did hatch (Ohlendorf et al. 1986b, 1989, 1990, Williams et al. 1989). Kesterson has since been filled with uncontaminated soil in order to displace breeding birds to wetlands of better habitat quality, and habitat managers are monitoring potential bioaccumulation through the new terrestrial food chain (Wu et al. 1995).

Selenium (Se) pollution has since been documented as widespread problem associated with irrigated agriculture in w. U.S. (Seiler and Skorupa 1995, Skorupa 1998, JPS; see Fig. 6); stilts much more sensitive to Se poisoning than closely related American Avocet (Skorupa 1998). Normally, sets of eggs average <3 ppm Se (dry wt.); at contaminated sites average up to 97 ppm (Skorupa and Ohlendorf 1991, JPS). Strong correlation between average waterborne Se (range 0.2–3,003 ppb) and average egg Se (range 1.4–97 ppm) for 36 breeding populations in Tulare Lake basin, CA (r = 0.901, p < 0.1; Skorupa 1998). Regression equation: log (egg Se [ppm]) = 0.44 + 0.434 log (water Se [ppb]). Kesterson Reservoir (California) and Salton Sea data fall on same regression line (Skorupa 1994, 1998).

There is a dose-response relationship between egg Se concentrations and incidence of embryo deformity. Embryo deformity (teratogenesis) 50% effect concentration is 58 ppm Se (dry wt.; logistic regression anal-ysis, n = 608 randomly sampled Black-necked Stilt eggs from the San Joaquin Valley, CA), compared to 105 ppm for American Avocets (Skorupa 1998). Discernible depression in stilt egg viability begins at 6–7 ppm Se (Skorupa 1998). Selenium-induced embryo deformities in 5–60% of eggs at 5 California sites (Ohlendorf 1989, Skorupa 1998). Background embryo deformity rate 0.15% (n = 654 embryos from Se-normal eggs; JPS). Recent agroforestry project (growing a series of salt tolerant crops as a disposal method for drainage water) also had unexpected side effects of pooling Se-contaminated water which led to 56.7% Black-necked Stilt embryo teratogenesis (n = 30; Skorupa 1998). Any time >10% of eggs in successful nests fail to hatch, possibility of Se poisoning warrants examination (JPS).

Significant Se-induced mortality of hatchlings at 2 California sites (Kesterson, Williams et al. 1989; Tulare Basin, C. Marn pers. comm.). Combined embryo and hatchling poisoning can cause complete reproductive failure.

Se impacts on adults virtually unstudied. Although evaporation ponds theoretically could have positive impacts (by providing food-rich stopover areas) as well as negative impacts (through Se bioaccumulation) on migrants, adults might be more susceptible to Se poisoning during winter (as is the case with ducks; Heinz 1996). Selenium-contaminated evaporation ponds in Tulare Basin are used by Black-necked Stilts migrating from uncontaminated sites in w. U.S. (Robinson and Oring 1996). Some birds reside in these ponds for as long as 48 d (Robinson and Oring 1996), long enough for significant amounts of Se to accumulate in their bodies (Heinz et al. 1990).

Adult females at breeding sites in Tulare Lake Basin (California) had significantly lower body weight (–14%; controlled for week of collection and reproductive status) at high-Se pond compared to reference pond (JPS). Emaciation is a classic symptom of Se poisoning.

Potential for significant population level impact at Se-contaminated Salton Sea (California) where estimated 100,000 wintering stilts occur (Setmire et al. 1993, Skorupa 1998). Potential exists for Se-poisoning in Mexico where high-Se Colorado River water also used for irrigated agriculture (Mora and Anderson 1995).

Other Trace Elements. Less studied than Se impacts, but studies of contaminants in irrigation drainage have documented trace element accumulation at levels that could produce detrimental effects. For example, the Carson River drainage of Nevada is contaminated with mercury from mid-1800s mining activity; bioaccumulation of mercury occurs in addition to any toxicants concentrated by irrigated agriculture. Juvenile stilt livers in Nevada exceeded effect criteria concentrations for boron, mercury, and Se; adult stilt livers exceeded effect concentrations for mercury and Se (Hoffman et al. 1990).

Other Contaminants. Nonnetted oil pits and gold-mining cyanide leach ponds probably attractive and lethal. Gizzard of Hawaiian Stilt contained ample grit, thus, ingestion of spent lead shot probable, but unconfirmed risk (Woodside 1979a); potential problems unstudied in Hawaiian Stilts. Probable risk for Black-necked Stilts as well.

Degradation Of Habitat

Wetland losses and conversion in North America (Dahl 1990) probably have led to population declines of Black-necked Stilts (Page and Gill 1994); small habitat gains from construction of artificial ponds and extensions of range (see Distribution: historical changes, above) might offset some, but not all, of effect of habitat loss. Many traditional breeding areas dependent on vernal flooding, which is now rare because of agricultural diversion of in-stream flows and urban flood control projects. Rangewide loss of historic perennial wetlands and coastal intertidal wetlands also severe (Helmers 1992). Use of salt ponds, sewage ponds, agricultural evaporation ponds, and wet agricultural areas such as rice fields and flood-irrigated fields may counterbalance some of historic wetland losses. These surrogate habitats, however, are not as likely as natural habitats to include predator-safe islands for nesting, or to have suitable shallow habitats for juvenile foraging.

Extant breeding and wintering habitat, natural and surrogate, subject to widespread agricultural and industrial degradation of water quality (see above). Major breeding, staging, and wintering areas in Pacific Flyway such as San Francisco estuary, San Joaquin Valley, and Salton Sea, are already seriously polluted (Fig. 6; Moore et al. 1990, Harvey et al. 1992).

Increased salinization (human-caused increases in salt content of water) from agricultural drain water, surface flow, and subsurface flow is a problem in at least some inland wetlands in every western state in U.S. (USFWS 1992). Because of multitude of ecosystem-level changes that salinization produces, salinization is likely to have significant impacts on populations of temperate-breeding shorebirds, including Black-necked Stilts (Rubega and Robinson 1996).

In Hawaiian Is., invasion of alien plants (in particular, California grass, water hyacinth [Eichornia crassipes], and mangrove [Rhizophora mangle]) poses a severe threat to the availability of suitable open water and mudflat habitat (USFWS 1985, Engilis and Reid 1995). Human encroachment by filling wetlands for development and agriculture has also greatly reduced availability of wetlands (Saito 1974, Griffin et al. 1989). In Hawaii, human encroachment is especially severe. Urbanization has impacts through both loss and degradation of habitat (Engilis and Reid 1995); it is possible that light pollution increases predation (JMR). Impact of scarcity of predator-safe nesting islands on reproductive success is exacerbated in Hawaii by human introduction of nonnative ground predators.

Disturbance At Nest And Roost Sites

At agricultural evaporation ponds, sewage ponds, and probably commercial salt ponds, routine grading of levees destroys many eggs (JPS). Loss measured for 1 season at 1 evaporation pond was 211 of 1,344 eggs (16%). Severe reproductive failure at another evaporation pond when about 125 chicks were trapped inside discarded motor vehicle tires placed at shoreline to stabilize earthen levees against wave erosion (JPS). In Hawaii, flooding taro fields can flood nests on embankments (JMR), human visitors also unknowingly disturb or step on nests and eggs (M. Morin unpubl. data).

Human/Research Impacts

Nests visited early—when only first egg present—have moderate probability of abandonment at some research areas but not others. No effect in Lassen Co., CA: 12.2% abandonment if found with 1 egg (n = 41), 12.1% if found with >1 egg (n = 224, p = 0.48; JAR and LWO). Possible effect in Tulare Basin, CA: 19.7% if found with 1 egg (n = 137), 3.7% otherwise (n = 1,064, p < 0.0001; JPS). Black-necked Stilt abandonment as a response to research activities is less prevalent than among American Avocets nesting in same area (36% if found with 1 egg and 9.5% if found with >1 egg in Lassen Co., CA [Robinson et al. 1997]; 31.7% if found with 1 egg and 3.3% if found with >1 egg in Tulare Basin, CA [n = 300 and 1,676 nests respectively, JPS]). However, both of these probabilities of abandonment must be considered maxima, because some 1-egg nests are abandoned before the researcher visit (JAR and LWO). When nest searching and/or monitoring occurs weekly, about 10% of nests contained 1 egg at first visit, thus, overall 1.9% rate of researcher-induced nest abandonment at Tulare Basin (JPS). In most cases, pairs probably renested.

At site where adults were captured for color-marking via nest-trapping, 33% of successful nests contained ≥1 inviable egg (Alberico 1995) compared to normal incidence of 5–10% at multiple sites without nest-trapping (JPS). This problem was avoided in subsequent years by use of wooden egg replacements during trapping (Robinson and Oring 1996, 1997).

Nest-trapping and banding probably influence movement behaviors of individuals. For example, 33.3% (3 of 9) of incubating birds disappeared from study area immediately after being captured (Robinson and Oring 1996) and were resighted at migration sites. Nest-trapping and banding probably affected dispersal distances in the year immediately after banding (as for American Avocets; Robinson and Oring 1997), and the lower return rates suggest that the effect might be greater for stilts than for avocets (JAR and LWO).

Hawaiian stilts collide with power lines along coastal highways (Fish and Wildlife Information Service 1996), but impact on populations probably are trivial.

Conservation Status

Hawaiian Stilt listed as federal and state (Hawaii) Endangered Species. All subspecies protected from unregulated destruction under general wildlife laws such as federal Migratory Bird Treaty Act and state Fish and Game codes.

Measures Proposed And Taken

Hawaiian Stilt Recovery. A guiding sentiment for continued management for Hawaiian stilts was articulated by George C. Munro (Munro 1946: 22), “[It is] a very fine endemic bird which should not be allowed to become extinct or even rare.” In the most recent recovery plan, Engilis and Reid (1995) identify reduced habitat availability, primarily due to urban encroachment, as the most important reason for decline. This hypothesis was supported by viability modeling, which predicted that given predator control, this subspecies would increase to meet habitat availability (Reed et al. 1998). For Hawaiian Stilts, predator control and maintaining core areas of permanent wetlands, i.e., areas cleared of alien plants, are fundamental management activities for the continued existence of this subspecies (Ueoka et al. 1976, Ohashi and Burr 1977, Engilis 1987, Engilis and Reid 1995).

In light of the importance of habitat availability, it is a concern that many of the wetlands currently used by Hawaiian Stilts are not secure over the long term. For example, none of the U.S. Fish and Wildlife Service refuges on O‘ahu are on land that is owned by the Fish and Wildlife Service. Measures proposed in recovery plans include: (1) securing specified habitats on five islands, (2) private lands protection easements, (3) water level, salinity, and predator management. Numerous recovery actions for Hawaiian Stilt accomplished or in progress. Most actions involve habitat preservation or restoration. Manipulation of nesting opportunities using water level management and pond contouring (Engilis and Reid 1995) likely to have positive impact. Efficacy of other activities such as artificial nest structures important in some wetlands (e.g., ‘Aimakapä), but not others; active research program evaluating management techniques and efficacy is ongoing (Fish and Wildlife Information Service 1996).

Currently, the recovery plan criterion for downlisting the Hawaiian Stilt is to observe population estimates of ≥2,000 individuals in 3 of 5 years of biennial censuses (USFWS 1985, Engilis and Reid 1995). Population viability analysis shows population persistence most sensitive to adult survival, a parameter for which no data exist (Reed et al. 1998). Until introduced threats (predators and wetland plants) are removed, constant management will be required for persistence.

Selenium Contamination. In California’s Tulare Lake Basin, operators of selenium-contaminated evaporation ponds now required via Waste Discharge Permits to provide mitigation habitat targeted for breeding recurvirostrids (California State Water Resources Control Board 1995). In 1995, four operators provided 600 ha of mitigation wetlands of various designs which supported 1,723 stilt nests (overall density of 2.9 nests/ha; Central Valley Region, California Regional Water Quality Control Board unpubl. data).

Effectiveness Of Measures

Hawaiian Stilt Recovery. Total Hawaiian Stilt numbers stable at 1,100–1,1500 birds (Banko 1988, Hawaii Div. of Forestry and Wildlife unpubl.)— still below minimum number (2,000) deemed adequate for delisting.

Selenium Contamination. The most attractive mitigation design was a 130 ha site with 2:1 ratio of shallow-water feeding areas (10–15 cm depth) to elevated nesting areas. Site arranged as 35 feeding lanes (each 20 m wide) alternating with 34 unvegetated and gradually sloping (12:1 slope) nesting lanes (each 10 m wide). All lanes 1.6 km long, and site perimeter bounded by electrified fence. Water managed as flow-through system. Site supported 1,303 stilt nests (density 10 nests/ha) with <1% nest predation (Tulare Lake Drainage District 1996). By comparison, the 1,200 ha of evaporation ponds being mitigated for in this case typically supported 550 stilt nests (density 0.5 nests/ha) with 15% nest predation (C. Marn pers. comm., JPS). Species response much more favorable than expected. Long-term sustainability of response unknown.