Demography and Populations

Age At First Breeding; Intervals Between Breeding

Two males and one female bred at age 1 yr in s. California (James 1995). One of 51 banded stilt chicks returned and bred at age 2 yr in n. Utah (Sordahl 1984). One record of Hawaiian Stilt female breeding at age 1 yr (within 1 m of natal site) but probably more typical at age 2 yr (JMR).

Clutch

See Breeding: eggs, above.

Annual And Lifetime Reproductive Success

Appendix 2 . At breeding sites potential reproduction is lost at both egg and chick stages, primarily due to predation and flooding. Nesting success (proportion of nests that hatched ≥1 egg) variable by location depending on whether nesting areas are on islands that protect nests from ground predators. In Tulare Basin, CA, nest success varied from 11.1% (n = 36 nests) to 100% (n = 47; mean across n = 14 sites with ≥30 nests each, 59.8% ± 26.8 SD; overall mean 59.7%, n = 1,706 nests, JPS). Success rates at sites where most nests are on islands are higher than at other sites (one-tailed t = 2.2, df = 12, p = 0.024, JPS). At Salton Sea, CA, 37.9% (n = 58; Grant 1982). In n. Utah, mean of 1.3 eggs hatched/nest (n = 18 nests, 72 eggs), and 32% of eggs hatched, with majority of nest failures due to egg predation; for eggs that remained in nest long enough to hatch, egg viability 95.8% (n = 24; Sordahl 1996b). Downy broods averaged 3.06 chicks ± 1.03 SD (n = 17) and Juvenal-plumaged broods averaged 2.41 chicks ± 0.79 SD (n = 97), a significant decrease in brood size (1-tailed t = 2.06, df = 112, p <0.05, Cache Co., Utah; Sordahl 1996b).

Chicks hatched/nest for Hawaiian Stilts: 2.18 chicks ± 1.6 SD (n = 982; compiled by JMR from years of USFWS monitoring: Chang 1990, Coleman 1981, Dougherty et al. 1978, Ohashi and Telfer 1977, Telfer 1972, 1974, 1983, 1984, 1985, Telfer et al. 1981, 1982, Ueoka et al. 1976, Ueoka and Telfer 1980, Woodside 1979b). Hawaiian Stilt fledging success (number of fledglings per brood): 0.934 fledglings ± 0.431 SD (weighted mean across 4 yr, 1985–1988, range 0.125–1.355, n = 131); 0.887 fledglings ± 0.550 SD (simple mean of means, n = 4 yr, USFWS unpubl.).

No lifetime reproductive success data available for stilts.

Number Of Broods Normally Reared Per Season

One. A pair of Hawaiian Stilts fledged one brood and laid a second clutch, so two broods per year possible in Hawaiian Is. (M. Morin unpubl. data).

Proportion Of Total Females That Rear At Least One Brood To Nest-Leaving Or Independence

Estimate not available because of difficulty in estimating total number of females in breeding area. Available studies (1) only marked a portion of females and (2) observed significant movement of both marked and unmarked individuals into and out of study area during breeding season.

Sufficient banding and long-term monitoring has not been conducted. However, based on records for R. americana and R. avosetta at least 10 yr would be expected (Robinson et al. 1996). In captivity, a Black-necked Stilt survived 19 yr (cited by Johnson 1987). Hawaiian Stilts alive in 1996 have survived 15 yr in captivity, and several wild stilts banded in 1978–1980 were still alive in 1995 (JMR).

Due to the length of the putative life span and the relatively recent emphasis on banding of adults, longitudinal life table analyses are not available. No systematic study of survivorship can yet be done, but may eventually become available from recoveries from extensive banding of this species in the 1990s.

From two Hawaiian Stilt cohorts, first year survival was 0.53 and 0.60; survival from first to second year for one cohort was 0.81 (Reed et al. 1998).

Disease

Black-necked Stilts and American Avocets are the shorebird species most frequently stricken with botulism (caused by the bacterium Clostridium botulinum type C_a; see review by Rosen 1971; also see discussion of botulism in Hawaiian Stilt, Morin 1996). Their diets, ranges, and habitat preferences all correspond to risk factors for the occurrence of botulism epidemics. In severe botulism outbreaks, number of stilt deaths would be sufficient to affect population dynamics (JAR). Large numbers of stilts are most likely to be affected in w. U.S. (especially California, Utah, and Nevada) in late summer and early fall, and in years when higher-than-normal precipitation produces an abundance of shallow, stagnant wetlands. Botulism toxin causes flaccid paralysis, evidenced by staggering and eventual loss of use of legs (Alcorn 1942, Rosen 1971). In contrast to Black-necked Stilts, botulism uncommon in Hawaiian Stilts even when other waterbird species are dying in the same ponds (Brock and Breese 1953), possibly because they do not eat the maggots on morbid carcasses; outbreaks do occur, however, under certain conditions (Morin 1996).

Avian cholera (Pasteurella multocida multocida) has also been identified as a cause of death in wild stilts (Hirsch et al. 1990).

Parasites

Internal Parasites. Well categorized, including 6 species of trematodes, 9 species of cestodes, and 10 species of nematodes (review and checklist in Hinojos and Canaris 1988). When collected, usually infected with at least 1 species of helminth parasite (34 of 35 birds collected contained helminths, Hinojos and Canaris 1988; also noted for Hawaiian Stilts by Perkins 1903, Schwartz and Schwartz 1951). A tapeworm (Diplophallus polymorphus) is the most common species observed. Although multiple tapeworms can be present in the small intestine early in infection, competition increases as the worms grow, and eventually only two large worms survive (Ahern and Schmidt 1976, Burt 1980, Hinojos and Canaris 1988).

External Parasites. Three of 35 (9%) collected by Hinojos and Canaris (1988) were infected with nasal mites (Rhinonyssus himantopus; identified from Black-necked Stilts in Texas, Cuba, and Louisiana; Pence 1972, 1975 and op cit.). Malcomson (1960) listed 2 species of lice (Mallophaga); augmented to 6 species by Hinojos and Canaris (1988) with 63% (n = 35) of stilts infected with lice. Lice also noted on Hawaiian Stilts (Schwartz and Schwartz 1951, JMR). Hawaiian Stilt chicks commonly infested with feather mites (Acarina) and lice (25% heavily infested, n = 56 chicks 7–17 d old; JMR).

Nest Parasites. Louse flies (Ornithomyia spp.) in nest of Himantopus sp. (probably H. himantopus, Séguy 1935 in Hicks 1962).

Adult mortality results from predation (see Behavior: predation, above) and botulism (see Diseases and body parasites, above). In addition to these factors, mortality of flightless young observed following unseasonable cold storms, after observed leg injury, and during movements from nest area to nursery (JAR and LWO); chick mortality also due to contaminants (see Conservation and management: effects of human activity, below). Eggs lost primarily to predation and flooding (Sordahl 1996, JAR and LWO; Hawaiian Stilt: e.g., Ohashi and Telfer 1977, Dougherty et al. 1978, Woodside 1979a, Coleman 1981).

Initial Dispersal From Natal Site

No suitable sample available.

Fidelity To Breeding Site And Winter Home Range

In a banding study in s. California (4 yr, >12 adults, about 157 chicks banded), 22 birds were resighted during the nonbreeding season, 3 cases of natal philopatry, and 2 cases of adult breeding site fidelity (James 1995). Two of 9 (22%) adult stilts returned the next breeding season in n. Utah (Sordahl 1984).

Home Range

No information available.

Numbers: Estimates Or Counts Of Density

Species ranked as highly vulnerable to habitat alteration relative to w. North American shorebirds as a whole (Page and Gill 1994). Population status of breeders is not known.

At least 25,000 H. m. mexicanus winter in North America west of the Rocky Mtns. and from Sinaloa, Mexico, north; these numbers are probably reduced relative to presettlement conditions due to wetland losses over the last 200 yr (Page and Gill 1994). Peak numbers of migrants approximately 2,000 in se. Oregon (Oregon Dept. of Fish and Wildlife unpubl.); 17,000 in Central Valley, CA (Shuford et al. 1994a); 100,000 at Salton Sea, CA (Setmire et al. 1993); 1,700 in Lahontan Valley, NV (USFWS 1995); 65,000 at Great Salt Lake, UT (Shuford et al. 1994b).

Identification of important wintering areas is limited by survey data for Mexico. At least 1,000 stilts were counted in aerial surveys in 1992 of coastal nw. Mexico (Morrison et al. 1992). Approximately 6,000 counted in Dec 1993 and again in Feb 1994 at Ensenada Pabellones and Bahía Santa María, Sinaloa, Mexico. These counts considered minimal estimates of numbers there because stilts used winter habitat in vegetated estuarine areas and were extremely difficult to census (Engilis et al. 1998).

Due to habitat loss and hunting, Hawaiian Stilts declined to as few as 200 birds in 1944 (Munro 1976), but rebounded by 1949 after hunting them was banned (Schwartz and Schwartz 1949). Current population size is around 1,100–1,500 individuals (Banko 1988, Hawaii Div. of Forestry and Wildlife unpubl.).

Trends

Breeding Bird Survey (BBS) data showed no significant trends 1966–1996 in the conterminous U.S., its regions, or the entire survey area (Sauer et al. 1997). A potentially significant decline was reported for Texas 1966–1979 (trend –13.7, n = 12, p = 0.08; Sauer et al. 1997). Consistent with recent expansions of breeding at the edges of the range (see Distribution: historical changes, above), BBS data suggest increases in populations in the U.S. (trend +3.3, n = 84, p = 0.08) and survey-wide (trend +3.0, n = 84, p = 0.09; Sauer et al. 1997). From 1980 to 1995 Hawaiian Stilt numbers appeared to be on a slow increase statewide, although the lack of good, long-term census records for Ni‘ihau make this uncertain (Reed and Oring 1993).

Although demographic data exist for some parameters (such as breeding biology), the structure and regulation mechanisms of Black-necked Stilt populations are not understood. Results from banding studies in ne. California have been limited by low adult return rates and marked pairs that breed there in some years and at unknown locations in other years (making inferences more difficult than for American Avocets banded in the same study). Hawaiian Stilts live in discrete local populations with quantifiable movement among wetlands and major islands (reviewed in Reed et al. 1994). Population modeling indicates that observed movements and colonization abilities should stabilize statewide population size (Reed et al. 1998). For both Black-necked Stilts and Hawaiian Stilts, the fate and success of such movements are unknown.