Demography and Populations

Age At First Breeding

Unknown; presumed to be 1 yr.

Clutch

Mean clutch size differs among subspecies (p = 0.0001), on basis of data from completed clutches in WFVZ collection and nest card records at Cornell Laboratory of Ornithology. Differences explained mainly by the larger clutches of Clapper Rails on East and Gulf Coasts of U.S. (p < 0.05). Clutch size for nests of different subspecies (mean ± SD, range, n): obsoletus (San Francisco Bay, CA), 8.3 ± 1.7, 4–14, 179; levipes (s. California and Baja California Norte), 7.3 ± 1.2, 5–11, 94; yumanensis (lower Colorado River and Salton Sea, CA), 6.7 ± 0.8, 5–8, 15; yumanensis (West Coast of Mexico), 5.5 ± 0.9, 3–7, 22; beldingi (Baja California Sur), 6.4 ± 1.0, 5–8, 9; crepitans (n. Atlantic Coast of U.S.), 9.2 ± 1.6, 4–16, 455; waynei (s. Atlantic Coast of U.S.), 9.4 ± 1.9, 5–15, 75; scotti (s. and w. Florida), 7.3 ± 2.0, 2–11, 19; insularum (Florida Keys), 6.5, 6–7, 2; saturatus (Gulf Coast of U.S.), 9.5 ± 1.5, 7–14, 20; and caribaeus (Jamaica), 6.5, 6–7, 2. Similarly, clutch size increases with latitude (p = 0.0133) at rate of 0.174 egg for every 1° of latitude (WRE).

One clutch from San Francisco Bay, CA, with 21 eggs probably represented a dump nest (DeGroot 1927). In Virginia, clutches in 149 first nests averaged 9.0 ± 0.2 (range 4–12), but 16 second clutches averaged only 5.6 ± 1.1 (range 3–9; Stewart and Meanley 1960). Clutch size averaged 7.9 in 42 first nests in S. Carolina, 6.7 in 15 second nests, and 6.2 in 6 third nests (Blandin 1963). No appreciable decrease in clutch size in second nests in Georgia (Oney 1954).

Reproductive Success

Nest success (nests hatching ≥1 young) variable, but often high in good habitat (Table 1). Egg success also high; 87.3% of 513 eggs hatched in New Jersey (Kozicky and Schmidt 1949). An average of 8.1 eggs ± 0.7 SD hatched/successful nest in New Jersey (n = 242 nests), 11.2 ± 2.0 SD (n = 174 pairs) chicks hatched/pair and (12.8 ± 3.2 SD) chicks hatched/ha (36.8-ha study area; Mangold 1974).

Nest success estimates are based on proportions of nests found (Table 1). Mayfield daily nest survival for 145 nests in New Jersey from nest card records from Cornell Laboratory of Ornithology 0.994063 (95% confidence interval [CI], 0.991988–0.995841; WRE). For 21-d incubation period (not including laying period), nest success is 88.2% (95% CI, 84.4–91.6%).

Most nest failures attributed to flooding by high tides or high winds, but predation also a factor; in s. California, 9 nests lost to high tides and 6 to predation (Zembal and Massey 1983). Eggs often lost to predation by Norway rats in San Francisco Bay, CA (Harvey 1988). Isolation of nesting areas from the mainland may determine success, because predators have poor access to isolated habitats (Meanley 1985).

Chick mortality high in first few weeks; observed brood size is low relative to number hatched and often decreases rapidly after hatching (Duhsé 1988). Age ratios in fall populations are variable; 3 juveniles/adult in Virginia (Meanley 1985), 2 in Louisiana (Sharp 1976), 4.9–5.8 juveniles/adult in N. Carolina, (Adams and Quay 1958).

Number Of Broods Normally Reared Per Season

Birds with failed nesting attempts renest up to 5 times; some evidence suggests up to 7 attempts (Blandin 1963), suggesting high reproductive potential despite losses to flooding and predation. About 50% of pairs in San Francisco Bay, CA, renest following nest failure (DeGroot 1927). Second nests after successful first nest documented in New Jersey, S. Carolina, and Florida (Blandin 1963, Duhsé 1988, Ferrigno 1990). Second nest placed on average 17 m from first nest (n = 21; Blandin 1963).

After-hatch-year male banded 24 May 1971 8 km SSE of Tuckerton, NJ, was shot 5 Dec 1977 6.4 km south of Tuckerton, making it at least 7 yr 6 mo old (Clapp et al. 1982). Annual survival rates of adults fitted with radio transmitters ranged from 49 to 67% in Arizona (Eddleman 1989). Composite annual survival rate for all years (1985–1987) was 26%, but effects of radio transmitters may have depressed survival. Most adult mortality occurred during fall and winter.

Diseases

Poorly known. One of 9 birds necropsied from Arizona died from ideopathic hepatitis caused by disease or ingestion of a toxin (Eddleman 1989). Six individuals died during a botulism outbreak in San Francisco Bay, CA, in 1974 (USFWS 1974).

Body Parasites

Forty-six species of internal parasites reported: 35 trematodes, 6 cestodes, 4 nematodes, and 1 acanthocephalan (Bateman 1965, Heard 1968, Deblock and Heard 1969, Byrd and Heard 1970, Heard 1970, Nickol and Heard 1970, Bates and Meade 1972, Brooks and Heard 1977, Taft and Heard 1978, Underwood and Dronen 1986, Wong and Anderson 1990). Type host for nematode parasite Ancyracanthus heardi (Wong and Anderson 1990) and trematode parasites Levinsiella byrdi (Heard 1968), Longiductotrema floridensis, Maritrema prosthometra (Deblock and Heard 1969), Renicola ralli, R. glandoloboides (Byrd and Heard 1970), and Notocotylus schmidti (Brooks and Heard 1977). Effects of parasites on individuals unknown.

See Behavior: predation, above. Ribbed horse mussels and other bivalves clamp onto Clapper Rail toes or bills, which prevents birds from feeding and causes them to starve (DeGroot 1927, Zembal and Fancher 1988). Storms devastate local breeding and wintering populations (see Population regulation, below; see also Conservation and management: effects of human activity, below). Susceptible to prolonged cold, especially unusually cold periods (Simmons 1914).

Initial Dispersal From Natal Site

Juveniles become independent at about 5–6 wk, then move erratically (Meanley 1985). Often chased by territorial adults (Zembal and Massey 1987). Natal dispersal patterns and degree of philopatry unknown.

Fidelity To Breeding Site And Winter Home Range

More than 17% of 191 birds banded on nesting areas in New Jersey were recaptured in subsequent years (Mangold 1974). Adults in Arizona and s. California that have established home range usually move little (Eddleman 1989, Zembal et al. 1989). Fidelity to winter home range unknown.

Dispersal From Breeding Site Or Colony

Highly vagile; individuals sometimes appear far from typical habitats (Crawford et al. 1983). However, poorly described. Occurrences of obsoletus, levipes, and yumanensis in atypical habitats summarized by Leipsic-Baron (1992). Most movements made by young birds, but dispersal in sw. Arizona occurred in 4 different ways: dispersal by juveniles, movement to new sites by males that had failed to attract a mate, movements from breeding areas by postbreeding adults, and movements to new areas by birds in late winter (Eddleman 1989). These dispersal movements allow birds to find new or ephemeral habitats, such as on lower Colorado River in the past (Ohmart et al. 1975). Fall dispersal also occurs in San Francisco Bay, CA, area (Linsdale 1936, Orr 1939). Has been suggested as a mechanism for occupying ephemeral habitat, new habitat, or habitats in which local extinction has occurred (Orr 1939, Remsen and Parker 1990). Dispersal distances of 3 individuals from s. California were minimum of 23.5, 32.2, and 12.9 km, respectively (Zembal et al. 1985). One banded male dispersed from Upper Newport Bay, CA, to Seal Beach National Wildlife Refuge, CA, between 5 Oct and 29 Dec 1982, a distance of 21.7 km.

Home Range

During breeding season, banded birds in S. Carolina occur in range with diameter of 183–274 m (Blandin 1963). Mean maximum movement of birds occurring along canals in Louisiana was 154 m ± 37 SD during Aug and 487 m ± 467 SD in Dec–Feb (Roth et al. 1972). Daily home range of 3 breeding adults in Louisiana averaged 0.44 ha, and total breeding-season home range averaged 0.53 ha (Sharp 1976). Home range in Arizona shows great seasonal variation: for males—largest (24.0 ha ± 15.7 SD, n = 6) in Jan–Feb and smallest (3.6 ha ± 2.8 SD, n = 9) during incuba-tion; for females—largest (21.0 ha ± 8.7 SD, n = 8) in Aug–Oct, and smallest (2.2 ha ± 1.8 SD, n = 4) during incubation (Eddleman 1989). Home ranges in s. California ranged from 0.4 to 1.7 ha, and adults generally used only a small portion of their entire range within a day (Zembal et al. 1989). Home range boundaries of adjacent individuals overlap considerably (Zembal et al. 1989, WRE).

Numbers

Density in ideal habitat 2.2–4.7 individuals/ha in Georgia (Oney 1954), 3.2/ha in New Jersey, and 8.4/ha in Virginia (Mangold 1977). Number of active nests ranged from 1.0 to 1.6/ha during 6-yr study in New Jersey (Mangold 1974). Nest density at Chincoteague, VA, was 4.2/ha (Stewart 1951). Density was 0.9–1.6/ha in San Francisco Bay, CA (Harvey 1988); density in s. California ranges from 0.12 pair/ha at Anaheim Bay to 2.0 individuals/ha in Upper Newport Bay (Zembal and Massey 1987). Density in habitats of different quality ranges from 0.09 to 0.79 individual/ha in Arizona (Smith 1974).

Trends

Few data available, except for endangered western subspecies, which have declined greatly during past 100 yr (see Conservation and management: effects of human activity, below). On basis of limited Breeding Bird Survey data, Clapper Rail populations showed no increasing or decreasing trend from 1966 to 1994 (Sauer et al. 1996). However, populations appear to have declined at rate of 8.2% in U.S. during 1966–1979 (p < 0.01). Declines appear greatest in Texas and Louisiana. Clapper Rails occurred on only 20 routes with usable data during this time, so this trend should be interpreted with extreme caution. Distribution of Clapper Rails in Maryland largely unchanged during past century, although numbers have probably been reduced because of habitat loss (Robbins and Blom 1996).

Along Atlantic Coast, Clapper Rail population limited by nesting losses of eggs and young caused by high tides in association with storms (Bent 1926, Kozicky and Schmidt 1949, Adams and Quay 1958). Ability to repeatedly renest allows populations in good habitat to recover rapidly (Blandin 1963). Storms affecting major wintering areas on s. Atlantic Coast may cause direct mortality of large numbers of adults (Ferrigno 1990). Breeding populations were at record lows in New Jersey in 1990 after Hurricane Hugo struck a major Clapper Rail wintering area in Oct 1989. Flooding also believed to be important factor for population regulation in coastal Texas (Simmons 1914). Sand deposition after Hurricane Frederic in 1979 affected salt marsh cordgrass distribution, and therefore Clapper Rail distribution (Holliman 1981). Populations probably recover after cordgrass reinvades such areas. Elevated beaches or tree lines may protect salt marshes from storm damage.

Other factors affecting populations in New Jersey include territoriality (in dense populations), predation, interspecific competition with Laughing Gulls, pesticides, winter kill, and habitat loss (Ferrigno 1990). Nest predation by raccoons, mink, or opossums is an important limiting factor in Georgia, Louisiana, and Texas (Simmons 1914, Oney 1954, Bateman 1965); and probably also elsewhere (Sharp 1976, Eddleman and Conway 1994). Populations of obsoletus in San Francisco Bay, CA, have declined mainly because of predation on adults by introduced Norway rats and red foxes (Harvey 1988). Predation is principal cause of mortality in adult yumanensis in sw. Arizona (Eddleman 1989). Availability of quality habitat is a principal limiting factor for West Coast subspecies (Eddleman et al. 1988, Fleischer et al. 1995). Nest-site availability thought to limit levipes populations in s. California (Zembal and Massey 1983).