Demography and Populations

Measures Of Breeding Activity

Age At First Breeding; Intervals Between Breeding

Most Snowy and Kentish plover young breed the first nesting season following birth (Warriner et al. 1986, Sandercock et al. 2005). At Monterey Bay, CA, the median age that first-time female breeders initiated first clutches was 309 days (n= 50, range 252-388) and males 307 days (n= 53, range 257-416) from 1984-2004. The median clutch initiation dates were 12 April (range 13 March-30 May, interquartile range 7-24 April) for females and 1 May (range 27 March-11 June, interquartile range 24 April-19 May) for males (LES). Although nearly all Snowy Plovers breed annually, one exception is a 10-year old male banded as a chick at Monterey Bay in 1998: although he was not monitored sufficiently to determine if he bred in 1999 and 2000, he was not known to nest in 8 subsequent years when he was closely monitored (LES).

Clutch

Mean clutch size for 171 central California coastal nests: 2.95 ± 0.25 SD (range 2–4; Warriner et al. 1986); for southern California coastal nests: 2.84 ± 0.04 SE (Powell and Collier 2000); for 125 interior California nests: 2.92 ± 0.27 SD (range 2–3; Page et al. 1979); for 70 interior Oregon and Nevada nests: 2.93 ± 0.31 SD (range 2–4; Herman et al. 1988); for 448 Great Salt Lake, UT, nests: 2.96 ± 0.12 SD (range 2–5; Paton 1995); for 41 Kansas nests: 2.78 ± 0.42 SD (range 2–3; Boyd 1972); for 512 Oklahoma nests: 2.86 (range 2–3; Hill 1985); for Playa Lakes, Texas nests: 2.71 and 2.48 in 1998 and 1999, respectively (Conway et al. 2005); and for 20 Puerto Rico nests: 2.7 (Lee 1989).

Number of clutches annually is dependent on length of breeding season, rates of polygamy, and rates of clutch loss. In coastal California, up to 6 nests per season for plovers with high levels of clutch loss, whereas completely successful males and females had up to 2 and 3 clutches, respectively (JSW and JCW). In Florida, both sexes also initiate multiple clutches (R. Pruner in litt.). On the Great Plains, successful birds of either sex are reported to have only 1 clutch/season (Boyd 1972). At the Playa Lakes region, TX, 5 males and 1 female hatched two clutches/season (Conway et al. 2005).

Annual And Lifetime Reproductive Success

Observed clutch hatching rates from locations where conservation efforts were not being employed to increase clutch hatching success vary widely from 12.5–86.8% in 20 studies (Appendix 1). Observed rates (mean = 47.4%, range 3.3–69) average 1.63 (range 1.03–2.50) times higher than rates calculated from Mayfield’s (1961) method (mean = 33.8 ± 20.0 SD, range 5.4–67) in 14 samples from 4 studies (Page et al. 1983, Hill 1985, Lee 1989, Paton 1995). Additional Mayfield estimates of annual clutch-hatching success include: 54% for 5 years combined in San Diego Co., CA (Powell et al. 2002), 53% on river gravel bars and 40% (n= 4 years) on neighboring beaches in Humboldt Co., CA (Colwell et al. 2005), and 57% in 2006 on the Florida coast (Himes et al. 2006). Hood and Dinsmore (2007b) recommend using the nest survival model in program MARK for calculating clutch survival because it provides for fitting complex models that can reveal seasonal variation and the effect of habitat variables at multiple scales.

A variety of factors are reported to influence clutch hatching rate. At Batiquitos Lagoon, San Diego County, CA, nest success peaked the first year after habitat restoration, then declined; this suggests high productivity may temporarily occur at newly-created sites, possibly because predators fail to cue into the presence of potential prey immediately (Powell and Collier 2000). Also in San Diego Co., the probability of nest success decreased as distance from Least Tern nests increased (Powell 2001). In se. Colorado, no significant relationship was apparent between nest fate and structural habitat variables at either the nest or at the 5 m scale (Mabee and Estelle 2000). In coastal Texas, daily nest survival declined slightly over the breeding season and was a function of both location and the daily age of the nest. Nests at inland lakes had lower daily survival than those at coastal sites. Nests with an object or debris in the immediate vicinity had higher daily survival than those that did not (Hood and Dinsmore 2007b). At Mono Lake, CA, nests next to objects were more likely to fail than nests under objects or in the open, possibly because predators there used objects as part of their search image (Page et al. 1985).

There is considerable annual and spatial variation in chick survival. Proportion of broods producing at least 1 flying young averaged 61% ± 10.9 SD (range 48–71%) in 4 studies (Page et al. 1979, Hutchinson et al. 1987, Wehtje and Baron 1993, M. Stern unpubl. data). Number of flying young/successful brood (producing at least 1 flying young) averaged 1.6 ± 0.21 SD, range 1.4–1.9) in 4 studies (Page et al. 1983, Wehtje and Baron 1993, Paton 1995, M. Stern unpubl. data). Proportion of chicks reaching flying stage averaged 0.39–0.43 at the Pajaro River mouth in Monterey Bay, CA, from 1977-1982 (Warriner et al. 1986). Chick fledging rate throughout the Monterey Bay area averaged 0.382 ± 0.014 SE (range 0.285-0.483) with a third of 2813 clutches producing ≥1 fledglings from 1984-1999 (Stenzel et al. 2007). At Point Reyes, CA, 0.46 of 83 chicks fledged in 1999-2000 (Ruhlen et al 2003). During a 4-year period in Humboldt Co., CA, 0.60 of the chicks fledged on river bars compared to only 0.29 on beaches; males were twice as productive on river bars (1.5 fledglings/male ± 1.4 SD) as on beaches (0.8 fledglings/male ± 1.0 SD); and chick survival increased seasonally (Colwell et al. 2005). Number of young reaching flying age was 0.8–0.9 per female at a central coastal California site (Warriner et al. 1986), averaged 0.35 ± 0.12 SD (n= 5 years) per female and 0.28 ± 0.10 SD per male in s. coastal California from 1994-1998 (Powell et al. 2002), and 0.5 per female at 1 interior California location (Page et al. 1983); rate at the interior site increased to 0.7 per female when only females making all of the season’s nesting attempts at the study site were considered (Page et al. 1983).

Most chick loss occurs during the first 2 weeks after hatch (Warriner et. al 1986). At Point Reyes, Marin County, CA, 88.9% and 80.5% of chicks disappeared before 14 days of age in 1999 and 2000, respectively (Ruhlen et al. 2003). At the same location from 2003-2008, on average 88% ± 7.9 SD (range 75-94%, n= 6 years) of the chicks were lost when they were 1-10 days old (Peterlein 2008); of chicks that did not fledge during the 6 years, 43.5% disappeared during daytime, 19.5% overnight, and 37.0% at an unknown time with respect to darkness (C. Peterlein in litt.).

Number Of Broods Normally Reared Per Season

Only 1/yr on Great Plains. More than 1 brood in Florida (R. Pruner in litt.) and probably coastal Texas. Rates of single, double, and triple brooding for birds present all breeding season at Monterey Bay, CA, were: females 40–44%, 54–60%, and 0–3%, respectively; and males 59–63%, 38–41%, and 0%, respectively, during 2 yr when no attempt was made to protect nests from predators. After erecting exclosures around most nests to protect them from predators for 1 yr, rates of single, double, and triple brooding were: females 17%, 67%, and 17%, respectively; males 54%, 46%, and 0%, respectively (JSW and JCW).

Life Span And Survivorship

A male, at least 15 yr old when last seen, is oldest known Snowy Plover (JSW and JCW). Most individuals do not approach this age; Paton (1994b) estimated a mean life span of 2.7 years for adults.

Apparent annual adult survival for Snowy Plovers ranged from 0.578 to 0.880 over 4 yr at Great Salt Lake, UT (Paton 1994b). Apparent survival was 64% for males and 57% for females (best-fitting model included sex dependence in survival) on the n. California coast; the difference may be attributed to greater mortality of females or greater breeding site fidelity of males (Mullin 2006).

Apparent survival of Kentish Plovers in the Netherlands was 0.65 ± 0.072 SE for males and 0.61 ± 0.060 SE for females in the first year after banding; for females it was 0.73 ± 0.170 SE in the second year after banding and 0.91 ± 0.085 SE for all years combined except the first year after banding (Foppen et al. 2006). At Tuzla, Turkey the best estimate of apparent adult survival was 0.64 with little difference between males and females (Sandercock et al. 2005).

For Snowy Plovers, mean survival rate from hatching to beginning of first breeding season was 0.179 ± 0.010 SE for chicks banded from 1984-1999 at Monterey Bay, CA (Stenzel et al. 2007) and 0.09 ± 0.01 SE for Kentish Plovers hatchlings at Tuzla Turkey (Sandercock et al. 2005).

Survival of Snowy Plover juveniles from 28 days of age to the beginning of their first breeding season averaged 0.463 ± 0.018 SE (annual range 0.283 ± 0.028 to 0.575 ± 0.061) for juveniles fledging from 1984 to 1999 on the central California coast (Stenzel et al. 2007). Apparent survival was 30% for juveniles on the northern California coast (Mullin 2006). For Kentish Plovers, apparent survival rate of juveniles in the Netherlands was 0.28 ± 0.072 SE (Foopen et al. 2006) and 0.15 ± 0.01 SE at Tuzla Lake, Turkey (Sandercock et al. 2005).

Disease And Body Parasites

Diseases

Adults susceptible to botulism in w. North America (Alcorn 1942, D. Gutherie upubl. data). On the California coast, a Snowy Plover found lying in the wet sand with its wings partially spread and appearing to be uncoordinated was diagnosed as probably having avian tuberculosis (R. Orr in litt). In 2005, >30 adult Snowy Plovers in San Diego County, CA died from an unknown cause characterized by the plovers’ inability to stand and maintain balance (B. Foster in litt).

Body Parasites

Feather lice can be found on many adults handled for banding on the Pacific coast (J. Erbes and D. George in litt.). A necropsy of an adult female Snowy Plover found dead at Oceano Dunes, San Luis Obispo County, CA, in 2006 did not identify a cause of death but did note that trematodes were established in the intestinal tract (D. George in litt.).

Causes Of Mortality

Eggs

Mainly predators (see Behavior: predation); strong winds causing dispersal or burial of eggs in sand (Stein 1993, Colwell et al. 2005); high tides (Warriner et al. 1986, Chase and Gore 1989); rain/hail (Boyd 1972, Hill 1985, Paton 1995, Koenen et al 1996a, Winton et al. 2000); parental desertion (JSW, JCW); and crushing by people, vehicles, or pets (Warriner et al. 1986, Chase and Gore 1989, Colwell et al. 2005). Eggs with well developed embryos are occasionally deserted by the parents because their hatching is delayed by more than 48 h from the rest of the brood (GWP, Fraga and Amat 1996).

Along the Pacific coast, Common Ravens, American Crows, coyote, feral red fox, and striped skunk are the most important predators of Snowy Plover eggs. Roosting flocks of gulls, terns and pelicans trample some nests (GWP). Drifting sand during high winds and tidal over wash also cause some clutch failures (Powell and Collier 2000, Neuman et al. 2004, Colwell et al. 2005, Lafferty et al. 2006).

At inland playa lakes, the main predators of eggs appear to be coyote, striped skunk, Common Raven, California Gull, and Ring-billed Gull. Rain and hail can cause considerable nest loss (Boyd 1972, Page et al. 1993, Koenen et al. 1996a, Winton et al. 2000). At Salt Plains NWR, OK, 8-9 cm of rain on 16 June 1996 destroyed 44 of 109 nests in one day (Winton et al. 2000). Also at Salt Plains NWR, numbers of Ring-billed Gulls have increased in recent years; their predation of Snowy Plover nests was first noted in 1993 and is expected to increase as gull numbers increase (Winton et al. 2000). At the Playa Lakes region of Texas, 44% of 185 failed nests were flooded by rain or destroyed by hail and 28% depredated -- most likely by mammals (Conway et al. 2005).

Chicks

Mainly predators (see Behavior: predation), separation from parent (Warriner et al. 1986), birth deformity (JSW and JCW), failure to break out of egg (JSW and JCW), drowning in high tides (JSW and JCW), trampling by horses (Lee 1989) or people (GWP, Colwell et al. 2005), people preventing adults from brooding chicks (Colwell et al. 2005), killed by unleashed dog (Lafferty et al. 2006), crushed by vehicles (P. Persons pers. comm.), blown away by strong winds (Lafferty et al. 2006), sticking to a tar ball (Lafferty et al. 2006), disease, entanglement in vegetation, death of attending parent, exposure after illness of adult (D. George in litt.), and being pecked to death by an adult defending a neighboring brood (Fraga and Amat 1996, Székely and Cuthill 1999).

Adults

Predators (see Behavior: predation), bouts of cold winter weather (Stenzel et al. 2007), disease, hail (Boyd 1972, Grover and Knopf 1982, Hill 1985), being run over crossing highways (D. George in litt.), being run over while incubating nests on beach (JSW and JCW), being struck by off-road vehicles (R. Glick, in litt.), becoming entangled in fishing line (JSW and JCW), becoming entangled in the net top of a nest exclosure (D. George in litt.), colliding with objects (M. Parker pers. comm.), and shooting (GWP).

Range

Initial Dispersal From Natal Site

Some young from Monterey Bay, CA, do not disperse but become year-round residents; others disperse as early as 1 mo after first flight to wintering areas as far south as San Carlos, Baja Sur, and as far north as Bandon, OR. Some return to Monterey Bay to breed, whereas others nest elsewhere along Pacific Coast from Silver Strand, San Diego CA, to Damon Point, WA (LES). One young female from Monterey Bay, CA moved inland to breed at Mono Lake, CA (Warriner et al. 1986). Two other females and 1 male were seen during summer in e. Oregon without evidence of breeding; the male was not present inland long enough for a nesting attempt (LES). Another male from the Monterey Bay area was observed once on 30 June 2008 at Great Salt Lake, UT (J. Cavitt in litt.).

Female Snowy Plovers are slightly more likely than males to winter in their Monterey Bay, CA natal area but males are more likely to nest there. From 1984 to 1999, an estimated 38.0% of juvenile males and 39.9% of females wintered then bred in the Monterey Bay area; 36.4% of the males and 19.5 % of the females wintered away and then bred at Monterey Bay; 6.3% of the males and 13.7% of the females wintered in the Monterey Bay area but nested elsewhere; and 19.3% of the males and 26.8 % of the females wintered and nested elsewhere (Stenzel et al. 2007). The estimated natal philopatry rate to the Monterey Bay area is 59% for females and 74% for males. The mean distance between the natal site and site of first breeding is greater for females (median 6.9 km, maximum 790 km, n= 238) than for males (median 4.2 km, maximum 360 km, n= 259). Overall, 64% of natal dispersal distances are <10 km and only 16% > 50 km. Among local recruiters, 35% of females and 27% of males breed within 1 km and 73% of females and 76% of males within 10 km of natal sites (Stenzel et al. 2007).

Of 432 banded Snowy Plover chicks, 14.4% returned to their natal area on the northern California coast; males tended to be more philopatric than females; 69.4% of the philopatric plovers bred in their natal habitats; those hatching from late nests were more likely to be year-round residents than those from early nests. Philopatric Snowy Plover males nested 18.2 km ± 33.6 SD and females 16.0 ± 29.2 SD from natal nests; the two furthest dispersal distances of females were 354 and 474 km (Colwell et al. 2007b).

The resighting rate of juvenile Snowy Plovers at natal areas the year following marking was 19% in San Diego County, CA, with males twice as likely to return as females (Powell and Collier 2000) and 14.6% at Lake Abert, OR (Stern et al. 1990). Return rate of juveniles estimated from survival analysis was 21.3% at Great Salt Lake (Paton 1994b).

In Kentish Plovers, only 4% of juveniles were recaptured on study site nests one or more years after their natal year at Tuzla Lake, Turkey; there was no evidence of a sexual difference in return rates (Sandercock et al. 2005). Juvenile return rate was 13% to a breeding area in the Netherlands (Foppen et al. 2006). Of 84 Kentish Plover chicks banded in Spain, 8.3% returned and bred the following year at their natal site (Fraga and Amat 1996).

Fidelity To Breeding Site And Winter Home Range

Adult Snowy Plovers show a high degree of breeding-site fidelity but also disperse among breeding sites both within and between years (Paton 1994a, Stenzel et al. 1994). Adult resighting rates at breeding sites between consecutive years: Monterey Bay, CA: males 76.8%, females 65.8% (Warriner et al. 1986); San Diego County CA: males 72% and females 62% (Powell and Collier 2000); Mono Lake, CA: males 77.8%, females 44.9% (Page et al. 1983); Lake Abert, OR: males 64.1%, females 40.9% (Stern et al. 1990). Lower female rates are due partly to sexual differences in detection rates (Warriner et al. 1986) and dispersal rates (Stenzel et al. 1994).

Adult Kentish Plovers also exhibit a high degree of fidelity to breeding areas. Return rates were at least 54.9% for males and 40.3% for females in s. Spain (Fraga and Amat 1996) and 41% for males and 39% for females at Tuzla, Turkey (Sandercock et al. 2005). For Kentish Plover, there was no evidence that polygamous individuals had lower return rates than monogamous individuals in Spain (Amat et al. 1999b).

Snowy Plovers also exhibit strong site fidelity to wintering areas; about two-thirds of males, females, and immatures from Lake Abert, OR, that were located on their coastal California or Baja California wintering areas were present for 2 consecutive yr, and about one-third for at least 3 yr (Page et al. 1995).

Dispersal From Breeding Site Or Colony

About 50% of adult females and 25% of adult males nesting at Monterey Bay, CA, disperse to other breeding sites at least once during their lifetime; dispersal distances range from 50 to 1,140 km. Dispersal to alternate nesting areas for partial breeding seasons is far more common than for entire seasons by regular nesters at Monterey Bay. Migratory birds are more likely than residents to make these long-distance, breeding-dispersal movements; no relationship between breeding success at Monterey Bay and long-distance breeding dispersal. Individual males and females on California coast moved up to 840 km and 660 km, respectively, for successive nesting attempts within 1 nesting season (Stenzel et al. 1994). See Behavior: spacing: territoriality.

Home Range

Variable with stage of nesting cycle. Males feed up to 4 km from their nests at Mono Lake, CA, and up to 8 km at Monterey Bay, CA (JSW and JCW). Adults with broods found up to 3.2 km from their nests at Lake Abert, OR, and Cheyenne Bottoms, KS (Boyd 1972, Stern et. al. 1990); up to 5 km at Morro Bay, CA (Hutchinson et al. 1987); up to 6 km as early as 2 d after hatching at Mono Lake, CA (GWP); and up to 7 km at Monterey Bay (JSW and JCW). At Great Salt Plains, OK, home range size of radio-tagged Snowy Plovers averaged 917 ha ± 623.7 (n= 14) and did not differ between sexes; birds that lost broods had the largest ranges (L. Hill unpubl. data). Individuals move up to 50 km between sites during winter on California coast.

Population Status

Numbers

Breeding: Information (much of it dated) suggests a breeding population of about 18,000 Snowy Plovers for the Gulf of Mexico coast, the interior of the US, and the Pacific coast of the U.S. and Baja, California, Mexico combined. (Appendix 2). A USFWS survey of Snowy Plovers in the U.S. and Mexico during the summers of 2007 and 2008 should soon be available to provide a more current estimate of the size of the North American breeding population.

Counts of plovers during the breeding season tend to underestimate the number of individuals present. At Mono Lake, CA, sightings of color marked birds on censuses, indicated on average 1.90 marked males and 2.95 females were present in the area for each one seen on a survey (Warriner et al. 1986). In the lower Laguna Madre region of Texas, Hood and Dinsmore (2007a) estimated a survey detection probability of 0.58 ± 0.04 SD and extrapolated a population of 416 plovers using an occupancy abundance estimation technique. Concurrently, Zdravkovic (2004) counted 456 plovers in the same area.

Winter: On the U.S. Pacific Coast there were an estimated 4,000 wintering plovers in the early 1980’s (Page et al. 1986) and 355-499 inland in s. California during the winters of 1993-94 and 1994-95; most of the interior birds were at the Salton Sea and in agricultural waste water ponds in the San Joaquin Valley (Shuford et al. 1995). Counts on the California coast each winter from 2003-04 to 2008-09 averaged 3708 ± 548 SD plovers; the number along the Pacific coast of the U.S. averaged 3849 ± 532 SD during the same period (GWP, USFWS unpubl. data). An estimated 1605 wintering Snowy Plovers are reported for the west coast of Baja, California, Mexico from shorebird surveys conducted in the early 1990’s (Page et al. 1997).

Along the Gulf of Mexico coast 1191 birds were tallied in Laguna Madre region of Tamaulipas Mexico on a 1997 survey (Mabee et al. 2001). A 2000-01 survey of the U.S. Gulf coast tallied the following winter numbers: Texas 690, Louisiana 36, Mississippi 13, Alabama 0, and Florida 311; also 16 birds were found in Cuba and 17 in Puerto Rico (Elliott-Smith et al. 2004). In 2002, state-wide winter surveys produced counts of 252-305 individuals in Florida (Doonan et al. 2006).

Trends

The breeding population has likely decreased on the Gulf Coast since the late 1800s owing to habitat alteration and increased recreational use of beaches (Chase and Gore 1989, T. Eubanks pers comm.). The number of pairs along the Florida coast was estimated as 167 in 1989, 213 in 2002, and 222 in 2006, suggesting a fairly stable population during the past 2 decades (Himes et al. 2006). There are no studies of recent trends for the coast of Texas (Hood and Dinsmore 2007a). On the Great Plains, numbers along the Cimmarron River in sw. Kansas and nw. Oklahoma have decreased since the mid-1980s owing to loss of river flow (R. Boyd pers. comm.). A historic population decline is indicated along the Pacific coast by the large number of pre-1970 breeding locations that were inactive by 1980 (see Distribution: historical changes). For California, Oregon, Washington, and Nevada combined, there was an approximate 20% decline in size of the breeding population on surveys between the late 1970s and late 1980s (Page et al. 1991).

For coastal Washington, Oregon and California combined, total adult plovers on coordinated breeding-season counts rose from 1,493 in 2002 to 2,017 in 2004 and subsequently declined to 1,537 and 1,541 in 2007 and 2008, respectively (GWP, USFWS). In California, coast-wide breeding season surveys in 1977-80 totaled 1,593 adult plovers. Follow up surveys in 1989 and 1991 produced 1,371-1,376 plovers. Only 976 plovers were tallied on a 2000 count. Between 2002-2004 numbers increased from 1,387-1,904 adults, then declined to 1,680-1,719 in 2005-2006, and decreased further to 1,362-1,394 birds in 2007-2008 (GWP).

Inland, at Owens Lake, CA, there was a steep decline on breeding season counts from 499 adult plovers in 1978 to 198 in 1988. Nine counts from 1990-2001 averaged 138 ± 11 SE adults (range= 101-203). After introduction of shallow-water flooding for dust control in 2002, numbers increased annually to a peak of 658 adults in 2004 (Ruhlen et al. 2006). From 2005-08, breeding season counts averaged 502 ± 75.6 SD (range 421-602, n= 4) (GWP). At Mono Lake CA, 240 km north of Owens Lake, a decline is apparent from surveys locating 384 adults in 1978, 342 in 1988, 119 in 2001, 98 in 2002, and 71 in 2007 (Ruhlen et al. 2006, GWP).

Population Regulation

Page et al. (1983) suggest that density-dependent rates of clutch destruction by predators could be a factor limiting population size. Reduction in the amount of suitable breeding habitat, particularly along the U.S. Pacific and Gulf coasts, has undoubtedly been responsible for a reduction in size of the breeding population since the late 1800s.

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