Recovery Plan: Population Status and Distribution

Trends Prior to 1985

Historical population trends for the Atlantic Coast piping plover have been reconstructed from scattered, largely qualitative records. Nineteenth-century naturalists, such as Audubon and Wilson, described the piping plover as a common summer resident on Atlantic Coast beaches (Haig and Oring 1987). By the beginning of the 20th century, uncontrolled hunting, primarily for the millinery trade, and egg collecting had greatly reduced the population, and in some areas along the Atlantic Coast the piping plover was close to extirpation. Following passage of the Migratory Bird Treaty Act in 1918 and changes in the fashion industry, piping plover numbers recovered to some extent (Haig and Oring 1985).

Raithel (1984) showed that Rhode Island piping plover numbers reached a 20th-century peak following the 1938 hurricane, which flattened dunes and destroyed shoreline developments. Rhode Island piping plover numbers declined after World War II, as habitat was lost to dune stabilization efforts and summer home construction. The population partially recovered following another severe hurricane in 1954 before beginning a steady decline which continued through the early 1980's.

Wilcox (1959) documented major fluctuations in piping plover numbers between Moriches Inlet and the village of Southhampton on Long Island, which he correlated with habitat changes. An increase from 20 pairs before the hurricane in 1938 to 64 pairs in 1941 attests to the piping plover's ability to rapidly colonize newly available habitat. The population then declined as habitat was lost to dune stabilization, summer homes, and road construction.

Available data suggest that the most recent Atlantic Coast-wide population decline began in the late 1940's or early 1950's (Haig and Oring 1985). Starting in 1972, the National Audubon Society's "Blue List" of birds with deteriorating status included the piping plover. Johnsgard (1981) described the piping plover as "... declining throughout its range and in rather serious trouble." The Canadian Committee on the Status of Endangered Wildlife in Canada designated the piping plover as "Threatened" in 1978 and elevated the species' status to "Endangered" in 1985 (Canadian Wildlife Service 1989).

Reports of local or statewide declines between 1950 and 1985 are numerous and many are summarized by Cairns and McLaren (1980) and by Haig and Oring (1985). While Wilcox (1939) estimated more than 500 pairs of piping plovers on Long Island, a 1990 survey recorded 197 pairs (Litwin et al. 1993). B. Blodget (Massachusetts Division of Fisheries and Wildlife, pers. comm. 1991) reports that there was little focus on gathering quantitative data on piping plovers in Massachusetts through the late 1960's, because the species was commonly observed and presumed to be secure. However, numbers of pairs of breeding piping plovers declined 50-100% at seven Massachusetts sites between the early 1970's and 1984 (Griffin and Melvin 1984). Further, recent experience of biologists surveying piping plovers has shown that counts of these cryptic birds sometimes go up with increased census effort. This suggests that some historic counts of piping plover numbers by one or a few observers, who often recorded occurrences of many avian species, may have underestimated the piping plover population. Thus, the magnitude of the species' decline may have been even more severe than available numbers imply.

Trends Since Listing under the Endangered Species Act

Table 4 and Figure 3 summarize 1986-1995 nesting pair counts furnished to the U.S. Fish and Wildlife Service by the state wildlife agencies and Canadian Wildlife Service (CWS). Table 5 compares 1991 and 1994 nesting pair counts shown in Table 4 with those obtained during the 1991 International Census and similar "window" censuses conducted in 1994. Estimates drawn from Table 4 are based on methodologies that vary slightly among the states and that, in most cases, may result in some double counting of birds that renest during the season. The 1991 International Census reflected a single survey of breeding sites conducted during the peak of the nesting season, June 1-9, 1991. A similar window census was conducted between May 28 and June 5, 1994. Most state coordinators believe that the International Census methodology undercounts their plover populations because some plovers that nest before or after are unpaired during the census window. The actual 1991 and 1994 nesting populations probably lie somewhere between the two figures shown in Table 5.

Table 5. Comparison of 1991 and 1994 Population Estimates Based on
"Window" Census with Estimates Based on 1990 Census Methodologies

[NOTE: For the most current population estimates, see the Annual Status Report.]

STATE/REGION	1991		1994
	Estimate Based on "Window" Census	Estimate Based on 1990 Census Methodology	Estimate Based on "Window" Census	Estimate Based on 1990 Census Methodology
Maine	18	18	33	35
Massachusetts	148	160	329	352
Rhode Island	22	26	29	32
Connecticut	30	36	25	30
New England	218	240	416	449
New York	181	191	209	209 1
New Jersey	122	126	102 2	124
NY-NJ Region	303	317	311	333
Delaware	5	5	2	4
Maryland	16	17	30	32
Virginia 3	131	131	96	96
North Carolina	30	40	51	54
South Carolina	1	1	-	-
Southern Region	183	194	179	186
U.S. Total	704	751	906	968
Atlantic Canada	236	236	182	182
Atlantic Coast Total	940	987	1088	1150
1 In 1994, New York adopted the window count as its standard census methodology. 2 The 1994 New Jersey window census was conducted by relatively inexperienced surveyors. State biologists believe that some birds were present but undetected during thew "window," and that the actual State population is cloer to the estimate based on 1990 methodology. 3 Virginia uses only a window census.

The apparent rangewide increase in numbers of pairs from 790 pairs in 1986 to 957 pairs in 1989 is thought to at least partially reflect the effects of increased survey effort following the proposed listing in 1985. Intensified survey effort may have played an especially important role in population estimates for three states:

North Carolina: 1986-87 estimates were made by compiling results of site surveys from previous years (R. Dyer, U.S. Fish and Wildlife Service, pers. comm. 1993). The first comprehensive state-wide field survey in North Carolina was conducted by volunteers in 1988 (Carter 1988). Piping plover research conducted on the two National Seashores that together account for more than 80% of the North Carolina population in 1989 and 1992-94 provided intensive search effort in those years (Coutu et al. 1990, McConnaughey et al. 1990, Collazo et al. 1994). LeGrand (1991) states that, while the North Carolina population trend over the last few decades is unknown, "it can be assumed that the apparent increase in the past 10 years is due to much better survey coverage, especially on the relatively remote Core Banks and Portsmouth Island."

New York: K. Wich (NY State Department of Environmental Conservation, in litt. 1993) states that although protection of beach nesting birds in New York increased after 1983, survey effort also intensified, especially at sites such as Breezy Point, Queens County and Westhampton Beach, Suffolk County. While the relative contributions of each cannot be determined, he believes that "the stability of more recent estimates probably accurately reflects the status of New York's plover population." Ducey-Ortiz et al. (1989) documented an increasing plover monitoring effort in New York between 1984 and 1988 and found that, when results from 54 uniformly monitored sites in that State were analyzed, the population trend did not increase or decrease significantly.

Downer and Leibelt (1990) likewise cite intensified survey effort as a major contributor to the increased estimate of the New York population between 1984 and 1989. Furthermore, inferences that the apparent 1986-88 New York population gain was caused by increased efforts to protect beach nesting birds in that State fail to explain why the State population estimate has remained static since 1989, despite continuing improvements in protection.

New Jersey: C.D. Jenkins (New Jersey Division of Fish, Game and Wildlife, in litt. 1993) conjectures that increased survey intensity accounts for one-quarter to one-third of the population increase observed between 1987 and 1989 in New Jersey.

The Recovery Team believes that increases in U.S. Atlantic Coast population estimates between 1989 and 1995 reflect the actual population trend. However, the net increase of 426 pairs was very unevenly distributed. The New England subpopulation increased 346 pairs (+168%), while the New York-New Jersey and the Southern (DE-MD-VA-NC) subpopulations gained 62 (+19%) and 18 (+9%) pairs, respectively.

Census data suggest that the overall piping plover population in Atlantic Canada is declining (Flemming and Gautreau in CWS 1994; B. Johnson, Canadian Wildlife Service, in litt. 1994). Estimates obtained during the 1991 International Census reflect by far the most intensive survey effort to date for the Canadian portion of the plover's Atlantic Coast range. During the second half of the 1980's and through 1991, numbers of breeding pairs appeared stable or slightly improving in Newfoundland, Quebec, New Brunswick, and Prince Edward Island (provincial summaries in CWS 1994). A decline from 66-71 pairs counted in Nova Scotia in 1983 to 48-54 pairs in 1987 seemed to have been arrested but not reversed as of 1991 (Austin-Smith et al. in CWS 1994).

A comprehensive census of all sites that were occupied by plovers in 1991 was conducted in 1994. Results of that census suggest that the Atlantic Canada subpopulation is currently experiencing a sharp decline, except in Newfoundland (eight pairs and one single adult in 1994 compared with three pairs and one single adult in 1991) and the Magdalen Islands (up to 48 pairs in 1994 from 38 in 1991). Substantial declines were recorded in New Brunswick (63 pairs and 19 single adults in 1994, compared with 203 adults (91 pairs) in 1991) and Prince Edward Island (26 pairs and eight single adults, compared with 110 adults (51 pairs) in 1991). Reports from Nova Scotia placed the Provincial population at 37 pairs and 8 single adults compared with 110 adults (51 pairs) in 1991.

Some of this apparent decline may be attributable to surveying only the sites that were occupied in 1991, and it is possible that some birds nesting at sites that were unoccupied in 1991 were undetected in 1994. Surveys conducted in 1995 showed an overall increase in the Atlantic Canada subpopulation, from 182 pairs in 1994 to 199 pairs in 1995 (the latter figure includes three pairs in St. Pierre-et-Miquelon) (D. Amirault, Canadian Wildlife Service, in litt. 1995). The possibility that some plovers that formerly nested in Atlantic Canada have shifted their breeding sites to New England or other parts of the range also cannot be conclusively ruled out, but information about plover dispersal patterns gained from studies of banded birds (see pages 22-23) suggests that this is unlikely to be a substantial contributor to the overall downward trend in Canadian plover numbers. It is anticipated that results of the upcoming 1996 International Census and comparison with 1991 data will furnish the most accurate indicator of the five-year trend in the Atlantic Canada subpopulation.

Productivity

Comparisons of productivity data reported prior to 1989 were confounded by inconsistent definitions of "fledged young" and reporting methods (e.g., some reports provided fledged chicks per nesting pair while others provided the number of nests fledging at least one young). Beginning in 1989, the U.S. Fish and Wildlife Service adopted "25 days of age or flying (whichever comes first)" as the standard definition of a fledged chick for the purposes of tracking plover productivity on the U.S. Atlantic Coast (USFWS 1988b). (It should be noted that 25-day-old chicks are often unable to fly, and therefore, may remain vulnerable to off-road vehicles and other sources of mortality.) Since the vast majority of chick losses in most studies occurred during the first 15 days post-hatch (Elias-Gerken 1994, Loegering 1992, Coutu et al. 1990, MacIvor 1990, McConnaughey et al. 1990), data on chick survival for periods of less than 25 days may be informative, but care should be exercised when making comparisons among data sets.

Population modeling by S.M. Melvin and J.P Gibbs (1994) (see Appendix E) yielded an estimate of 1.24 chicks fledged per pair needed to maintain a stationary population. However, modeled populations with this productivity rate remained highly vulnerable to extinction (35% probability of extinction within 100 years for a 1,200-pair population with mean productivity of 1.25 chicks per pair). Modeling also revealed that extinction probabilities are very sensitive to changes in productivity. For example, extinction probability over 100 years for a 2,000-pair population with observed survival rates was 4% when average productivity was 1.50 chicks per pair; this extinction probability increased to 22% when other parameters were held constant and average productivity was 1.25 chicks per pair. The probability that the population would drop below 500 pairs over 100 years increased from 26% when average productivity was 1.5 chicks per pair to 82% when average productivity was 1.25 chicks per pair.

Table 6 and Figure 4 summarize productivity data from 1987 to 1995. Averages reflect data from 95% of nesting pairs in New England, 73% in New York-New Jersey, and 61% in the southern states. In general, the seven-year weighted averages correlate with population trends observed since 1989. New York and North Carolina productivity figures, which are below those needed to effect population growth, support the concept that the apparently large increases in those States' population estimates between 1986 and 1989 are due to increased survey effort (see discussion on page 22). Average productivity figures for Atlantic Canada appear to be high for a declining population, but productivity data are available for only 43% of nesting pairs. Since productivity data are often gathered at sites that are also the most intensively protected, available data may not be representative.

Survival

Resightings of 103 adult plovers and 61 chicks color-banded on Outer Cape Cod between 1985 and 1988 yielded estimates of mean annual survival of 0.74 for birds > 1 year old and 0.48 for chicks (see Appendix E). Loegering (1992) estimated annual survival rates of 0.67-0.72 for 53 adults and 0.41 for 29 chicks banded on Assateague National Seashore in Maryland between 1987 and 1989. R. Cross (Virginia Department of Game and Inland Fisheries, unpubl. data) estimated annual survival rates of 0.75 and 0.83 for adults and 0.44 for chicks.

Population viability modeling (Melvin and Gibbs 1994; Appendix E) shows that extinction probabilities are also very sensitive to changes in survival rates (such long-term declines in survival rates could occur due to continuing declines in availability or quality of wintering or migration habitat; increased human disturbance on wintering grounds; increased mortality due to disease, parasites, or environmental contaminants; increased predation; or reduced longevity or fitness due to unforeseen genetic factors). For example, modeling showed a 4% extinction probability over 100 years for a 2000-pair population with average productivity of 1.5 chicks per pair and survival rates observed on outer Cape Cod, Massachusetts between 1985 and 1988. When declines in adult and chick survival rates of 5% and 10%, respectively, were modeled holding other parameters constant, the extinction probability increased from 4% to 32%, and the probability that population size would drop below 500 pairs increased from 26% to 90%.

Current Breeding Distribution

Breeding Site Fidelity and Dispersal

In New York, Wilcox (1959) recaptured 39% of the 744 adult plovers that he banded in prior years (many were recaptured during several successive seasons and all but three of them were retrapped in the same nesting area), but recaptured only 4.7% of 979 plovers that he banded as chicks. He also observed that males exhibited greater fidelity to previous nest sites than females. Strauss (1990) observed individuals that returned to nest in his Massachusetts study area for up to six successive years. Also in Massachusetts, 13 of 16 birds banded on one site were resighted the following season, with 11 nesting on the same beach (MacIvor et al. 1987). Of 92 adults banded on Assateague Island, Maryland, and resighted the following year, 91 were seen on the same site, as were 8 of 12 first-year birds (Loegering 1992). R. Cross (unpubl. data) reports that 10 of 12 juveniles banded on Assateague Island, Virginia and resighted one and/or two years later were on the Virginia or Maryland portions of Assateague Island, while the other two were observed on other Virginia barrier islands.

On the Atlantic Coast, almost all observations of inter-year movements of birds have been within the same or adjacent states. Of 316 birds color-marked in Massachusetts between 1982 and 1989 (L.H. MacIvor, C.R. Griffin, and S.M. Melvin, unpubl. data; Strauss 1990), only one instance of subsequent nesting outside of that state (in Connecticut) has been observed (S.M. Melvin pers. comm. 1993). Two of 121 plovers banded on Assateague Island were resighted breeding in New Jersey; one resighting took place during the same breeding season as the banding, while the second bird moved to New Jersey the following year (Loegering 1992). Because banding of Atlantic Coast piping plovers ceased after 1989 (see discussion on page 87), it is possible that more birds are now dispersing from highly productive states, although a strong correlation between high productivity and an increasing subpopulation continues in New England. If populations in some areas approach carrying capacity of available habitat, it is possible that dispersal rates will increase.

Habitat Carrying Capacity

The carrying capacity of habitat to support breeding plovers is subject to fluctuation with the dynamic coastal formation processes that affect topography, vegetation, and other habitat characteristics. These fluctuations can be affected by natural factors, such as long-shore sand transport patterns and storm frequency, and by human intervention through shoreline development and stabilization projects (see discussion of loss and degradation of breeding habitat, pages 34-37). For this reason, estimates of carrying capacity, especially on a local basis, may be subject to change over time, and may require periodic revision to reflect changes in habitat conditions.

While it is expected that carrying capacity will fluctuate locally, and perhaps even within a state over time, it is anticipated that long-term carrying capacity of the Atlantic Coast's piping plover habitat (and that of regional subpopulations, which correspond to the recovery units laid out on page 55) will be maintained if natural coastal habitat formation processes are not interrupted . Shoreline development and stabilization projects may, however, erode carrying capacity locally and regionally (see pages 34-37) and, therefore, have potential to compromise the survival and recovery of the population.

Appendix B provides estimates of carrying capacity of current and potential U.S. breeding sites in 1993. These estimates, made by the state plover coordinators in consultation with the Recovery Team and, in some cases, biologists who manage specific sites, were compiled to appraise the carrying capacity for the entire U.S. Atlantic Coast portion of the plovers' range in order to facilitate population viability analysis (see Appendix E). In some cases, estimates were based on knowledge that a site supported a larger population at one time on a site where habitat characteristics have remained similar during the intervening years. Other estimates were based on information about current activities on a site, recent productivity data, and knowledge of population densities at other sites with comparable habitat. Biologists based their projections on the assumption that most traditional human uses of the site would continue, although increased intensity of management efforts (including curtailing of off-road vehicle use) might be needed to attain capacity estimates on some sites. Estimates were also designed to be below levels at which density-dependent effects on productivity would be triggered. The recovery team believes that the carrying capacity of more than 1,925 pairs estimated for U.S. Atlantic Coast in 1993 (Appendix B) is very conservative. For example, revised estimates made by Massachusetts Division of Fisheries and Wildlife (MDFW) in 1995 place the carrying capacity of habitat in that State at over 1100 pairs (MDFW 1996). Furthermore, this upward revision of Massachusetts' carrying capacity was primarily due to an increase in projected nesting densities to 16-24 pairs per linear mile in the highest quality habitats, based on observations of productive nesting pairs approaching those densities in a rapidly increasing population, not on changes in habitat characteristics (S.M. Melvin pers. comm. 1995). However, all carrying capacity estimates in Appendix B, including those for New England, are based on much lower projections of nesting densities. Furthermore, in order to allow for the possibility that plover habitat requirements may be more stringent at the edge of the range than the core, estimates for the southern recovery unit are substantially more conservative than those for New England.

In Atlantic Canada, no systematic effort to estimate carrying capacity of all breeding habitat has been conducted; however, available information suggests that recent population numbers are far below carrying capacity. Based on analyses of nesting patterns between 1987 and 1992 in the Magdalen Islands, Shaffer and Laporte (1992) have projected capacity for 65 pairs, where 48 were counted in 1994. On Prince Edward Island, 57 beaches with suitable piping plover habitat were surveyed in 1991, but plovers were located at only 20 of these sites (McAskill et al. in CWS 1994). K. Knox (Newfoundland Wildlife Division, in litt. 1993) estimated that three sites where seven pairs bred in 1993 could support 20 pairs, while a currently unoccupied beach adjacent to one site could support another 8-10 pairs. Biologists surveying 24 sites in Antigonish, Pictou, and Shelbourne Counties in Nova Scotia estimated that these beaches could furnish habitat for more than 65 pairs, compared with the 20 pairs they actually located there in 1994 (M. Goldin, The Nature Conservancy, in litt. 1994; S. von Oettingen, U.S. Fish and Wildlife Service, in litt. 1994). R. Williams (Nova Scotia Department of Natural Resources, in litt. 1993) estimated that six sites where 10 pairs nested in Queens County, Nova Scotia in 1993 could support 19 pairs if the regional population were to expand.

Data from Outer Cape Cod where the number of breeding pairs quadrupled between 1988 and 1993 show that relatively high nesting densities can be achieved without a loss of productivity (Figure 5). The breeding population at the Sandy Hook Unit of Gateway National Recreation Area in New Jersey grew from 18 pairs in 1990 to 36 pairs in 1994, and, again, productivity increased steadily over that time period, from 1.17 chicks per pair in 1990 to 1.94 in 1994 (Jenkins 1993, C.D. Jenkins in litt. 1993 and 1994). In Maryland, the plover population on Assateague Island increased from 19 pairs in 1993 to 44 pairs in 1995, yet high productivity -- 2.41 and 1.73 chicks per pair -- was achieved in both 1994 and 1995, respectively. Other examples of population increases attended by high productivity in New England are cited under Nesting Densities.

Vulnerability to Extinction

Demographic Factors

The population viability analysis (PVA) conducted by Melvin and Gibbs (1994) to assess the risk of population extinction (Appendix E) estimated probabilities of extinction as well as probabilities that the population would fall below thresholds of 50, 100, and 500 pairs during the next 100 years. Important model inputs, including fecundity (number of chicks fledged per pair) and mean annual survival rates for immature (less than one year old) and mature piping plovers, were based on actual field data.

Melvin and Gibbs (1994) calculated a mean fecundity of 1.21 chicks fledged per pair during the five-year period 1989-1993 for the U.S. portion of the Atlantic Coast population. The modeled scenarios that most closely approximate the current status of the Atlantic Coast piping plover population -- 1,200 and 1,500 pair populations with average productivity of 1.25 chicks per pair -- showed, respectively, extinction probabilities of 35% and 31% over 100 years, and 95% and 92% probabilities of the population dropping below 500 pairs during the same time period. Furthermore, the overall vulnerability to extinction is exacerbated by the fact that increases in both annual Atlantic Coast average fecundity and population over the last five years are largely attributable to the New England portion of the range. Because of their smaller size, subpopulations face an even larger risk of extirpation, and this is especially true in areas outside New England where average fecundity has been substantially below the coast-wide average.

The PVA indicates that extinction probabilities for Atlantic Coast plovers are very sensitive to changes in fecundity and survival rates and variability within these parameters (see pages 24 and 27). While extinction probabilities are less sensitive to initial population size, this does not diminish the importance of population size to population survival. Increasing population size will delay time to extinction, allowing implementation of measures to improve survival and productivity rates. The larger and more dispersed the Atlantic Coast population is, the less will be the overall effects of environmental stochasticity, catastrophes, or inconsistent management.

Genetic Factors

In addition to effects of demographic factors, modeled in the PVA, populations may be vulnerable to extinction due to loss of genetic diversity. The risk of loss of genetic diversity is related to effective population size (Ne), i.e., the number of individuals actually passing their genes on to the next generation. An Ne of 500 was cited by Franklin (1980) and Frankel and Soulé (1981) as the minimum necessary to maintain long-term genetic fitness and evolutionary potential. No formal estimates of Ne/N have been made for the Atlantic Coast piping plover. It appears that a large percentage of breeding plovers fledge young that are subsequently recruited into the breeding population, but the species' sparse distribution results in highly non-random mating that may pose a barrier to gene flow.

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