Population Trends of North American Shorebirds Based on the International Shorebird Survey Marshall A. Howe, Paul H. Geissler* &

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Biological Conservation 49 (1989) 185-199

Population Trends of North American Shorebirds Based on the International Shorebird Survey

Marshall A. Howe, Paul H. Geissler* Patuxent Wildlife Research Center, US Fish and Wildlife Service, Laurel, Maryland 20708, USA & Brian A. Harrington Manomet Bird Observatory, Manomet, Massachusetts 02345, USA

(Received 16 May 1988; revised version received 10 January 1989; accepted 12 January 1989)

A BS TRA C T

Shorebirds Charadrii are prime candidates Jor population decline because of their dependence on wetlands that are being lost at a rapid pace. Thirty-six of the 49 species of shorebirds that breed in North America spend most of the year in Latin America. Because populations of most species breed and winter at remote sites, it may be most feasible to monitor their numbers at migration stopovers. In this study, we used statistical trend analysis methods, developed .for the North American Breeding Bird Surw,y, to analyze data on shorebird populations during southbound migration in the United States. Survey data were collected by volunteers in the International Shorebird Survey ( ISS). The analyses indicate that whimbrels Numenius phaeopus, short-billed dowitchers Limnodromus griseus, and sanderlings Calidris alba have undergone statistically significant declines. Methodological concerns over both the ISS and the trend analysis procedures are discussed in detail and biological interpretations of the results are suggested.

INTRODUCTION

Reliable data on population size and change are basic to evaluating the conservation status of species. Because absolute numbers are usually * Present address: Migratory Bird Management Office, US Fish & Wildlife Service, Laurel, Maryland 20708, USA. 185 Biol. Conserv. 0006-3207/89/$03"50 ,,~'~ 1989 Elsevier Science Publishers Ltd, England. Printed in Great Britain 0045007

186 Marshall A. Howe, Paul H. Geissler, Brian A. Harrington impossible to obtain for large populations, one is often forced to rely on indices that reflect proportional changes in population from year to year. The North American Breeding Bird Survey (BBS) is one of the best examples of a broad geographic population sampling scheme that provides indices to population change (Bystrak, 1981). The Route Regression Method (Geissler & Noon, 1981), developed by Patuxent Wildlife Research Center, uses raw BBS data to generate estimates of rate and direction of species-specific population change over any given range of years. This method is based on the assumption that changes in populations sampled along roadsides are representative of changes in the entire population of each species. The validity of this assumption is enhanced by the stratified random selection of BBS routes and by the spatial stability of most breeding bird populations. The BBS is primarily suited to species that are both detectable from roadsides and uniformly or randomly distributed within suitable habitat. It is considered an effective index of population change for about 240 species of North American birds. Other methods are required to survey species not detectable from roadsides or whose distributions are highly clumped. Shorebirds fall into this category because their breeding grounds are not readily accessible and their distribution patterns in migration and winter tend to be highly clumped. It is important that reliable monitoring methods for shorebirds be developed, as most species are dependent upon vulnerable wetland habitats. These habitats have been disappearing at a rapid pace (Howe, 1987). Forty- six percent of the wetlands that existed in the United States at the time of European settlement have been lost to a variety of man's activities (Tiner, 1984). Total wetland area destroyed between the mid-1950s and the mid- 1970s alone is equal to twice the size of the State of New Jersey (Tiner, 1984). Comparable rates of wetland loss have recently been documented for Latin America, where most species spend the winter (Scott & Carbonell, 1986). Shorebirds often congregate in vast numbers in certain of the remaining wetlands, from which they procure food critical for fueling long migration flights or for winter survival. If wetland habitats continue to be lost or degraded, we should eventually expect to see this pattern reflected in declining shorebird populations. The difficulty of sampling shorebird populations on a regular basis in remote arctic breeding grounds or Latin American wintering grounds (where 36 of the 49 species that breed in North America spend most of the year) leads one to consider the possibility of sampling populations during migration through the United States. Although migration counts are not typically used as an index to total populations, two studies (Svensson, 1978; Hussell, 1981) have found parallels between migration counts of passerines and breeding counts conducted the same years. A substantial data set of 0045008

Population trends in shorebirds 187 shorebird surveys at southbound migratory stopovers, during 1972-83, already exists. These data were collected by volunteer collaborators as part of the International Shorebird Survey (ISS) administered by Manomet Bird Observatory (MBO) in Massachusetts and the Canadian Wildlife Service. In this paper, we describe an analysis of the United States subset of these data, complete through 1982 and including part of 1983, to derive estimates of population trends of North American shorebirds.

METHODS

Structure of the International Shorebird Survey

Unlike the BBS, sampling areas in the ISS are neither preselected randomly nor of a transect design. The purpose of the ISS has been to identify important stopovers and to enhance knowledge of migration routes, rather than to obtain statistically defensible estimates of population change. To accomplish ISS goals, volunteer participants attempt to visit specific stopovers at 10-day intervals between 1 July and 1 December, the inclusive period of most southbound shorebird migration. In reality, few people are able to achieve such intensive coverage and many sites are surveyed only a few times annually or even missed entirely in some years. Survey sites are distributed throughout eastern North America. The great majority lie on the Atlantic coast and a few observers report from the Caribbean and South America. The highest density of sites is between Maine and North Carolina. Special attention is paid to ensuring coverage of sites that traditionally attract large numbers of shorebirds. At each site the total number of each species is estimated by direct count. In tidal areas, an effort is made to survey the sites during the same segment of the tide cycle each time to minimize the effect of local movements in response to tidal fluctuations. Each survey is summarized on forms provided by M BO. The forms are checked and edited by MBO staff and the data are computerized.

Editing of the ISS data set for trend analysis

To be suitable for trend analysis, the ISS data must meet the same assumption that is made for the BBS, that the sample of birds counted is consistently representative of the total population. For several reasons this assumption cannot be made for ISS data: (1) survey sites are not selected in a random or other statistical manner (emphasis is placed on sites known to be traditionally important and some important sites are relatively inaccessible and not covered); (2) because migrating shorebirds move through at varying 0045009

188 Marshall A. Howe, Paul H. Geissler, Brian A. Harrington

rates determined by physiological and climatological factors, major flights in and out of a stopover could take place between consecutive sampling times; and (3) many species prefer inland freshwater sites, opportunistically selecting wetlands that have the proper water conditions at any given time. Despite these drawbacks, acceptance of certain assumptions about the migration ecology of shorebirds and careful selection of species to be analyzed can increase the probability that ISS samples are representative of species' whole populations. If analyses are confined to coastal sites and to species that are obligate users of the coast, the chance of overlooking major segments of the migrating population is decreased. We know from several studies not only that many stopovers traditionally attract large numbers of shorebirds (Senner & Howe, 1984) but also that returns of individual birds of at least some species to the same stopovers in successive years occur much more often than would be expected by chance alone (Smith & Houghton, 1984; B. A. Harrington, unpublished data). This fact lends credence to the notion that approximately the same breeding populations are being sampled at a given migratory stopover in successive years. Clearly, however, the time that the birds are sampled is of critical importance. Most species exhibit sharp population peaks (or multiple peaks representing differential migration of adult males, adult females, and juveniles) during migration. Therefore, the interval between sampling visits should be short enough to intercept the peaks, which may vary somewhat from year to year. Several studies suggest that, early in southbound migration, lengths of stay of individual birds at stopovers average two to three weeks. From these limited data, we tentatively conclude that the ten-day sampling interval the ISS attempts to achieve is adequate. Based on the above considerations, we conducted two subsetting procedures on the ISS data set for 1972-83 as follows: the first eliminated all but Atlantic coastal sites in the United States; the second deleted data for species that frequently use inland freshwater sites. The remaining data set consisted of surveys of 12 species of shorebirds at Atlantic coastal stopovers only. The species in this data set are black-bellied plover Pluvialis squatarola, semipalmated plover Charadrius semipalmatus, greater yellowlegs Tringa melanoleuca, lesser yellowlegs T. flavipes, willet Catoptrophorus semipalmatus, whimbrel Numenius phaeopus, ruddy turnstone Arenaria interpres, red knot Calidris canutus, sanderling C. alba, semipalmated sandpiper C. pusilla, least sandpiper C. minutilla, and dowitcher Limnodromus spp. Dowitchers sampled were presumably almost entirely short-billed dowitcher L. griseus but some individuals of the similar- appearing long-billed dowitcher L. scolopaceus were undoubtedly included in the data set. 0045010

Population trends in shorebirds 189

To determine the peak periods of migration for each species, survey sites were pooled within each 2-degree latitude interval from Maine to Florida for each year and population curves constructed over the period July through November based on the average of all counts conducted within successive five-day blocks (Fig. 1). The peak date for each latitude interval in each year was determined by visual inspection and that date used as the peak for all sites within that latitude interval. In instances where peaks were not easily identified for a given year, an average peak for that latitude interval was used, based on all years combined. Years were averaged only when absolutely necessary, because peaks often varied by several weeks among years. In all instances where multiple peaks were evident, the earliest (usually the largest) peak was chosen because this should consist almost entirely of the adult population and avoid the confounding effect of annual recruitment. Effective recruitment is reflected in the adult population of subsequent years. In the trend analyses, the individual survey site is analogous to the individual route in the BBS. For any given BBS route, the one count per year for a species is the population value for that species on that route. For the ISS site, there may be as many as 100 counts in a year, covering a period during which populations fluctuate dramatically. The problem of deriving a population count that is representative of a given site in a given year is not a

SEMIPALMATED SANDPIPER MEAN COUNT

1250

1000

750

500

250

10 20 3'0 9 19 29 8 18 28 8 18 28 7 17 27 JULY AUGUST SEPTEMBER OCTOBER NOVEMBER Fig. I. Seasonal pattern of abundance of migrating semipalmated sandpipers on the US Atlantic coast between 41 and 43' North latitude, 1 July-1 December 1972 83 (based on averages of all counts at all sites within this latitude range pooled over 5-day intervals). 0045011

190 Marshall A. Howe, Paul H. Geissler, Brian A. Harrington trivial one. After experimenting with four different measures of population, we determined that the best estimate is obtained by averaging the highest three counts (or two if only two surveys were conducted) within a 21-day period centered on the peak population date for each species in each year and interval. For each year, sites that did not meet the minimum criterion of two data points per 21-day period centered on the peak time were deleted from the analysis. Although this reduced the number of sites used in the trend analyses, it eliminated the extraordinarily high variances associated with inclusion of counts outside of the peak period.

Trend analysis procedures

The analyses applied to these average counts were essentially the same as that used for BBS data. Population indices, combining data from all sites, are calculated for each species each year. These annual indices (shown in Figs 2-7) represent the geometric mean number of birds per site. Viewed over a period of years, the graphed indices depict the direction and relative magnitude of population change at the survey sites from year to year. A calculated adjustl'nent for sites not surveyed in a particular year assures that omission of a traditionally important site does not unduly bias the annual index. Population trends, representing the average proportional increase or decrease of the population over the range of years examined, are computed independently of the annual indices. Trends are computed for each site and averaged over all sites, each weighted according to the magnitude of its population. By calculating and averaging site-specific trends (instead of calculating trends based on data pooled for all sites), the influence of a declining population at one major site, due to habitat deterioration or other local factors, on the overall trend estimate is minimized. Procedures for deriving both annual index and trend values are described in more detail elsewhere (Geissler & Noon, 1981; Geissler, 1984). The only significant departures from procedures described previously are: (1) replacement of jackknife estimates of the trend variance (Geissler, 1984) by bootstrap estimates, which are more precise; (2) adjustment for missing counts in the estimation of annual indices; and (3) using the bootstrapping method to depict variance in the estimate of annual indices. This technique (shown in Figs 3, 5, and 7) generates alternative annual index values by randomly sampling with replacement the population values for all the sites for a given year. Because of data requirements for annual indices, values could not be calculated for years 1972 74, when relatively few sites were surveyed. Therefore, annual indices are calculated for years 1975-83 and population trend lines are based on the entire period 1972-83. 0045012

Population trends in shorebirds 191

RESULTS

The actual number of individuals of each species observed per year was impossible to estimate, because the number of surveys varied greatly among sites and turnover rates of the migrating populations were not determined. The average peak number of individuals per site ranged from 540 for semipalmated sandpiper to only 5 for whimbrel (Table 1). In most cases the number of individuals observed probably represents only a small fraction of the hemispheric population of these species. Populations of three of the twelve shorebird species decreased (P<0"05) over the years 1972-83: whimbrel, dowitcher species (primarily short-billed dowitcher), and sanderling (Figs 2-7). Black-bellied plovers exhibited a nearly significant (P < 0"10) decline. The remaining species showed no significant population change (Table 1). The whimbrel was recorded infrequently and in very small numbers at most ISS sites, perhaps because it migrates relatively early compared to other species. Therefore, the trend estimate for the whimbrel is particularly vulnerable to distortion by population anomalies at the few sites that

TABLE ! Population Trends of Selected Shorebird Species, 1972-1983, Based on Surveys of United States Atlantic Coastal Migration Stopovers (International Shorebird Survey)

Species No. Mean peak Mean 95% Statistical Cumulative c~[ count~site~ annual % confidence significance % change sites )'ear change limits 1972 1983

Black-bellied plover 55 164 -5"4 -13.3~0'4 P<0.10 -45-8 Semipalmated plover 47 138 -9.5 -14.5 12.3 ns -66.7 Greater yellowlegs 52 18 -3.1 - 15.5 16.0 ns -29.3 Lesser yellowlegs 43 38 +3.5 - 13.1 15.0 ns +46.0 Wilier 30 21 + 0-2 - 3.7-6.3 ns + 2.2 Whimbrel 22 5 -8.3 - 18.3 - 1.3 P< 0"01 -61.5 Ruddy turnstone 56 26 -8.5 - 17.3 3"0 ns -62"4 Red knot 33 105 -11"7 -20"2 7"7 ns -74"6 Sanderling 52 241 -13.7 -23.8 -5"5 P < 0.01 -80.2 Semipalmated sandpiper 63 540 -6.7 - 14.9 12-9 ns -53-4 Least sandpiper 45 56 +2.9 --3.9 13-4 ns +37.0 Short-billed dowitchera 59 220 -5"5 - 13"7-- -0-1 P< 0-05 -46"3

Some misidentified long-billed dowitchers may have been included in the short-billed dowitcher data set. ns = not significant. 0045013

192 Marshall A. Howe, Paul H. Geissler, Brian A. Harrington

WHIMBREL INDEX 6 "

0 I I I I I I I I 75 76 77 78 79 80 8 t 82 83

YEAR

Fig. 2. Annual population indices (asterisks) and trend line for the whimbrel based on analysis of International Shorebird Survey data (see Methods). (The trend line is calculated independently of annual indices and is therefore not a regression line through those points.)

WHIMBREL INDEX 40

I I "~'-..-.. ~ ~_4~ ~'4~ ~ ~ ~=',,-...~..

10 ~.~ -.~..____ ~._.__~_. _~______--.~

I I I I I I I I I 75 76 77 78 79 O0 8t 82 83

YEAR Fig. 3. Five bootstrap replicates of the annual index estimates for the whimbrel (see Methods). Each replicate is offset by an index value of 5 for easier viewing and comparison. 0045014

Population trends in shorebirds 193

DOWITCHER SPP INDEX

I I I I I I I I 75 76 77 78 79 BO Bl 82 83

YEAR

Fig. 4. Annual population indices (asterisks) and trend line for the short-billed dowitcher (possibly including some long-billed dowitchers) (see Fig. 2).

DOWITCHER SPP INDEX

40 :I -I., .#/%..I \\ +'-.-, \ ,.,,..~ v. ,;'-. ~ ',,• I ,'I / ,~~.~

I I | I I I I I I 75 76 77 78 79 80 B~ 82 83

YEAR Fig. 5. Bootstrap replicates of annual indices for the short-billed dowitcher (see Fig. 3). 0045015

194 Marshall A. Howe, Paul H. Geissler, Brian ,4. Harrington

SANDERLING INDEX 80

I I I -I I I I I 75 76 77 78 79 80 8t 82 83

YEAR Fig. 6. Annual population indices (asterisks) and trend line for the sanderling (see Fig. 2).

SANDERLING INDEX i25 ~ii,¢t" //

100 /f~

75 ~ \ "~'----. ",, .~. Ja / , ,~ j "---. ~_ ,, ~:>---o, \ .- ,. /,, / ~---C..~'" ..... - ....~-\.', .- .~. -..! 7,, 50 4 ,, \.. "-,,-I .//\\ ,, ,.,,.f .-.'~"~'-:t,K/"

0 t I I I 1 75 76 77 7B 7g 80 81 82 83

YEAR Fig. 7. Bootstrap replicates of annual indices for the sanderling (see Fig. 3). 0045016

Population trends in shorebirds 195 supported substantial numbers of birds. The short-billed dowitcher and sanderling, however, were recorded much more widely and frequently and, as a result, we have greater confidence in the trend estimates for these species.

DISCUSSION

In addition to the assumption that the data used in these analyses are representative of hemispheric or major regional populations, two aspects of the data set are cause for concern. First, the variance of trend estimates is large compared with trend estimates derived from the BBS. This raises concern about conformance of the data to statistical assumptions on which the trend analysis procedures are based. Second, inconsistency of coverage of ISS sites both within and between years necessitates elimination of many sites from the analyses, leaving a less than desirable sample on which to base estimates of population change.

Problems associated with statistical assumptions

The trend analysis procedure used in this and other studies is based on the assumption that the counts of birds are distributed lognormally. We examined the counts of each species in the final data set analyzed and generally found that they fit lognormal distributions. Thus, we feel this assumption is met satisfactorily. Our second concern was the influence of high variance on model assumptions, because the variance is an important component of the back- transformation to the original scale based on the lognormal model. To evaluate these possible effects, we conducted trend analyses on several simulated data sets, each with variances comparable to those in the true data. In the first simulation, the actual counts for semipalmated sandpiper were permuted randomly, simulating a zero trend between 1972 and 1983. The analysis of the true data had indicated a nonsignificant decrease of 6"7% per year. The result of this simulation was a zero trend as predicted. In the second simulation, a trend analysis was run on the actual semipalmated sandpiper data, but in reverse. The predicted result was the reciprocal of the 6.7 % annual decrease, an increase of 7.2%. The result of this simulation was a 3'5% increase, not significantly different from the predicted 7"2%. Next, two artificial data sets with between-year variances comparable to that of the actual data were generated. Into one data set was built a 10% annual increase and into the other a 10% annual decrease. Because of the small number of sites in the true data set, the trend analysis might not distinguish statistically between a 10% increase and a 10% decrease. 0045017

196 Marshall A. Howe, Paul H. Geissler, Brian A. Harrington

Therefore, 1000 hypothetical sites were used in the simulated data sets as a means of narrowing the confidence limits on the trend lines (i.e. enhancing the ability to distinguish between plus 10% and minus 10%). Trend analysis on these data detected the built in trend values effectively, suggesting that the high year-to-year, within-site variances typical of ISS data do not introduce serious bias into the trend estimation procedure. Finally, much of the within-site variance results from the high number of zero counts in ISS data. This was a source of concern because of the multiplicative nature of the trend model. To test for the effects of zeros, simulated data sets consisting of at least 25% zeros and having known trends were created. Trend analysis on these data sets generated absolute values smaller than the actual values, regardless of the sign of the trend. Therefore, we concluded that a high proportion of zeros in ISS data results in a conservative estimate of the magnitude of the trend.

Problems associated with inconsistency of site coverage

Relatively few sites had population estimates for all years. Many had estimates for only two or three years. Because the trends are computed site by site, we examined the data for the sanderling, which showed a significant population decline, to determine if certain years were contributing disproportionately to the combined trend estimate. For each site, we recorded all pairs of years involved in the trend estimate. For example, if population estimates for a given site were available for 1978, 1980 and 1982, three pairs of years would be represented: 1978-80, 1978-82 and 1980-82. We found all possible pairs of years to be uniformly represented across the entire data set, with the exception of all pairs including year 1983, for which the data set was incomplete. From this examination we conclude that the trend estimates were primarily reflecting population change over the years 1972-82 and are not biased toward any subset of years within that range. However, the precision of the estimates would undoubtedly be better if population estimates had been available for all sites for all years.

Biological interpretation

The frequency (3 of 12 species) and magnitude (5.5-13.7%/year) of population declines detected by this analysis are greater than would be typical of passerine populations on BBS routes. However, it is to be expected that species nesting in arctic or subarctic sites may experience more dramatic population fluctuations than species of more temperate latitudes. Severe spring weather in some years in the arctic occasionally kills large numbers of adult shorebirds locally (Evans & Pienkowski, 1984). Also, because the time 0045018

Population trends in shorebirds 197 available for breeding is only a few weeks at high latitudes, a delay in the onset of breeding may eliminate recruitment in an entire population. Therefore, substantial short-term declines are probably normal events, particularly if two or more bad years occur in succession. The long-term impact of such a situation is compensated to some degree by the relatively high annual adult survivorship of 75-80% in those species studied (Evans & Pienkowski, 1984). Although the decline of short-billed dowitchers and sanderlings between 1972 and 1983 may be part of a normal, long-term cycle, it is also the type of pattern one would expect to find in most instances of man-induced population decline. We see no obvious reason why these two species with different ecological requirements might be declining for man-induced reasons while others are not; their populations should be monitored carefully in the future. The apparent increase in sanderlings in 1983 gives some cause for encouragement. The increase is not reflected strongly in the trend estimate because of the small 1983 sample of sites. The possibility exists, however, that the apparent increase is simply an artifact of the small sample. The possibility that the population trends we have calculated are not truly representative of the entire migrating populations of the species examined must be considered. The disproportionate effect that one important site can have on the trend estimate for an infrequently recorded species (e.g. whimbrel) has been mentioned. Other factors are also potential sources of concern. Coastal habitats change periodically, prey distributions may change accordingly, and local shifts in shorebird distributions may bring birds out of range of detection by ISS volunteers. ISS sites that have impoundments with water conditions dependent upon rainfall would be particularly susceptible to year-to-year population shifts unrelated to overall population levels. Changes in migration routes among years could also lead to erroneous conclusions about population change. Nonetheless, significant trend estimates result only when the pattern of population change is consistent across sites. Therefore, unless changes in local distribution patterns or migration routes are affecting most sites in a temporally consistent manner (an unlikely event), our significant trend estimates for short-billed dowitcher and sanderling appear to represent bonafide population phenomena.

CONCLUSIONS AND RECOMMENDATIONS

Our analysis of International Shorebird Survey data suggests that at least two (short-billed dowitcher and sanderling) and possibly a third (whimbrel) 0045019

198 Marshall A. Howe, Paul H. Geissler, Brian A. Harrington species of shorebird experienced substantial and statistically significant declines between 1972 and 1983. Arguments are presented to dispel concerns about the influence of high variances, zero counts, and inconsistent site coverage on the validity of the population trend estimates. Declines in these species may be natural population phenomena or may be the consequence of environmental perturbations that are presently unclear. Our results point to a need both to continue shorebird population surveys and to improve their quality. The effectiveness of the ISS could be improved in several ways. Intensive within-year coverage and consistent between-year coverage should be established and maintained. This is especially important for stopovers that attract large numbers of shorebirds, even if coverage of some less important sites has to be abandoned. Major concentration points for migrating shorebirds not presently surveyed (such as some Canadian coastal sites) need to be incorporated into the ISS. Attention should be more clearly focused on peak periods of migration. If possible, additional searching, including aerial surveys, in the neighborhood of important sites is desirable, so that local shifts of population centers are not overlooked. Finally, more research is needed to determine the degree of fidelity that species show to migratory stopovers, as site fidelity increases our ability to detect trends and to relate population declines of migrating populations to general breeding or wintering areas.

ACKNOWLEDGEMENTS

We gratefully acknowledge the assistance of Kathy Marois, Lynne McAllister, and Ellen Snyder in data analysis. Pete Myers, Chandler Robbins, and Jim Spann reviewed the manuscript and provided many helpful comments and insights.

REFERENCES

Bystrak, D. (1981). The North American Breeding Bird Survey. Stud. Avian Biol., 6, 3441. Evans, P. R. & Pienkowski, M. W. (1984). Population dynamics of shorebirds. In Behaviour of Marine Anirnals, 5. Shorebirds: Breeding Behaviour and Populations, ed. J. Burger & B. L. Olla. Plenum Press, New York & London, pp. 83-123. Geissler, P. H. (1984). Estimation of animal population trends and annual indices from a survey of call-counts or other indications. Paper presented at Proc. Section on Survey Research Methods (Amer. Statistical Assoc.), Philadelphia, Pennsylvania, 13 16 May. 0045020

Population trends in shorebirds 199

Geissler, P. H. & Noon, B. R. (1981). Estimates of avian population trends from the North American Breeding Bird Survey. Stud. Avian Biol., 6, 42-51. Howe, M. A. (1987). Wetlands and waterbird conservation. Amer. Birds, 41,204-9. Hussell, D. J. T. (1981). The use of migration counts for monitoring bird population levels. Stud. Avian Biol., 6, 92-102. Scott, D. A. & Carbonell, M. (1986). A Directory of Neotropical Wetlands. IUCN, Cambridge, and IWRB, Slimbridge. Senner, S. E. & Howe, M. A. (1984). Conservation ofnearctic shorebirds. In Behavior of Marine Animals, 5. Shorebirds: Breeding Behaviour and Populations, ed. J. Burger & B. L. Olla, Plenum Press, New York & London, pp. 379-421. Smith, P. W. & Houghton, N. T. (1984). Fidelity of semipalmated plovers to a migration stopover. J. Field Ornithol., 55, 247-9. Svensson, S. E. (1978). Efficiency of two methods for monitoring bird population levels: breeding bird censuses contra counts of migrating birds. Oikos, 30, 373 86. Tiner, R. W., Jr (1984). Wetlands of the United States: Current Status and Recent Trends. US Fish and Wildlife Service, National Wetlands Inventory.