0045006 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