Development of a North American Survey for Monitoring Shorebird Populations
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DEVELOPMENT OF A NORTH AMERICAN SURVEY FOR MONITORING SHOREBIRD POPULATIONS Report to: U.S. Fish and Wildlife Service Patuxent Wildlife Research Center Laurel, MD 20708 Prepared by: Brian A. Barrington Gary W. Page Manomet Bird Observatory Point Reyes Bird Observatory P.O. Box 1770 4990 Shoreline Highway Manomet MA 02345 Stinson Beach CA 94970 Assisted by; James E. Lyons & Janet E. Kjelmyr & u. Cartar w. David Shuford Manomet Bird Observatory Point Reyes Bird Observatory September 1991 1 SYNOPSIS Tasks The purpose of this report is to investigate the feasibility of a survey for North American shorebird populations that will provide reliable annual indices to population sizes of as many species as possible and allow analysis of long-term population changes to be conducted. Specific tasks were: 1. To identify all known spring and fall migration stopovers in North America for which a typical migration peak is at least 5000 shorebirds and to provide information on ownership, accessibility, special logistical considerations and habitat stability. (We added to this the identification of all sites likely to hold 5000 or more shorebirds during winter.) 2. To propose a sampling protocol that focuses on the identified sites and considers census frequency, best time of year for counts, ef feet of migration timing on counts, · and need for team census approach. The potential effectiveness of the sampling protocol must be analyzed for all species of North American shorebirds. The sampling protocol must include a rational for omitting any significant sites from the survey network. Methods We solicited information on sites likely to hold peak numbers of at least 5000 shorebirds through: advertisements in the Western Hemisphere Shorebird Research Network newsletter; questionnaires sent to wildlife refuges, bird watchers and employees of government agencies involved in non-game management; examination of American Birds migration season accounts; examination of Audubon Society Christmas Bird Counts; and examination of data from Manomet Bird Observatory's International Shorebird Survey and Point Reyes Bird Observatory's Pacific Flyway Project. After their initial response to requests for information many people were telephoned to get more precise data on shorebird numbers and physical characteristic of wetlands. The results of this effort are presented in Appendices 1-4 and 6-9. Analysis of data from The International Shorebird Survey provided the bulk of the information on the timing of shorebird migration east of the 105th Meridian ( summarized in Appendix 5) • Comparable information is not available for the region west of the 105th Meridian. For the western region we had to rely on published literature, anecdotal information, and the authors' knowledge of migration timing. We identified 11 counting methods that could be employed to monitor shorebirds, assuming counting techniques should be considered separately if they involved coverage of different habitat types. These methods are described on pages 10 to 13. Also identified were existing census programs that could be suitable for monitoring 2 shorebird populations. The abundance, distribution, preferred habitats, migration routes, and location of breeding and wintering grounds were then considered to identify the best counting method for each species. Results We identified 53 species of North American shorebirds that might occur with sufficient regularity that they should be considered for monitoring. South of Alaska there are at least 149 U.S. sites supporting 5000 or more of these shorebirds in spring, winter or fall. Of those 15 are in region 1, 15 in region 2, 12 in region 3, 21 in region 4, 35 in region 5, and 51 in region 6 (Fig. 1). In Alaska there are an additional 5 7 sites holding thousands to hundreds of thousands of shorebirds. There were 59 Canadian sites with the potential of attracting 5000 or more shorebirds. The composition of species in each site is presented in Appendices 3 and 6. More sites supporting 5000 or more shorebirds are likely to be found in the future, especially in the Sacramento Valley of California and the prairie provinces of Canada where we felt information on important shorebird habitats was incomplete. The best method(s) for monitoring each species are summarized in Table 4. Table 4 also indicates the best season, the best dates and the most important regions for surveying. Four species, Common Ringed Plover, Eskimo Curlew, Curlew Sandpiper, and Sharp-tailed Sandpiper, breed too rarely in North America to warrant a monitoring program. The status of the Northern Jacana as a U.S. breeder was not well enough known to us that we felt qualified to recommend a monitoring scheme. Government agencies and private organizations currently are monitoring breeding populations of the Snowy Plover, Piping Plover and American Woodcock or have baseline information suitable for future monitoring of American Black Oystercatchers, Bristle-thighed Curlews, Bar-tailed Godwits and Black Turnstones. Similar projects, focused on the accessible and restricted breeding populations Wilson's Plover and American Oystercatcher would be desirable. Two existing census programs provide useful information for monitoring shorebird populations. The U.S. Fish and Wildlife Service's Breeding Bird Survey (BBS) is likely the best method of monitoring the Killdeer and Upland Sandpiper (Table 1). The BBS may be useful, if not the best method, for monitoring seven additional species (Table 1). The Christmas Bird Counts (CBC) of the National Audubon Society are probably the most practical method for monitoring Black Turnstones, Purple Sandpipers, Rock Sandpipers and Common Snipes. CBC's are suitable but not the best method for two additional species (Table 1). A census program focused on counts in wetlands during migration and winter is necessary for monitoring most shorebirds. This requires r;[_ A I- .- ~- d • - -- .... .-.- - .. "I[_ B w Figure 1. Boundaries of regions used for analysis of ISS data east of the 105th Meridian. Region VI is the area of the U. S. and Canada west of the Rocky Mountains and south of Alaska. 4 an organized network of wildlife biologists and birders that can count shorebirds during peak periods of migration or in winter. The effectiveness of the census program will vary among species and be most effective for those that are concentrated on the coast, show a preference for the most limited range of habitats, and assemble in accessible locations with other shorebirds that we wish to monitor simultaneously. We also believe that winter is a more suitable time for monitoring than spring or fall migration. If monitoring must occur during migration then spring is preferable to fall. The criteria noted above were used to rank the value of winter-time and migration-time counts for shorebirds into six groups ( Table 1) • Top ranked is a winter count which provides the best source of information for four species: American Oystercatcher, American Avocet, Marbled Godwit and Dunlin. During winter counts of these species useful information could also be gathered for four additional species listed under the third rank. Winter counts would need to extend into Mexico but this would be possible if shorebirds could be counted during the annual FWS waterfowl survey in Mexico. The Mountain Plover could also be counted in winter. A winter count for Mountain Plovers is not ranked higher because the necessary focus on agricultural habitat for this species would not provide useful information for any other species. Second in importance is a spring count which would provide the best source of information for nine species (second rank, Table 1) the second best source of information for the Marbled Godwit, and partial information for five species which are forth-ranked. The value of a spring count is lower (forth-ranked) for Lesser Yellowlegs, Willet and Short-billed Dowitchers because it would only be useful in the eastern or western region of the country. It is also ranked lower for the Whimbrel · and Sanderling because extensive coverage of non-wetland habitat would be required for spring counts of these species to be as complete as those for higher-ranked shorebirds that are more concentrated in wetland habitats. There are an additional 11 shorebirds for which a spring census is ranked low (sixth rank) because it would only provide information from a small portion of the population (Black-necked Stilt) or require counting in so many different sites from those chosen for the second- and forth-ranked species that the feasibility of monitoring would be low. In this category are species such as Budsonian Godwit, Baird's Sandpiper and Pectoral Sandpiper which may stage in substantial numbers at unidentified Canadian prairie sites. Fall counts do not have nearly the utility of winter or spring counts. We only identified six species for which a fall count either provided information that would not be obtained by another method or information of higher quality than in another method. The value of a fall count for five of the six species was limited by its applicability to only a portion of the species' range. Fall would be the best time to monitor Wilson's Phalaropes. This would require counts dedicated solely to the Wilson's Phalarope since 5 this species migrates earlier and uses highly saline habitats avoided by most other species. We identified two approaches to monitoring. during migration. Trend Analysis I uses peak annual counts of migrating shorebirds from ISS-monitored sites to examine population trends of target species (Howe et al. 1989). Annual population indices for target species are calculated as the geometric mean of the peak annual counts. Population trends representing the average proportional increase or decrease of the population over the range of time examined are computed independently of the annual indices. For population trends peak counts are regressed on year for each site and averaged over all sites, each weighted according to the magnitude of the population.