Distribution, abundance and habitat associations of fall- and spring-migrating shorebirds and waterfowl in the southwest Lake Erie marsh region
Tara E. Baranowski, MS
Advisor: Robert Gates
The southwest Lake Erie marsh region (SLEMR) has long been known as an important waterfowl staging area and was recently designated as a significant stopover site for shorebirds (Charadriiformes). Waterfowl and shorebirds were counted weekly during spring (March-June) and autumn (July-November) 2002-2003 using a stratified random sample of lake-affected, managed marsh and agricultural plots (0.0625-0.25 km2). Plots contained various wetland and upland habitat types, but were classified according to their dominant water regime (i.e. lake-level influenced, controlled, precipitation-driven). All habitat types were surveyed within 90 plots (30 plots per stratum) except for spring 2002 when only 60 plots were sampled (20 plots per stratum). Plots were divided evenly among two study sites, one coastal site and one embayment site. Based upon a 7-day stopover period I estimated a total of 313,451 and 171,852 shorebird use-days in 2002 and 2003 respectively and a total of 250,844 and 299,208 waterfowl days. Managed marsh plots supported the most shorebird use-days followed by lake-affected plots (managed marsh: 318,752, lake-affected: 148,011, agriculture: 18,541 both years combined). Waterfowl use-days were also highest in managed plots followed by lake-influenced plots (managed marsh: 399,080, lake-affected: 138,902, agriculture: 12,070 both years combined). Shorebird population estimates extrapolated by sampling stratum out to the site level ranged from 57,596 (SE + 34,703) to 226,760 (SE +112,592) while extrapolated waterfowl estimates ranged between 35,342 (SE + 7,593) to 267,859 (SE + 60,151). Total waterbird (shorebird and waterfowl) abundance (birds/km2) varied (P < 0.026) among the three wetland sampling strata during all seasons and years. In particular the coastal, lake-affected estuarine wetlands were most significant in terms of shorebird abundance for all shorebird guilds during all seasons except spring 2002. Redundancy analysis of shorebird abundance and environmental variables in the SLEMR also showed that shorebird selection of plots was driven largely by the amount of lake-affected estuarine habitat contained within the plot. Lake-affected estuary plots also attracted the most diverse assemblage of shorebirds for both autumn seasons and suggest the importance of conserving this limited habitat within the region.
The repeated importance of estuarine habitat to shorebirds throughout all of the analyses illustrates the association shorebirds have with freshwater estuarine habitat within the SLEMR. Of the habitat types in the region, the estuarine habitat is the least available and yet shorebirds seem to prefer it over the other choices available to them. The positive response of shorebirds to waterfowl habitat management techniques such as marsh drawdowns was also evident in this study and has been well documented by others. Plots dominated by drawn-down marsh habitat contributed the most diversity to the waterfowl community during both years and both seasons and also contributed the most shorebird diversity for both spring seasons. Early spring drawdowns most benefit shorebirds by supplying shallow water and moist soil habitat at a time when these conditions are rare in the natural wetlands and regional freshwater estuaries. Although current management plans do not target shorebirds per se, they benefit as a byproduct of management for waterfowl habitat. The Western Hemisphere Shorebird Reserve Network categorizes a site as internationally important if it supports at least 100,000 shorebirds annually or >10% of the biogeographic population for a species. The embayment site alone supported over 100,000 shorebirds and this just during spring 2002 (105,822, SE + 52,543). On an annual basis, the most conservative of my migrant shorebird population estimates ranks the SLEMR as a stopover site of international importance for migrating shorebirds. Based upon the results of this study the SLEMR’s designation as a site of regional importance to migrating shorebirds should be reevaluated and the designation of international importance should be seriously considered.