Studies of endangered species such as the piping plover (Charadrius melodus) face a common problem: individuals of rare species cannot, by definition, be found in large numbers. It is therefore difficult to obtain adequate sample sizes for the statistical analyses usually applied in studies of common species. However, biologists need data about endangered species to guide the formulation of policy appropriate for their conservation. This study explores the value of applying the comparative method to provide such data. The piping plover was listed in the Federal Register (January 10, 1986) as endangered in the Great Lakes region and threatened throughout the remainder of its range. Historically, this small shorebird bred along the shores of Lake Michigan. Many of the traditional nesting sites, however, have been abandoned in recent years. Piping plovers still nest at Waugoshance Point in Wilderness State Park, Emmet County, Michigan (45 45 N, 84 53 W). The population of plovers on Waugoshance Point has decreased from an average of 7 breeding pairs in the 1970’s to 4 breeding pairs in 1985 and 1986 (Ed Pike, personal communication). Spotted sandpipers (Actitis macularia) also breed at Waugoshance Point. This shorebird is common throughout Michigan and nests at the Point in much larger numbers than piping plovers. This study has three major purposes. The first is to describe the major similarities and differences between the foraging and breeding ecologies of piping plovers and spotted sandpipers. The second is to determine what the comparison can teach us about managing the few piping plovers that still nest at Waugoshance Point. Third, and more generally, I conducted a comparative study of these two species to determine if information about a more common shorebird using the same habitat could help determine what limits piping plover reproduction. The comparative approach involves studying two or more species and asking how the biology, behavior, or ecology of each species differs. This approach has been used widely in behavioral ecology as a method of studying the degree to which selective pressures have affected different species. In studies of two species, the comparative approach is particularly useful when one of the two species is endangered and therefore difficult to study. The comparative approach requires some basic assumptions about the species involved. Different species have different life histories, and some of these differences may confound a comparison of their ecology. My interest in this study, however, is not simply to show that the piping plover and the spotted sandpiper differ with respect to some variables. This could be inferred from the fact that they are two different species. My aim is to understand how several specific aspects of their behavior and ecology differ, and whether these differences affect the reproductive success of each species. While these are large and general questions, an attempt to answer them may prove valuable in understanding the factors which limit the reporduction of piping plovers. There are several reasons why a comparison of spotted sandpipers and piping plovers is logical and useful. Both species belong to the same order, Charadriiformes, and so are reasonably close relatives. Many spotted sandpipers nest at Waugoshance Point. I found 13 nests in 1985 and 27 in 1986. The microhabitat that each species uses for nesting differs considerably, but the two species nest within several meters of each other. Thus, the potential egg predators in the area forage near nesting sites of both species. In addition, both species use the same microhabitats for foraging. This means that extremes of weather and food availability are similar for both species. Thus, by collecting specific information about spotted sandpipers, and making careful comparison with the same data for piping plovers, I may shed light on the relative importance of factors which affect the reproduction of the piping plover at Waugoshance Point. In this study, I focused on two major ecological comparisons. First, I compared the use of habitat for foraging by both species in two ways: the degree to which each species utilizes each habitat type, and the foraging rates of each species when it uses each habitat type. The results of these studies are reported in Chapter 2. The second major comparison involved the relative rates of predation on both species. To determine the relative impact of various potential predators, I measured predator activity in the nesting area and a variety of nest parameters for each spotted sandpiper nest. I made inferences about the threats to piping plover nests from information about the predators of spotted sandpiper nests. These results are reported in Chapter 3. I also report in Chapter 4 the results of some preliminary experiments I conducted to test the application of taste-aversion conditioning as a technique for controlling egg predation.