Nest Site Selection of American Oystercatchers (Haematopus palliatus) on the Upper Texas Coast with Comments on Field Sexing Techniques
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The American Oystercatcher, hereafter oystercatcher, is a shorebird species of high conservation concern that requires intertidal shellfish beds for breeding and wintering habitat. Considerable attention has been paid to obtaining site-specific productivity data and determining factors contributing to their reproductive success on the Atlantic coast; however no data exists for populations along the Texas coast. I monitored breeding oystercatchers on the upper Texas coast and discovered 58 and 83 nests during 2011 and 2012, respectively. During 2011 productivity was 0.78 based on 36 chicks fledging from 46 breeding pairs. During 2012, productivity was 0.21 based on 10 chicks fledging from 48 breeding pairs. Oystercatchers in Texas nested in 4 types locations: 52% of nests (n=74) were on emergent shell island, 37% (n=53) on the periphery of colonial waterbird rookery islands, 6% on shell spits connected to the mainland at low tide (n=8), and 5% on the mainland (n=7). The most common plants at oystercatcher nests on the Texas coast were sea purselane (Sesuvium portulacastrum), Carolina wolfberry (Lycium carolinianum), saltwort (Batis maritima), and sea ox eye daisy (Borrichia frutescens). I investigated nest site selection at two scales, microhabitat at the nest site and landscape scale at the presumed territory. I measured the same variables at nests and at randomly selected unused locations within the study area and then performed logistic regression to determine if oystercatchers were selecting nest locations non-randomly. Fifteen percent of nests surveyed were on shell substrate with no vegetation whereas overall, nests averaged 30% live vegetative cover. There was no significant difference between the amount of live vegetation (df= 147, r!<0.001, p=0.874) or shell (df= 147, r!=0.002, p=0.602) at nest locations and random locations. There was also no significant effect of the amount of vegetation (df= 73, r!=0.093, p=0.078) or shell (df= 73, r!=0.093, p=0.220) at nest locations on the likelihood of fledging.
At the landscape scale, I used a geographic information system (GIS) to measure elevation, the percent shell, rock or sand substance within territories, and distance to nearest oyster reefs, beach access points, urban landcover and the intracoastal waterway. I used these relationships to parameterize a model that predicts the presence of oystercatcher nests on the Texas coast. The best-supported logistic regression model illustrated that oystercatchers prefer nesting closer to oyster reefs, urban landcover, and with more shell substrate (w=0.44, r!=0.80). I found a relationship between oystercatcher nest site selection and habitat features, demonstrating that both land formations and urbanization influence oystercatcher nests, and further spatial analysis is suggested. My results suggests that landscape scale spatial analysis of the structure of coastal bays can inform land managers regarding projects aimed at restoring and developing dredge islands and oyster reefs in an effort to support and stabilize oystercatcher populations in rapidly changing coastal ecosystems.