Since the 1970s, artificially stabilized water levels, increased presence of common carp (Cyprinus carpio) and invasion by a cattail hybrid (Typha × glauca) have changed the nesting environment for Western Grebes (Aechmophorus occidentalis) at Delta Marsh, Manitoba. To evaluate the impact of these changes, nest survival rates, causes of nest mortality, wind conditions, locations of nests and vegetation structure at nests in 2009–2010, and chick-adult ratios were compared to similar data for Western Grebes at Delta Marsh from 1973–1974. Apparent nest survival rates were lower in 2009–2010 than 1973–1974, and between low-water years (1973, 2010) and high-water years (1974, 2009). Lower apparent nest survival rates in 2009–2010 (49% in 2009 and 43% in 2010, compared to 46% in 1973 and 84% in 1974), and chick-adult ratios (0.55 in 1973 and 0.88 in 1974, compared to 0.55 in 2009 and 0.39 in 2010) were attributed to increases in destruction of nests primarily by wave action and secondarily by common carp, which were not observed destroying Western Grebe nests in 1973–1974. The replacement of native bulrushes by cattails in Western Grebe nesting habitat may have caused the observed increase in proximity to openwater edge, and this proximity may have increased destruction of nests by waves. Restoring stands of emergent bulrush by varying marsh water levels and reducing carp in the marsh might improve nest survival of Western Grebes.
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Vol. 37 • No. 1