In 1986 and 1987, the New York State Department of Environmental Conservation and the Hudson River Foundation sponsored a study of avian breeding habitats in six tidal marshes on the Hudson River Estuary. Local concern prompted a repeat of this study at Iona Island Marsh in 2004 and at four of the marshes in 2005 (Iona Island Marsh, Constitution Marsh, Tivoli North Bay, and Stockport Flats). This study had three main objectives: (1) to document bird species breeding in these four marshes, (2) to determine how the marsh-breeding populations have changed since the 1986–87 study, and (3) to relate the spatial distribution of marsh-nesting species to measurable habitat variables within marshes. A total of 3522 observations of birds, representing 83 species, were recorded from April 28, 2005, to June 30, 2005. These observations were made by sampling 109 fixed observation stations five times using both visual and vocalization sampling methods. Nineteen of those species are dependent on emergent marsh habitats. The most common marsh-dependent species encountered during this study were Red-winged Blackbird (Agelaius phoeniceus) and Marsh Wren (Cistothorus palustris). These two species each accounted for 23–47% of the marsh-dependent guild at Constitution Marsh, Tivoli North Bay, and Stockport Marsh. Marsh Wrens were nearly absent from Iona Island Marsh (<1.0%); there, Red-winged Blackbirds accounted for more than 77% of the marsh bird community. Red-winged Blackbirds also dominated the marsh avian communities at Constitution and Stockport Marshes. Bird species diversity decreased significantly since 1986–87 at Iona Island and Constitution Marshes. Decreased diversity corresponds with an increase in the density of Red-winged Blackbirds. At Iona Island Marsh, this shift in the avian community to almost entirely Red-winged Blackbirds coincided with a shift of the plant community dominance from narrowleaf cattail (Typha angustifolia) in 1986–87 to common reed (Phragmites australis) in 2004–05. This shift was not evident at Constitution Marsh, Tivoli North Bay, or Stockport Marsh, although the number of Phragmites australis has also expanded at these sites. In addition to our survey, we found a total of 230 nests in 2005. Major findings of the bird nest searches were (1) the very low density of nests found at Iona Island Marsh (five nests total in 2004 and 2005), (2) the most common nest encountered at the other three marshes was that of the Marsh Wren (83% of total nests observed), and (3) the highest bird nest density occurred at Tivoli North Bay (65 nests ha−1).

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