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Avian Use of Purple Loosestrife Dominated Habitat Relative to Other Vegetation Types in a Lake Huron Wetland Complex

Results


As the season progressed water depths at coastal sites (coastal cattail and coastal bulrush) increased and those at inland sites decreased while vertical cover generally increased at all sites (Table 1). We surveyed 258 plots and observed 39 breeding bird species in Saginaw Bay wetland habitats (Table 2). Six breeding species were observed in the sampled vegetation type, but not on survey plots: Pied-billed Grebe, Wood Duck (Aix sponsa), Redhead (Aythya americana), Northern Harrier (Circus cyaneus), Forster's Tern (Sterna forsteri), and Black Tern (Chlidonias niger). We also observed 10 species breeding in loosestrife dominated habitats (Table 2).

Marsh Wren (n = 20), Swamp Sparrow (n = 16), and Red-winged Blackbird (n = 21) were the most commonly observed nests on all plots (Table 3). We observed Mallard, Blue-winged Teal (Anas discors), Virginia Rail, and Red-winged Blackbird nests while traversing between plots in loosestrife-dominated vegetation zones, but not on the plots.

Avian density and diversity.__Our final model indicated that avian density differed only in relation to vegetation (ANOVA: F = 14.45, df = 7,181, P < 0.001; Table 4). Avian density was higher (orthogonal contrast: F = 8.87, df = 1,181, P = 0.003) in loosestrife-dominated vegetation types [46.9 ± 3.8 (SE) birds/ha] than in other vegetation types (34.7 ± 1.6). Avian diversity also differed only in relation to vegetation (ANOVA: F = 12.76, df = 7,181, P < 0.001, Table 4). Avian diversity was lower (orthogonal contrast: F = 4.74, df = 1,181, P = 0.03) in loosestrife-dominated vegetation types (0.42 ± 0.08) than in other vegetation types (0.60 ± 0.03). Effects of year, period, and all interactions were not significant (P > 0.05 for all tests) for both avian density and diversity. Scrub-shrub contained the highest bird species diversity and wet meadow/loosestrife and coastal bulrush the lowest (Table 4).

Species abundance.__The vegetation period year interaction was significant (ANOVA: F = 2.34, df = 7, 157, P = 0.03) in our initial Sedge Wren model. Thus, vegetation related differences in Sedge Wren abundance were not consistent among periods and years (Table 5). Within periods and years, Sedge Wren abundance did not differ (orthogonal contrasts: P > 0.05 for all tests) between loosestrife dominated vegetation types and other vegetation types.

Marsh Wren abundance differed among vegetation types (ANOVA: F = 30.72, df = 7, 181, P < 0.001; Table 5). Marsh Wren abundance was lower (orthogonal contrast: F = 10.73, df = 1, 181, P = 0.001) in loosestrife-dominated vegetation types (0 ± 1.8) than in other vegetation types (6.2 ± 0.7). Yellow Warbler abundance differed among vegetation types, but differences were not consistent between mid- and late periods (ANOVA: vegetation period interaction, F = 2.08, df = 7, 173, P = 0.048; Table 5). The interaction was due to significantly higher (P < 0.001) numbers of Yellow Warblers observed in late-period scrub-shrub compared with mid-period scrub-shrub (Table 5). Yellow Warbler abundance did not differ (orthogonal contrast: P > 0.05 for both tests) between loosestrife-dominated and other vegetation types in either period. Common Yellowthroat abundance differed among vegetation types (ANOVA: F = 6.04, df = 7, 181, P < 0.001; Table 5). Common Yellowthroat abundance did not differ (orthogonal contrast: F = 1.20, df = 1, 181, P > 0.05) between loosestrife-dominated and other vegetation types.

Swamp Sparrow abundance differed among vegetation types (ANOVA: F = 39.03, df = 7, 180, P < 0.0001; Table 5) and between periods (ANOVA: F = 6.88, df = 1, 180, P = 0.009). Swamp Sparrow abundance was higher during the late period (19.1 ± 1.1 birds/ha) compared with mid-period (15.2 ± 1.1 birds/ha). Swamp Sparrow abundance was higher (orthogonal contrast: F = 133.06, df = 1, 180, P < 0.001) in loosestrife-dominated vegetation types (36.0 + 2.0) than in other vegetation types (10.8 ± 0.8). Swamp Sparrows accounted for 95% and 65% of the overall avian density at wet meadow/loosestrife and wet meadow/scrub-shrub/loosestrife plots, respectively. Abundance of Red-winged Blackbird differed among vegetation types, but differences were not consistent between mid- and late periods (ANOVA: vegetation period interaction, F = 2.14, df = 7, 173, P = 0.04, Table 5). The interaction was due to significantly higher (P < 0.001) numbers of Red-winged Blackbirds observed in mid-period scrub-shrub compared with late period scrub-shrub (Table 5). Red-winged Blackbird abundance did not differ (orthogonal contrasts: P > 0.05 for both tests) between loosestrife dominated and other vegetation types in either period.


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