Effect of density and nearest neighbours on simulated waterfowl nests: can predators recognize high-density nesting patches?

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In North America, expansion of agriculture has led to the fragmentation of grasslands used as nesting habitat by waterfowl. Consequently, low availability of suitable nesting areas may concentrate nesting birds in small patches, leading to density-dependent nest predation. If predators recognize high-density patches, then density effects may intensify with time. We tested this hypothesis in a fragmented prairie-farmland landscape using simulated waterfowl nests deployed at three densities (2.5, 10, and 25 nests/ha). Density effects did not occur in the early nesting season (15 May-13 June), but were significant during the late nesting season (15 June-14 July), when Mayfield nest success decreased from 16.0% in low, to 4.0% in high and 0.0% in intermediate nest density areas. During both the early and late season, nearest neighbour effects were present at both intermediate and high densities, but rarely observed at low densities. Furthermore, nearest neighbour effects occurred sooner during the late season than during the early season, suggesting that predators, mostly striped skunks, recognized and keyed on high-density nesting patches. However, density-dependent nest predation is probably not a major factor affecting waterfowl at current densities (typically < 2.5 nests/ha). Instead, other factors such as composition of the predator community and distance to predator dens (or nests) may have a stronger influence on the fate of waterfowl nests in fragmented landscapes.

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