Optimal Niche Space of the Redwinged Blackbird: Spatial and Temporal Patterns of Nesting Activity and Success

Nesting colonies of redwings in marshes are larger and tend to be more synchronous than colonies in upland habitats of comparable area, but the duration of nesting activity is longer in large colonies. These variations in timing are probably correlated with the phenology of the vegetation in nesting habitat and not with the size of the breeding colony. Earlier nesting may be selectively advanageous because of a seasonal trend of increasing predation. The survival value to redwings of large colony size and synchronous nesting, in the colonies studied, was a reduced predation rate on a percentage basis. In most large and small colonies, the percentage of nests predated was lower during the peak of nesting activity than later. The Holling predation model is applied as a possible interpretation of predation rates on different—sized nesting colonies. Larger colonies in uplands might reduce predation on individual nests, but colony size is probably limited by the abundance of certain resources, such as availability of suitable nest—sites or food supply. Solitary pairs of smaller colonies might also suffer less predation, but genetic exchange with marsh populations, where colonial nesting is advantageous, may prevent their occurrence.