Cape Sable seaside sparrows prefer to nest in graminoid prairie vegetation whose stands are composed mostly of Muhlenbergia filipes interspersed with thin stands of Cladium jamaicense. These stands provide ideal nesting as well as foraging sites for the sparrow. They average a meter in height and also contain vegetation whose density within the lower portion (~ 3 cm above ground level) of the stands are relatively obscured from view. These obscured portions of the grass stand provide protective cover from predators for the sparrow's nest. Within these stands, sparrows typically construct a cup nest but some sparrows will construct a dome nest.
The nesting season for the sparrow begins as the high water levels recede during the onset of the dry season (between late February and mid-March). Sparrows entire nesting cycle encompasses a 33 to 44 day period. The graph at left documents the relative contribution of each period to the nesting cycle. Portions of the cycle not included in the graph are courtship and nest building. Depending on the timing of the wet season, the sparrows may be able to complete up to three broods within a breeding season. Generally, the dry period within the marl prairies only allow for rearing of two broods. If it is an extremely wet year and water levels do not drop below nesting height then the sparrow will be forced to forgo breeding for the year.
In the model simulation, SIMSPAR, a nesting cycle of 33 days is used after a nest building period of 7 days and the end of egg laying. In total this represents an average period of 43 days if the pair lays 3 eggs.