Findings from studies of the nesting ecology of the coastal plain subspecies of the Swamp Sparrow
The nest is usually quite well-concealed. It is often anchored to the base of a High Tide Bush (Iva frutescens), a shrub often found in Swamp sparrow habitat. A dense tuft of Salt Hay (Spartina patens) often camouflages the nest and provides some structural support.
Smithsonian researchers are conducting studies of Swamp sparrow nests in Delaware. Finding the well-hidden nests might seem to be an impossible task, since the nests are so well disguised, but the female will often give away the nest's location.
When leaving the nest, about every 15 minutes, the female utters a distinctive series of "chip" notes.
By cueing in on this vocalization, a nest can be found fairly easily.
It is not known why females should advertise the presence of their nest, this would seem to be counterproductive as predators could cue in on the female's call.
However, Swamp sparrows often nest colonially and the males are particularly vigilant in defending their tiny bit of marsh. Since male and female sparrows are quite similar in appearance, it is possible that the female is telling any males within earshot that she is not a male and should not be harassed.
Predation is the number one cause of nest loss (70-80% of nests fail overall), flooding accounts for about 10% of failed nests. The primary nest predator is probably small mammals. Sparrows often renest later in the summer. They seem more likely to renest than their inland relatives and lay smaller clutches of eggs (they don't "put all their eggs in one basket".)