Traditionally, scientists seeking to estimate the population of a bird would survey the number of singing males in a given area. This method has worked well, but does present some issues.
For example, unmated males sing a lot, but their presence does not indicate the number of breeding females. Furthermore, males typically sing most at specific times of the day and season and during good weather.
Surveying females might give a more accurate picture of the true population size but they are mostly silent and hidden. However, the female coastal plain swamp sparrow utters a distinctive series of chip notes when she leaves the nest, either during incubation or brooding.
Scientists counted both male and female coastal plain swamp sparrows in Delaware marshes during the summer of 2000. They found singing males about twice as often as calling females, largely because the male's song can be heard from farther away.
In some situations, the female's nest departure call may be an effective survey technique. The survey time does need to be longer since females leave the nest every half hour or so, but the survey can be done any time during the day.
This article summarizes the information in this scientific paper:
Greenberg, Russell S. 2003. The use of nest departure calls for surveying Swamp Sparrows. Journal of Field Ornithology, 74(1): 12-16.
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