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 publication:
Greenberg, Russell S. 2003. The use of nest departure calls for surveying Swamp Sparrows. Journal of Field Ornithology, 74(1): 12-16.
Female Swamp Sparrows (Melospiza georgiana) give a loud series of chips as they leave their nest during incubation and brooding. I tested the efficacy of basing breeding population surveys on the nest departure call (NDC). For a subset of point-count surveys designed to estimate the abundance and distribution of Swamp Sparrows in the coastal mid-Atlantic States, I surveyed singing males and (in a longer survey) the number of different females uttering NDCs. A set of 31 points was surveyed in early June and a subset of 21 in early July. The number of NDCs was well correlated with the number of singing males detected in the early season. The number of females giving NDCs was consistently smaller than the number of singing males. In part, this is because the detection distance for NDC is significantly shorter than for male song. However, even within a small fixed circular plot, more males were detected. Singing-male surveys provide more data over a shorter period of time and are appropriate for large-scale surveys. However, female NDCs provide an index of actual reproductive activity with no unmated birds included. Although a longer survey period is required, the surveys can be conducted throughout the day. It is suggested that female vocalizations related to nesting activity are more widespread than is generally appreciated and, when possible, should be used at least as complementary data to singing-male surveys.
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