Virginia marsh songbird rope-drag data – winter 2014

Bird species that are restricted to tidal marshes during one or all of
their life stages are under increasing pressure from sea-level rise. To
date, most of the research focused on this group has been conducted during
the breeding season despite the fact that more than half of the annual
cycle is spent on wintering grounds and the high likelihood that the
winter period is the most critical time for adult survival. We used a
double-pass rope-drag technique to estimate the winter abundance of
sharp-tailed sparrows (Ammospiza nelson and A. caudacutus collectively),
seaside sparrows (A. maritimus) and marsh wrens (Cistothorus palustris)
within tidal marshes of Virginia along 102 60×250 m transects between
January and March 2014. We used the first pass to remove birds from the
transect and the second pass was used to estimate detection probabilities.
The technique was highly effective producing detection rates of 98% for
sharp-tailed sparrows, 95% for seaside sparrows, and 91% for marsh wrens.
We conducted three rounds of surveys and found that species-specific
detection rates were comparable when we restricted our analyses to two
survey rounds. Availability and abundance estimates deviated to a greater
degree than detection rates when restricting data to that collected during
only two rounds but confidence intervals overlapped for all three taxa,
regardless of which two survey periods were used for the comparison.
However, results were less precise when we restricted our analyses to two
of three rounds with confidence intervals averaging 13%, 45%, and 14%
larger for detection, availability, and abundance respectively. The
double-pass rope-drag technique provides an effective, unbiased sampling
technique to estimate winter songbird abundance in saltmarsh habitat
provided that at least two rounds are used and increasing the number of
survey rounds will result in more precise estimates.