DOI: 10.5061/dryad.prr4xgxr0
title: Virginia marsh songbird rope-drag data – winter 2014

description: 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.
description: We used double-pass rope-drag
 transects to survey sparrows and wrens within marsh sites. Transects were
 60m wide and their length varied from 200–250 m depending on the available
 space and placement within the marsh patch. The 60m rope (0.635 cm
 diameter) was weighted in the middle by two plastic containers filled with
 stones to create additional vegetation disturbance. A transect was
 surveyed at approx. 0.8 m/s<sup>-1</sup> by three people where
 two were stationed on either end of the rope and the third walked
 &lt;5 m behind the center of the rope. The third observer mapped the
 transect tracks to ensure that the survey was performed in a straight
 line. Once the entire transect length was surveyed (first pass) the
 observers immediately reversed direction to pass over the same area a
 second time (second pass) following the GPS track and observer footprints
 to ensure the exact same path was followed on the return pass. When birds
 were flushed, we recorded whether the bird landed within or outside the
 transect area. If birds landed within the transect area ahead of the rope,
 we monitored their landing location until the rope passed over them again
 and repeated the process until the bird flushed outside the transect. All
 birds that landed within the transect after flushing were eventually
 flushed off of the transect. Mapping allowed observers to distinguish
 between birds detected on the first pass from new birds detected on the
 second pass. Flushing birds were recorded by species group. Seaside
 sparrows and marsh wrens could be readily identified to species. Nelson’s
 sparrows and saltmarsh sparrows were combined into a single group referred
 to as “sharp-tailed sparrow” due to the difficulty in distinguishing these
 two species visually when birds quickly fly to concealing
 vegetation.