Vascular Plant and Vertebrate Inventory of Saguaro ... - USGS

the relative abundance by repeat-visit transect and
year. Because relative abundance is the closest
index to true population size that we employ (see
Chapter 1 for more detailed discussion), we use it
to note the “abundance” of species.
Relative Frequency of Detection. Relative
abundance is the least biased index to abundance
because we control a number of variables that
account for differences among transects (e.g.,
the ability to see or hear a bird). However,
we also wanted an index that accounted for all
of the species observed within a transect (i.e.,
including birds seen > 75 m, flyovers, and birds
seen outside of the 8-minute count period) and
which also conveyed some relative abundance
information. Relative frequency of detections
incorporates these observations and differs from
relative abundance in that it is clearly biased
toward those species that are highly visible or
vocal. Therefore, it can be thought of as an index
of the number of birds that we saw and heard at
typical stations on the transect (i.e., most similar
to an observer’s “experience”). This method also
enables us to convey other important information
regarding species’ presence at the transect level.
Community Classification. Using data from
repeat-visit VCP transects, we sought to identify
bird communities within the district and to
compare bird characteristics among communities.
We did not use the original stratification of
random transects for this analysis because we
were more interested in identifying communities
than drawing inference to a larger area. To group
transects, we used Ward’s hierarchical cluster
analysis using bird and vegetation data. Cluster
analysis is a multivariate technique that groups
like entities (in our case transects) that share
similar values. We performed separate cluster
analyses for the bird and vegetation data (see
Chapter 3 for results of cluster analysis using
vegetation data). To identify groups from the
bird data we used mean relative abundance for
each transect and all visits in both years. We
attempted to include reconnaissance VCP surveys
into this analysis but the results were inconsistent,
most likely because we had no vegetation data for
these transects and there was likely insufficient
sampling effort for birds.
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Comparing Communities. We compared
species richness and relative abundance among
community types. To compare species richness
we used a subset of data from all transects so
that each transect consisted of four visits to
four stations (n = 16); the minimum number
of visits and stations to repeated-visit transects
(see Table 5.2). We used only those detections
< 75 m from stations (n = 2,476 observations)
and excluded flyovers and birds seen outside
of the eight-minute count period. To compare
relative abundance among communities, we
used observations from all visits and stations
and did not choose a subset of observations (as
for species richness) because relative abundance
is scaled by survey effort. We tested for
differences among all communities using oneway
analysis of variance (ANOVA) and searched
for pairwise differences between communities
using the Tukey-Kramer procedure. We logtransformed
relative abundance data to better
meet assumptions of normality.
Line-transect Surveys
Field Methods
We used a modified line-transect method (Bibby
et al. 2002) to survey for birds from November
2002 to February 2003. Line transects differ
from VCP transects in that an observer records
birds seen or heard while the observer is walking
an envisioned line rather than while standing at
a series of stations. The line-transect method is
more effective during the non-breeding season
because bird vocalizations are less conspicuous
and frequent and therefore birds tend to be more
difficult to detect aurally (Bibby et al. 2002).
We established three transects in the
district (Fig. 5.2). Transects were broken into
sections, each approximately 250 m in length. As
with VCP transects, we alternated direction of
travel to reduce biases and did not survey during
periods of excessive rain or wind (see VCP
survey methods for details). We began surveys
about 30 minutes after sunrise and continued until
we completed the transect. As with VCP surveys,
we recorded weather conditions at the beginning
and end of each survey. We timed our travel so
that we traversed each section in ten minutes,