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

during which time we assigned all birds seen
and/or heard into one of the following distance
categories: ≤ 100 m, > 100 m, or “flyover.”
When possible, we noted the sex and age class of
birds. We recorded birds observed before or after
surveys as “incidentals” (see section below) and
we did not use techniques to attract birds (e.g.,
“pishing”).
Effort
The number of sections along each transect
ranged from eight to 13 (Table 5.2). We surveyed
each transect four times in the winter of 2002 and
2003.
Analysis
Due to the low number of observations
within 100 m of the transect lines, we used
all observations (except unknown species) to
estimate relative abundance (see Methods section
of VCP surveys for more details).
Nocturnal Surveys
Field Methods
To survey for owls we broadcast commercially
available vocalizations (Colver et al. 1999) using
a compact disc player and broadcaster (Bibby et
al. 2002) and recorded other nocturnal species
(nighthawks and poorwills) when observed. We
established nine transects (Fig. 5.2). The number
of transects per elevation stratum was lowest
for the middle elevation (n = 2) and highest for
the high elevation areas (n = 4) (Table 5.2). The
number of stations per transect varied depending
on logistical constraints but all stations were a
minimum of 500 m apart. For transects that we
visited multiple times, we attempted to reduce
sampling biases by varying direction of travel
along transects. We began surveys approximately
45 minutes after sunset.
We began surveys at each station with a
three-minute “passive” listening period during
which time we broadcast no calls. We then
broadcast vocalizations for a series of two-minute
“active” periods. We broadcast vocalizations
of species that we suspected might be present,
based on habitat and range information. The
species that we broadcasted changed based on the
elevation stratum of the surveys:
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• Low elevation: elf, western screech,
burrowing, and barn owls;
• Middle elevation: elf, northern pygmy,
flammulated, and whiskered screech
owls;
• High elevation: northern pygmy,
flammulated, northern saw-whet, and
whiskered screech owls.
We excluded the great horned owl from the
broadcast sequence because of its aggressive
behavior toward other owls (though we recorded
them incidentally). Also, we did not survey for
the Mexican spotted owl or the cactus ferruginous
pygmy-owl because that would have required
use of specific protocols and because park staff
survey periodically for these species.
We broadcast recordings of owls in
sequence of species size, from smallest to largest,
so that smaller species would not be inhibited by
the “presence” of larger predators or competitors
(Fuller and Mosher 1987). During active periods,
we broadcast owl vocalizations for 30 seconds
followed by a 30-second listening period. This
pattern was repeated two times for each species.
During the count period we used a flashlight to
scan nearby vegetation and structures for visual
detections. If we observed a bird during the
three-minute passive period, we recorded the
minute of the passive period in which the bird
was first observed, the type of detection (aural,
visual or both), and the distance to the bird. If a
bird was observed during any of the two-minute
active periods, we recorded in which interval(s)
it was detected and the type of detection
(aural, visual, or both). As with other survey
methods, we attempted to avoid double-counting
individuals recorded at previous stations. We
also attempted to use a different observer for each
visit, alternate direction of travel along transects,
and not survey during inclement weather.
Effort
The number of stations and visits to each transect
differed among transects (Table 5.2). Although
we had the most transects in the high elevation
stratum, we had most (56%) of our survey effort
in the low elevation stratum because of greater
ease of accessing stations.