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Mono Lake Phalarope Surveys 2020 Report
By Ryan Carle1 & Margaret Rubega2
1Oikonos - Ecosystem Knowledge, ryan@oikonos.org, www.oikonos.org
2 University of Connecticut, margaret.rubega@uconn.edu
Summary
Population trends of Wilson’s and red-necked phalaropes are poorly understood, though their
habitat at saline lake migratory staging areas is threatened by water diversion and climate
change. We conducted a second consecutive year of phalarope monitoring at Mono Lake in
2020, with the goal of better understand timing, habitat use, and population trends of Wilson’s
and red-necked phalaropes at Mono Lake. We conducted eight boat- and shore-based whole-lake
surveys during 2020, with pairs of surveys 1-2 days apart every two weeks from mid-June to
early September. Maximum counts during 2020 were 2,765 ± 96 Wilson’s phalaropes and
10,891 ± 581 red-necked phalaropes. Timing of phalaropes was consistent with past years, with
Wilson’s phalarope numbers peaking in late July, and red-necked phalarope numbers staying
relatively high from late July through early September. In 2020, both Wilson’s and red-necked
phalaropes used habitat in the remote north-northeast quadrant of Mono Lake, and Wilson’s also
concentrated at South Tufa. Wilson’s phalarope peak numbers were similar in 2019 and 2020.
However, there were >10,000 thousands fewer unidentified phalaropes during peak Wilson’s
phalarope surveys in 2020 than in 2019, suggesting thousands fewer Wilson’s phalaropes were
present in 2020. Red-necked phalarope numbers were greater in 2020 than in 2019, with the
2020 peak nearly double the 2019 peak. Both species’ numbers in 2019 and 2020 were lower by
multiple thousands of birds than results from surveys in 1990-1991 conducted with a similar
methodology to ours, but trends remain uncertain without more survey years with comparable
data. Our results underscore the need for rigorous annual surveys with a stable, standardized
methodology at Mono Lake and other major staging sites, as well as the need for tracking studies
to understand phalarope residence times at staging sites and movement patterns between sites.
Introduction
Wilson’s and red-necked phalaropes are banner species of saline lakes in the interior west of
North America, where they stop in flocks of up to hundreds of thousands. Although the saline
lake and wetland habitats that phalaropes rely on are globally threatened, the conservation status
of each species of phalaropes is poorly understood (Lesterhuis and Clay 2009, Rubega et al.
2020). This data gap is largely due to the difficulty of counting these unusual shorebirds that
spend much of their time swimming far from shore on large lakes. Given their reliance on saline
lakes, many of which are existentially threatened by water diversion and climate change (Moore
et al. 2016, Wurtsbaugh et al. 2017), there is an urgent need to better understand the phalarope
population trends. Indeed, the National Audubon Society considers Wilson’s phalaropes “climate
endangered” (Langham et al. 2015) and red-necked phalaropes “highly vulnerable” (Wilsey et al.
2019) to climate change based on modeled loss of wetland habitats predicted with climate
change.
Phalaropes breed in northerly latitudes of North America, and during southward migration
significant numbers of Wilson’s and red-necked phalaropes stop at just a few hyper-saline lakes
(Jehl 1986, 1988). Smaller numbers of phalaropes are also seen at wetlands across a broad
geographic front in western North America, and whether and how either species’ movements
vary seasonally or annually is unknown. Both species spend boreal winters in the southern
hemisphere–Wilson’s phalaropes in hyper-saline interior wetlands in South America, and red-
necked phalaropes on the ocean off the western coast of South America. Historically, Great Salt
Lake, Utah, Mono Lake, California, and Lake Abert, Oregon, have hosted large concentrations
of phalaropes, and surveys at these sites offer a good opportunity to understand population trends
of the species overall (Jehl 1986, 1999). However, phalaropes have not been regularly surveyed
at Mono Lake since the 1990s (Jehl 1999), and there is a need for a standardized, repeatable
annual survey method that can be compared year to year, and across major staging sites.
In 2019, we began annual surveys of phalaropes at Mono Lake in order to create a standardized
time-series to evaluate population trends in Wilson’s and red-necked phalaropes, and to better
understand the current timing and spatial use of phalaropes at Mono Lake. Also in 2019, we
founded the International Phalarope Working Group with partners from around the western
hemisphere. The working group has worked to coordinate phalarope censuses at Mono Lake with
other Great Basin sites, and to survey Wilson’s phalaropes in South America, with the goal to
update the world population estimate for the species (Castellino and Lesterhuis 2020). 2020 was
our second consecutive year of surveys at Mono Lake, using a methodology based on surveys by
Rubega and Keimel in 1990, 1991, and 2017. The timing of our Mono Lake surveys was
coordinated with phalarope surveys at five other western North American staging sites during
2020. Results across all sites will be described in an upcoming report.
Methods
During 2020, we conducted eight full-lake censuses of Mono Lake from July 16th to September
5th, with two surveys in a 2-3 day window roughly every two weeks. Two planned mid-
September surveys were cancelled because of heavy smoke in the Mono Basin from wildfires
during September. We did one shore survey on September 20th without a boat survey. Boat
surveyors were Ryan Carle, Ashli Lewis, and Sarah Hecocks, the same boat survey team that
conducted 2019 surveys. Shore surveyors were Nora Livingston, Dave Marquart, Bartshe Miller,
David Carle, Janet Carle, and Catherine Jones.
Our 2020 protocol followed methods developed during the 1990s by Rubega and Keimel (2017),
that we modified during 2017 and 2019. The 1990s protocol calls for surveying phalaropes by
boat at defined survey points (Fig. 1A), where two observers in the same boat count phalaropes
in a 360° span within a fixed-distance radius. Any single point set of counts differing by more
than 10% is repeated. Depending on flock size, flocks are counted in groups of 1s, 10s, 100s, or
1,000s and reported in corresponding round numbers. Count times are held to under 5 minutes to
minimize recounting birds. Boat survey directions are alternated on sequential survey days, in
order to minimize time-of-day effects on detection, starting either at point 1 and ending at point
45, or vice versa.
Figure 1A. Mono Lake in 1991. Numbered points are the original 45 phalarope boat
survey points. The lake’s elevation was approximately 6,375 feet above sea level in 1990-
1991. Note the dry areas in the north and east of the lake, which were flooded during 2019-
2020 surveys.
Figure 1B. Mono Lake in 2020. The 2019-2020 elevation of the lake was 6,381-6,382
feet above sea level– 6-7 vertical feet higher than in 1990-1991. White dots are original
1990-1991 phalarope boat survey points, yellow dots are points added in 2017 and 2019,
and blue dots are points added in 2020. Numbered dots are boat survey points, named dots
are shore survey points. Note that areas in the north and east of the lake that were dry in
1990-1991 (Fig. 1A) were flooded in 2019-2020.
During 2019, we updated the survey methods because Mono Lake’s water level had risen six
vertical feet since 1990-1991. The lake’s surface area in 2019-2020 was much larger than in
1990-1991 (Fig. 1A, 1B), meaning nearshore points in 1990-1991 were now farther offshore, and
some large areas were flooded that were once dry, especially the north and east parts of the lake
(Figs. 1A, 1B).
A detailed description of modifications to the protocol during 2019 are described in our 2019
report (Carle and Rubega 2019). Major modifications were:
● The addition of nine boat survey points in 2017, and two in 2019. The number of boat
survey points was 45 in 1990-1991, 54 in 2017, and 56 in 2019.
● The modification of the 1990-1991 boat count radius from 75m (17,671 m2 total count
area per point) to 400m (503,000 m2 total count area per point), starting in 2019. During
1990-1991, a 75m radius was guessed at without measuring tools. In 2019, few birds
were recorded within 75m, so we used a rangefinder to determine that 400m was a count
radius more likely to result in detections without being so large that accurate
identifications became difficult. 400m radius counts began on August 2nd, 2019.
Comparison of count numbers suggest that the 75m count radius during 1990-1991 was
likely overestimated. We again used a 400m count radius in 2020.
● The addition of shore survey points at County Park, Old Marina, Lee Vining Tufa, and
South Tufa in 2019. Boat survey points established in the 1990s near shore in these areas
historically known to host phalaropes (Jehl 1986, 1988), were no longer near shore in
2019. These areas also could not be fully covered by new boat points because of shallow
water, views blocked by tufas, and nesting osprey disturbance.
The shore survey protocol, added in 2019, consisted of one observer counting with a
spotting scope at standardized GPS points at shore locations. At each shore survey point,
count area and distance were standardized using fixed landmarks and maps (see
Appendix 2). We estimated the total lake area surveyed at each shore point by measuring
the areas of polygons of survey coverage created in Google Earth Pro (Table A2.1,
Appendix 2), using a satellite image from November 2019. Lake area covered varied by
shore survey site (Table 1), based on the specific views afforded by each site. The
estimated survey area for each shore survey location was less than the 0.5 km2 area of
each boat point (Table 1).
Table 1. Total estimated lake surface areas (km2) of phalarope survey points at Mono Lake
during 2019. See Appendix 2 for maps of shore survey areas.
Phalarope Survey Point
Total Count Area (km2)
Nov 2019 photo
Boat point – 400 m radius
0.50
County Park shore survey
0.20
Lee Vining Tufa shore survey point 1
0.13
Lee Vining Tufa shore survey point 2
0.16
Old Marina shore survey
0.47
South Tufa shore survey point 1
0.22
South Tufa shore survey point 2
0.25
Black Point shore survey
0.66
During 2020, we made the following additional modifications to the protocol:
● The addition of three more boat survey points to provide greater overall coverage of the
lake. These were located on the west side of Negit Island near the Black Point Shoals
(Point 58; added August 13th), near the Mill/Wilson Creek deltas (Point 60; added August
15th), and in a part of northwest of the lake that was previously without any points (Point
59; added August 15th; blue dots in Fig. 1B). The Mill/Wilson Creek delta and Negit
Island points were in near-shore, shallow areas with submerged tufa, characteristics
which appear to attract phalaropes. These areas were dry when the survey protocol was
created in the 1990s. The point in the middle of the lake was in an area that was
previously not well covered by the survey, which we had observed was often used by
phalaropes. The addition of these points complicates year to year comparisons, so we
excluded them in direct comparisons of 2019 and 2020 results. However, the addition of
these points gives a more complete account of the total number of phalaropes on the lake
during a survey, though it is important to note that the result is still an index, not a
comprehensive count of every bird on the lake.
● The addition of a shore survey point at Black Point. This area was dry during the 1990-
1991 surveys, but in 2020 had a shallow inlet of water between shore and the land bridge
island (also known as Gaines Island). This inlet was invisible from the boat points and
was too shallow to boat into. Similar to the additional boat points added in 2020, we
dropped this point for direct comparisons with 2019, but used it to report maximum 2020
counts.
● Refinement of shore survey boundaries and observer training (see Appendix 2), and the
addition of a second observer during shore surveys for better error estimation.
● Change of the error threshold for repeating counts from 10% difference between
observers to 15%. We decided to do this because of the tradeoff between making time-
consuming repeat counts of large flocks until we were within 10% observer error, and
finishing the survey without frequent large-flock recounts, and within the limited daily
window when counting is possible. Moreover, recount results often differed from original
counts because of movement of birds during the time-consuming counts of large flocks.
Observer error was often within 15%, but not 10%, in original counts, so we chose the
15% threshold.
To create single-day totals for each species, we combined each day’s boat and shore survey
results. First, we adjusted boat survey totals in places with overlapping shore and boat points by
estimating the percentage of overlap area between the boat and shore points, and subtracting that
proportion from the boat point’s count (see Appendix 2). We used this method for boat points
that overlapped with shore counts at South Tufa, Lee Vining Tufa, Old Marina, and County Park.
To quantify the total number of phalaropes observed per species on each survey, we used the
equation
+ =
where is the mean of the two observers’ totals from each survey type.
We calculated a standard deviation for the combined total using the equation:
(
) =
We report errors throughout as ± 1 standard deviation.
Results and discussion
Maximum numbers of phalaropes
The season maximums for phalaropes on Mono Lake during 2020 were 2,765 ± 96 Wilson’s
phalaropes on July 31st, 10,891 ± 581 red-necked phalaropes on August 15th, and 1,385 ± 25
unidentified phalaropes on July 31st (Fig. 2). The greatest combined count of phalaropes of all
species during 2020 was 11,130 birds on August 15th.
Figure 2: Total number of Wilson’s, red-necked, and unidentified phalaropes
recorded on Mono Lake from combined boat and shore surveys during 2020. Data
includes all points counted in 2020, including points not counted in 2019. Totals shown for
each date are the sum of the averages of the two boat surveyors’ totals and the averages of
the two shore surveyors’ totals. Error bars are the combined standard deviation of these
two averages (see equation in text). With the exception of August 15th, SDs were too small
to be visible on the scale of the graph. For four points with overlapping shore and boat
surveys, we adjusted boat survey totals to account for the percent of overlap (see text,
Appendix 2). Points close together are from surveys within 1-3 days of each other. Raw
data from all dates and surveyors, as well as overall whole-lake totals are in Appendix 1.
To compare 2020 counts with 2019, we dropped the counts from the three boat points (58, 59
and 60) and one shore point (Black Point) added in 2020. Excluding those points did not change
high count dates, but changed maximum count totals of red-necked phalaropes to 10,597 ± 567,
and unidentified phalaropes to 400 ± 14. Exclusion of those points did not change the Wilson’s
phalarope high count. The remainder of the results reported in the results section exclude the
points added in 2020 unless otherwise noted. We compare 2020 counts only to 2019 counts with
400m boat count radii (August 2nd 2019 and on).
The July 31st high count of 2,765 ± 96 Wilson’s phalaropes was similar to 2019’s August 2nd
high count of 2,712 ± 75 (Fig. 3). These numbers remarkably are similar; however, there were
0
2000
4000
6000
8000
10000
12000
14000
Number phalaropes
Wilson's
Phalarope
red-necked
phalarope
unidentified
phalarope
far more additional unidentified phalaropes on the peak Wilson’s phalarope count day in 2019
(12,817 unidentified phalaropes) than in 2020 (400 unidentified phalaropes; Fig. 4). If
unidentified phalaropes counted on July 31st, 2020 at Black Point (which was not surveyed in
2019) were included, the maximum combined Wilson’s and unidentified count would be 4,150
for 2020, vs. a combined unidentified and Wilson’s high count of 15,582 in 2019. A portion of
the “unidentified” birds in 2019 were a flock of 8,000-9,000 phalaropes at South Tufa that were
repeatedly identified as Wilson’s during late July-early August 2019, but were recorded as
unidentified on the official August 2nd, 2019 shore survey. Including these birds in the 2019 high
count results in an estimate of approximately 11,200 Wilson’s phalaropes. Thus, it appears that
the number of Wilson’s Phalaropes at Mono Lake was many thousands of birds lower in 2020
compared with 2019.
Figure 3. Comparison of Wilson’s phalarope high counts during survey windows in
2019 and 2020. For each survey window, the highest single-day total is shown. There
were two surveys in each window in each year. Mid-July surveys are not shown because
the mid-July 2019 surveys used a different boat count radius than was used on subsequent
surveys. Differences in annual Wilson’s phalaropes are best interpreted along with
unidentified phalarope counts on the same dates (Fig. 4).
0
500
1000
1500
2000
2500
3000
3500
7/31 to 8/2 8/13 to 8/20 9/2 to 9/5
Number Wilson's phalaropes
Survey window
2019
2020
Figure 4. Comparison of unidentified phalarope high counts during survey
windows in 2019 and 2020. For each survey window, the highest single-day total is
shown. There were two surveys in each window in each year. Mid-July surveys are not
shown because the mid-July 2019 surveys used a different boat count radius than was used
on all subsequent surveys.
The maximum count of red-necked phalaropes in 2020 was on August 15th (10,597 ± 567), and
was nearly double the 2019 maximum (5,573 ± 177, September 2nd, 2019; Fig. 5). Indeed, counts
on four of the 2020 dates exceeded the maximum 2019 count. Red-necked phalarope numbers
historically remained near peak-levels into mid-September (Jehl 1986), so it is possible that we
missed a peak of red-necked phalaropes because we could not survey in mid-September 2020.
The maximum count of unidentified phalaropes in 2020 was 400 ± 14 on July 31st, much less
than the 12,817 ± 3,197 maximum on August 2nd 2019 (Fig. 4). Far fewer birds were unidentified
in 2020, mainly because shore survey observers had greater identification skills and regularly
identified birds to species at Lee Vining Tufa and South Tufa shore survey points. The exception
was the Black Point shore point, which was too distant from the water to tell species apart during
most surveys. Black Point was excluded for comparison with 2019, but the highest overall 2020
counts of unidentified birds (985 ± 21 on July 31st, 418 (one observer, no SD) on August 2nd)
were from Black Point.
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
7/31 to 8/2 8/13 to 8/20 9/2 to 9/5
number unidentified phalaropes
Survey window
2019
2020
Figure 5. Comparison of red-necked phalarope high counts during survey windows
in 2019 and 2020. For each survey window, the highest single-day total is shown. There
were two surveys in each window in each year. Mid-July surveys are not shown because
the mid-July 2019 surveys used a different boat count radius than was used on all
subsequent surveys.
Timing and spatial use of phalaropes
Wilson’s phalaropes
Wilson’s phalarope numbers increased from 658 ± 18 on July 16th to 2,475 ± 535 on July 18th,
peaking on July 31st at 2,765 ± 96 (Fig. 2 for trends, exact values in Table A1.2 Appendix 1).
Numbers dropped to 1,128 ± 57 by August 2nd, and declined through mid-August (totals in the
hundreds) and early September (totals of 15 and zero birds). The attendance pattern and timing
of Wilson’s Phalaropes in 2020 was similar to 2019 and to patterns during 1980-1986 (Jehl
1988) and 1990-1991 (Rubega and Keimel 2017). The pattern in all survey years was that
numbers rapidly grew to a peak from mid-July to early August, followed by a rapid drop in the
numbers by mid-August, with most birds departed by September (Fig. 2).
0
2000
4000
6000
8000
10000
12000
7/31 to 8/2 8/13 to 8/20 9/2 to 9/5
Number red-necked phalaropes
Survey window
2019
2020
Figure 6. Distribution and magnitude of Wilson’s phalaropes on Mono Lake across
the 2020 season, from boat and shore surveys. Circle size indicates the magnitude of
counts at a given location on each date. Blue dots are boat surveys and gray dots are shore
surveys. Note that all boat survey points are directly comparable, with a standardized count
radius, whereas shore survey count areas are variable. See Table 1 for count areas of each
shore survey point. The peak Wilson’s phalarope count was July 31st.
Spatial use of Mono Lake by Wilson’s phalaropes in 2020 was generally similar to 2019, with
the largest flocks near shore at South Tufa and smaller flocks at Lee Vining Tufa and in the
north-northwest part of the lake (Fig. 6). In 2020, Wilson’s phalaropes were often in mixed
flocks with red-necked phalaropes (see cover photo of this report) in the north-northwest part of
the lake, whereas in 2019 the two species were often more segregated by species. In 2019, some
Wilson’s phalaropes moved to the west shore of the lake in mid-August, which did not occur in
2020. The use of both the south shore tufa areas and the north-northwest shore of the lake by
Wilson’s phalaropes was consistent with patterns in 1980-1986, though Wilson’s phalaropes also
used the west shore more during those years than during 2019-2020 (Jehl 1988). In 2017,
Wilson’s phalaropes were concentrated in the north-northwest shore and were not observed at
South Tufa or Lee Vining Tufa during several partial surveys over three days in mid-July
(Rubega and Keimel 2017).
Red-necked phalaropes
Small flocks of red-necked phalaropes were present during the mid-July surveys (≤120 birds
total). Numbers increased to 5,597 ± 122 on July 31st and 8,220 ± 134 on August 2nd, and peaked
at 10,597 ± 567 on August 15th (see Fig. 2 for trends, exact values in Table A1.2 Appendix 1).
Red-necked phalarope abundance did not follow a consistent trajectory; counts on back to back
days sometimes differed by thousands (Fig. 2). A count of 1,337 ± 18 on August 13th was
followed by a count of 10,587 ± 567 on August 15th, and a September 4th count of 3,428 ± 17
was followed by a September 5th count of 7,023 ± 70. These results showed the importance of
the back to back counts for capturing the daily variability of results and increasing the likelihood
of capturing peaks. Whether day-to-day variability is related to birds being present at points
during some survey days and not others, and/or to arrivals and departures of birds from Mono
Lake, is unknown. Day-to-day numbers of red-necked phalaropes also varied in 2019, but by a
lesser magnitude. Residence time of red-necked phalaropes at Mono Lake and other staging sites
is unknown, though it has been suggested that it is shorter than that of Wilson’s phalaropes (Jehl
1988). Our results on back to back days are suggestive of birds leaving and arriving, though we
cannot be certain it is not a survey effect without direct measurement of residence time.
Generally, the arrival of major numbers of red-necked phalaropes to Mono Lake in late
July/early August, followed by a protracted peak throughout August and September, was
consistent with both our 2019 results and those of surveys from 1981-1984 (Jehl 1986).
During the July 31st and August 2nd 2020 surveys, red-necked phalaropes were concentrated in
the north-northwest of the lake in shallow nearshore areas (Fig. 7). During mid-August and early
September surveys, red-necked phalaropes remained in that north-northwest part of the lake, but
also spread southwest into a greater swath of the eastern lake (Fig. 7). Small numbers (<300
birds) appeared at County Park on the northwest shore in September (Fig. 7). This movement
pattern was remarkably consistent in 2019 and 2020, and was relatively similar to movements
during 1981 and 1982 (Jehl 1986), when the lake’s elevation was ~10 vertical feet lower. In
those years, red-necked phalaropes concentrated in the north part of the lake in late July-early
August, and then spread out to more locations by mid-August, though they used the west and
south shores more often than in 2019 and 2020.
Figure 7: Distribution and magnitude of red-necked phalaropes at Mono Lake
across the 2020 season, from boat and shore surveys. Circle size indicates the
magnitude of counts at a given location on each date. Blue dots are boat surveys and gray
dots are shore surveys. Note that all boat survey points all directly comparable, with a
standardized count radius, whereas shore survey count areas are variable in area. See Table
1 for count areas of each site. The peak red-necked phalarope count was August 15th.
Unidentified phalaropes
Unidentified phalaropes were located only in the northern part of Mono Lake in 2020 (Fig. 8).
This was because it was usually not possible to differentiate phalarope species at the Black Point
shore survey point because of its distance from the lake, and because often the light was often
challenging looking straight east. Likewise, it was not always possible on the boat survey to
identify every bird in large flocks at points in the north of the lake. Unlike 2019, birds at South
and Lee Vining Tufas were regularly identified to species, due mainly to better observer
identification skills in 2020.
Figure 8: Distribution and magnitude of unidentified phalaropes at Mono Lake
across the 2020 season, from boat and shore surveys. Circle size indicates magnitude
of counts at a given location on each date. Blue dots are boat surveys and gray dots are
shore surveys. Note that all boat surveys are directly comparable, with a standardized count
radius, whereas shore survey count areas are variable. See Table 1 for count areas of each
site. The peak unidentified phalarope count was July 31st.
Comparison of survey results on consecutive days
Phalaropes move around Mono Lake on a scale from hours to days (Jehl 1988), and individuals
may arrive or depart from staging sites from one day to the next. Daily movements affect count
totals as they relate to the “true” total number of birds present. To compare counts during our
paired surveys (within 1-2 days of each other), we divided the absolute difference of the two
counts by the total from the highest count (Table 2). Daily counts differed by 59-100% in
Wilson’s phalaropes, with absolute differences sometimes over a thousand, and 32-87% in red-
necked phalaropes, with absolute differences of up to 9,260 (Table 2). This highlights the
importance of doing multiple surveys during survey windows at regular intervals throughout the
summer. For example, a single survey in a given year would be unlikely to accurately capture a
peak total of either species given day to day variability. This variability may be related to birds
moving around the lake and being visible at the fixed survey points one day and not the next, or
to birds arriving and departing the lake, or both. Moreover, we lack information about whether
birds move between Mono Lake and nearby wetlands, such as Owens Lake. The striking
difference in red-necked phalarope totals between August 13th and 15th (>9,000 more on the 15th)
was noticed during the survey; our sense was that there were many more birds on the lake overall
on the 15th than on the 13th, and it was not entirely a survey-effect. Better understanding of
residence time of phalaropes, especially of red-necked phalaropes, would help us interpret day to
day variability in count numbers.
Spatially, locations of phalaropes were fairly similar during each survey window (Figs. 6, 7, 8),
though there were enough differences to illustrate the importance of whole lake surveys. The
differing locations of Wilson’s phalaropes on August 13th and 15th (Fig. 6) was a good
illustration of how birds can be concentrated in one area (the north-northwest shore near the land
bridge island) one day, and mostly absent from that area two days later.
Table 2. Day to day differences in total counts of Wilson’s, red-necked, and
unidentified phalaropes during 2020. All counts shown were either on back to back days
or were two days apart. Percent differences were calculated by dividing the absolute
difference between the consecutive daily counts by the higher of the two count totals. Totals
used for each day were the combined average counts of the two boat observers and two
shore observers, excluding four points added in 2020 that were not counted on all 2020
dates.
Survey
dates
WIPH totals
(first survey,
second
survey)
WIPH
Absolute
difference
WIPH %
difference
RNPH totals
(first survey,
second
survey)
RNPH
absolute
difference
RNPH %
difference
7/16 and
7/18
658 ± 18,
2,475 ± 535
1,817
73%
82 ± 0,
129 ± 7
47
36%
7/31 and
8/2
2,765 ± 96,
1,128 ± 57
1,638
59%
5,597 ± 122,
8,220 ± 134
2,623
32%
8/13 and
8/15
637 ± 8,
203 ± 12
408
67%
1,337 ± 18,
10,891 ± 581
9,260
87%
9/4 and
9/5
15 ± 0,
0 ± 0
15
100%
3,938 ± 11,
8,251 ± 15
3,604
51%
Comparison of counts between boat observers: A key factor in bird surveys is error associated
with differences among individual surveyors. Table 3 shows absolute and percent differences
between observers on boat surveys. During most dates, the two observers’ results were within
10% of each other. Exceptions (e.g., Wilson’s phalaropes on August 15th; Table 3) were usually
driven by one observer seeing a flock of birds fly by that the other observer did not see. These
instances are methodologically important, but occurred most often with small groups of birds, so
they did not affect overall interpretation of results greatly. Though observers’ overall survey
sums were usually within 10% of each other, at individual points with large groups of birds it
was often difficult to efficiently achieve counts within 10% of each other. Indeed, trying to hit
the 10% threshold often felt like it biased the results by extending the count time as observers
attempted to get similar counts. This was why we changed the threshold to 15% in 2020, which
was an easier threshold to achieve.
Table 3. Absolute and percent differences between boat observers total survey
counts. Percent difference is the absolute difference divided by the highest count on each
date.
Date
WIPH totals
(obs. 1, obs. 2)
Absolute
difference
WIPH
%
difference
WIPH
RNPH totals
(obs. 1, obs.
2)
Absolute
difference
RNPH
%
difference
RNPH
7/16
0, 0
0
0%
82, 82
0
0%
7/18
152, 141
11
7%
122, 132
10
8%
7/31
1037, 1061
24
2%
5683, 5511
172
3%
8/2
641, 581
60
9%
8314, 8125
189
2%
8/13
573, 564
9
2%
1350, 1324
26
2%
8/15
96, 83
13
14%
11301, 10480
821
7%
9/4
15, 15
0
0%
3949, 3921
28
1%
9/5
0, 0
0
0%
7715, 7681
34
0%
Shore surveys vs. boat surveys: As in 2019, shore surveys were an important complement to
boat surveys. Though few or no phalaropes were registered at County Park, Old Marina, or Lee
Vining Tufa during 2020, shore surveys registered flocks of >1,000 Wilson’s phalaropes at
South Tufa in July, which were not recorded by the boat survey points (Table 4). Phalaropes at
South Tufa were located close to shore and often blocked from the view of boat points by tufa
towers. The additional shore point at Black Point, added in 2020, also registered groups of
hundreds to a thousand phalaropes. 2020 results also underscored the fact that both boat and
shore points are crucial for getting a complete picture of phalarope numbers at Mono Lake. In
both 2019 and 2020, the vast majority of phalaropes were located on the north and eastern parts
of the lake, often far out on the water in areas accessible and visible only by boat. With Mono
Lake’s dynamically fluctuating lake elevations, it is clear that survey protocols must balance
flexibility to adapt to the changing conditions (e.g. the addition of new shore and boat points in
previously dry areas) with maintaining standardization for time-series comparisons (e.g. keeping
historical boat points in the same places).
Table 4. Total counts of Wilson’s and red-necked phalaropes from overlapping
both boat and shore survey points at Mono Lake, 2020. Boat survey numbers shown
are raw, unadjusted counts. Note that boat and shore points shown were adjacent to each
other, with varying degrees of overlap. See Appendix 2. for overlap proportions in each
area. Areas with shore and boat counts that registered no birds are not shown (e.g. zero
birds the whole season at Old Marina, and zero Wilson’s phalaropes at County Park). Red-
necked phalaropes were seen at South Tufa only once (2 birds on the July 18th shore
survey, zero on the boat survey). Black Point is not shown because it did not overlap with a
boat survey point.
Wilson's Phalarope
Red–necked phalarope
Date
South Tufa
Boat
South Tufa
shore
Lee Vining
Tufa boat
Lee Vining
Tufa shore
County
Park boat
County
Park shore
7/16/20
0 ± 0
658 ± 18
0 ± 0
0 ± 0
0 ± 0
0 ± 0
7/18/20
0 ± 0
2328 ± 535
0 ± 0
0 ± 0
0 ± 0
0 ± 0
7/31/20
10 ± 0
1447 ± 90
291 ± 13
333 ± 4
0 ± 0
0 ± 0
8/2/20
12 ± 1
502 ± 33
158 ± 8
49 ± 2
0 ± 0
0 ± 0
8/13/20
7 ± 3
69 ± 6
0 ± 0
0 ± 0
0 ± 0
0 ± 0
8/15/20
11 ± 1
114 ± 7
0 ± 0
0 ± 0
0 ± 0
0 ± 0
9/4/20
0 ± 0
0 ± 0
0 ± 0
0 ± 0
51 ± 3
9 ± 0
9/5/20
0 ± 0
0 ± 0
0 ± 0
0 ± 0
215 ± 7
91 ± 0
Phalarope trends at Mono Lake
There have been phalarope surveys at Mono Lake since the 1970s (e.g., Winkler et al. 1977, Jehl
1986, Jehl 1999), but it is difficult to directly compare these surveys to our own, or to each other,
due to important differences in methodologies and coverage. We are working on a separate
report examining the details of how to best interpret historic data from Mono Lake, Great Salt
Lake, and Lake Abert. Here, we focus on comparing our 2019 and 2020 Mono Lake surveys
with each other, and to Rubega and Keimel (2017)’s surveys in 1990, 1991, and 2017, which
established the protocol ours is a refinement and extension of.
When comparing contemporary results with Rubega and Keimel’s 1990 and 1991 surveys, it is
noteworthy that the lake’s elevation was 6-7 vertical feet lower in 1990-1991 than in 2017, 2019,
or 2020 (Figs. 1A, 1B). The 1990-1991, 2017, and 2019-2020 surveys followed similar
protocols, but modifications were made as the lake level changed (see details in Methods). Given
the addition of more boat survey points, shore survey points, and a greater boat count radius
during 2019-2020 it might be expected that, all things equal, the 2019-2020 surveys would
register more birds than the 1990-1991 surveys did. However, it is also possible that phalaropes
were more concentrated in a smaller area in 1990-1991, and it may be easier to miss them on the
survey under current conditions, despite the additional survey points.
Wilson’s phalarope trends
2019 and 2020 surveys registered far fewer Wilson’s phalaropes than the 1991 peak of 35,225
birds on July 18th (Rubega and Keimel 2017; Fig. 9). Rubega and Keimel’s 1990 survey was not
timed to capture Wilson’s phalarope peak numbers, though they registered 9,037 on August 7th.
Contemporary surveys (2017, 2019, 2020) have recorded no more than 16,000 birds even when
combining confirmed Wilson’s phalaropes with unidentified birds seen on the peak Wilson’s
phalarope count date (Fig. 9).
Trends of peak Wilson’s phalaropes are best examined along with the totals of unidentified birds
on the same dates (Fig. 6), because in some years there were many more unidentified birds than
confirmed Wilson’s phalaropes. In 2017, surveys were conducted only on July 17th-19th, and
none was a complete survey (Rubega and Keimel 2017). However, the July 19th survey
registered 1,168 Wilson’s phalaropes and 14,096 unidentified phalaropes (no error rates
reported; Rubega and Keimel 2017; Fig. 6). Given this timing, it seems likely that many of the
unidentified birds were Wilson’s phalaropes in 2017 because mid-July surveys in 2019 and 2020
registered few red-necked phalaropes (i.e., <150 birds).
Figure 9. Peak annual numbers of Wilson’s phalaropes, and unidentified phalarope
totals on the same day, at Mono Lake, 1991-2020. Red squares are Wilson’s
phalaropes and blue diamonds are unidentified phalaropes. Survey date is shown above the
data points for each year. 1991 and 2017 data are from Rubega and Keimel (2017). All
surveys shown were based on the same protocol, but 2019-2020 surveys had additional
boat and shore surveys and a greater count radius than 1991 or 2017 (see text).
During 2019, the peak confirmed Wilson’s phalarope count was 2,712 ± 75 on August 2nd, but
there were also 12,817 ± 3,197 unidentified birds on the lake that day (Fig. 6). A flock of 9,457
confirmed Wilson’s phalaropes were counted from shore a few days prior at South Tufa. 8,500
unidentified phalaropes were at South Tufa on July 31st, and were almost certainly Wilson’s.
There were also substantial numbers of red-necked phalaropes at the lake on August 2nd 2019
(2,612 identified as red-necked). Thus, the true number of Wilson’s in 2019 was likely
somewhere in the range of 11,000-14,000 birds.
Jul 18
Jul 19Aug 2
July 31
0
5000
10000
15000
20000
25000
30000
35000
40000
Peak number Wilson's phalaropes
Year
Peak Wilson's phalarope counts at Mono Lake
1991-2020
Wilson's
phalaropes
unidentified
In 2020, the peak count of Wilson’s phalaropes was 2,765 ± 96 on July 31st. An additional 400 ±
14 unidentified birds could have been Wilson’s, but even with these unidentified birds, the total
count would only be 3,165 ± 97. Despite the difficulties of modified methodologies and the
uncertainty of Wilson’s vs. unidentified phalaropes in some years, the differences between the
1991 and contemporary counts are striking, and cause for concern. There were half as many or
fewer Wilson’s phalaropes at Mono Lake in 2017, 2019, and 2020 than there were in 1991, and
2020 counts were especially low (Fig. 9).
Red-necked phalarope trends
Our 2019 and 2020 surveys registered lower peak red-necked phalarope totals than 1990 and
1991 surveys, though the magnitude of the difference was not as great as the difference in
Wilson’s phalarope numbers between those years. The 1990 and 1991 surveys recorded peaks of
17,000-18,000 red-necked phalaropes (Rubega and Keimel 2017), whereas the 2019 survey had
5,573 ± 177 and the 2020 survey had 10,597 ± 567. A better understanding of red-necked
phalarope movements among lakes and residence times at Mono Lake, both of which are
unknown, is needed for both an accurate estimate of the total number of birds using Mono Lake,
and overall species population trends.
Figure 7. Peak annual numbers of red-necked phalaropes at Mono Lake, 1990-
2020. Survey date is shown above the data points for each year. 1990, 1991, and
2017 surveys were carried out by Rubega and Keimel (2017). All surveys shown
were based on the same protocol, but 2019-2020 surveys had additional boat and
shore survey points and a greater count radius than 1990, 1991, and 2017 (see
text).
Aug 7
Aug11
Sep 2
Aug 15
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
20000
1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014 2016 2018 2020
Peak number red
-necked phalaropes
Year
Peak red-necked phalarope counts at Mono Lake 1990-
2020
Conclusions
One take-away from the 2020 surveys at Mono Lake is the continued importance of rigorous
annual surveys with a stable, standardized methodology. With 2019 and 2020 data, we now have
two years that are fully comparable. These surveys are comparable enough to the 1990, 1991,
and 2017 surveys to be of some use for comparisons over time. Each additional year of surveys
using the current method will add value to the time-series and bring us a better understanding of
phalarope trends at Mono Lake.
Another take-away is that the timing of phalaropes at Mono Lake appears unchanged since
surveys in the 1980s and 1990s. This is important as it suggests that climate change has not
altered the timing of migration of phalaropes at Mono Lake; whether phalarope arrival or
departure from staging sites will begin to lag annual peaks in prey abundance remains to be seen.
This information is also helpful for planning dates of future surveys and interpreting data from
years like 2017, when there was only a mid-July survey.
We have learned a great deal about the current spatial use of phalaropes at Mono Lake, partially
answering an original question of this project—why do people have a general impression that
there are few phalaropes at Mono Lake in recent years? One answer is that many of the
phalaropes at Mono Lake in 2019 and 2020 were simply in places where it is difficult for casual
observers to see them. The vast majority of red-necked phalaropes, and many Wilson’s
phalaropes, were located in the north-northwest quadrant of Mono Lake in 2019 and 2020, where
shore access is difficult. Furthermore, birds were often located far out on the water, not visible
from any point from shore (Fig. 8). This underscores the need for the full-lake surveys that
include boat points, and that casual surveys such as eBird, which are biased toward accessible
shore sites like South Tufa and County Park, will never tell a complete story about Mono Lake
phalaropes.
Figure. 8. Red-necked phalaropes far out on the water in the eastern part of Mono Lake,
which is accessible only by boat.
Comparison of 2019-2020 and 1990-1991 data suggests declines in both phalarope species using
Mono Lake, and especially of Wilson’s. However, more data are needed to evaluate if these are
real population trends and how much inter-annual variability and/or movement among lakes is
influencing count numbers. Combining Mono Lake data with results from other locations is vital
for understanding species-level trends. We are continuing to work with partners at Great Salt
Lake (Utah), Lake Abert (Oregon), Owens Lake (California), Lake Chaplin (Saskatchewan), San
Francisco Bay (California), and elsewhere to collate data from coordinated surveys in 2020.
Finally, efforts facilitated by the International Phalarope Working Group to survey Wilson’s
phalaropes in South America and to install Motus towers for phalarope tracking at multiple sites
will help us contextualize and understand results from Mono Lake, and lead us toward a better
understanding of how to conserve phalaropes across the hemisphere.
Funding
Funding for this project came from California State Parks, The Tracy Aviary Conservation Fund,
the Jeff Maurer Chautauqua Research Grant, Friends of Mono Lake Tufa State Reserve, and
private donations.
Acknowledgements
Many thanks to the multitude of partners that contributed to the success of this project, including
California State Parks for funding and staff and logistical support, and the Mono Lake
Committee for use of the research boat, and staff and logistical support. Thanks especially to
Bartshe Miller and Claire Landowski for help with boat coordination. Thanks to phalarope boat
surveyors Sarah Hecocks and Ashli Lewis, and shore surveyors David Carle, Janet Carle,
Catherine Jones, Nora Livingston, Dave Marquart, and Bartshe Miller. Thanks to Jonathan Felis
for making beautiful visualizations of the data. Thanks to Jessica Espinosa for comments that
improved the draft report.
Literature Cited
Carle, R. & M. Rubega. Phalarope surveys at Mono Lake 2019 report. Unpublished report to
California State Parks. Available at: https://oikonos.org/wp-content/uploads/2020/03/Phalarope-
Surveys-at-Mono-Lake-2019-Report.final_.pdf
Castellino, M. & A. Lesterhuis. 2020. Wilson’s phalarope simultaneous census 2020.
Unpublished report by Manomet, Plymouth, Massachusetts, USA.
Jehl, J.R. 1986. Biology of Red-necked Phalaropes (Phalaropus lobatus) at the western edge of
the Great Basin in fall migration. Great Basin Naturalist 46: 185-197.
Jehl, J. R. 1988. Biology of the Eared Grebe and Wilson's Phalarope in the nonbreeding season:
a study of adaptations to saline lakes. Studies in Avian Biology 12.
Jehl, J. R. 1999. Population studies of Wilson's Phalaropes at fall staging areas, 1980-1997: A
challenge for monitoring. Waterbirds 22: 37-46.
Langham, G. M., Schuetz, J. G., Distler, T., Soykan, C. U., & C. Wilsey (2015). Conservation
status of North American birds in the face of future climate change. PloS one, 10(9).
Lesterhuis, A. J., & R.P. Clay. 2009. Conservation plan for Wilson’s phalarope (Phalaropus
tricolor). Manomet Center for Conservation Sciences, Manomet.
Moore, J.N. 2016. Recent desiccation of western Great Basin saline lakes: lessons from Lake
Abert, Oregon, USA. Science of the Total Environment 554: 142-154.
Rubega, M. A., D. Schamel, and D. M. Tracy. 2020. Red-necked Phalarope (Phalaropus lobatus),
version 1.0. In Birds of the World (S. M. Billerman, Editor). Cornell Lab of Ornithology, Ithaca,
NY, USA. https://doi.org/10.2173/bow.renpha.01
Rubega, M. and B. Keimel. 2017. Final report on a 2017 census of phalaropes at Mono Lake,
California. Unpublished report to California State Parks.
Wilsey, C, B Bateman, L Taylor, JX Wu, G LeBaron, R Shepherd, C Koseff, S Friedman, & R.
Stone. 2019. Survival by Degrees: 389 Bird Species on the Brink. Report by the National
Audubon Society: New York.
Wurtsbaugh, W.A., Miller, C., Null, S.E., DeRose, R.J., Wilcock, P., Hahnenberger, M., Howe,
F. and Moore, J. 2017. Decline of the world's saline lakes. Nature Geoscience 11: 816-821.
Winkler, D.W., Weigen, C.P., Engstrom, F.B., Burch, S.E. 1977. Ornithology in An Ecological
Study of Mono Lake (D. Winkler, Ed.). Institute of Ecology Publication No. 12. University of
California, Davis.
Appendix 1.
Table A1.1. Mono Lake 2020 shore survey totals. Results shown are averages of the
two shore survey observers ± combined SD. Counts without an SD reported were made by
a single observer.
Wilson's phalarope
Date
South Tufa
Lee Vining Tufa
Old Marina
County Park
Black Point
7/16/20
658 ± 18
0 ± 0
0 ± 0
0 ± 0
No survey
7/18/20
2328 ± 535
0 ± 0
0 ± 0
0 ± 0
0 ± 0
7/31/20
1447 ± 90
333 ± 4
0 ± 0
0 ± 0
0 ± 0
8/2/20
502 ± 33
49 ± 2
0 ± 0
0 ± 0
0 ± 0
8/13/20
69 ± 6
0 ± 0
0 ± 0
0 ± 0
0 ± 0
8/15/20
114 ± 7
0 ± 0
0 ± 0
0 ± 0
0 ± 0
9/4/20
0 ± 0
0 ± 0
0 ± 0
0 ± 0
0 ± 0
9/5/20
0 ± 0
0 ± 0
0 ± 0
0 ± 0
0 ± 0
9/20/2020
0 ± 0
0 ± 0
0 ± 0
0 ± 0
No survey
Red-necked phalarope
South Tufa
Lee Vining Tufa
Old Marina
County Park
Black Point
7/16/20
0 ± 0
0 ± 0
0 ± 0
0 ± 0
No survey
7/18/20
2 ± 0
0 ± 0
0 ± 0
0 ± 0
0 ± 0
7/31/20
1 ± 1
0 ± 0
0 ± 0
0 ± 0
0 ± 0
8/2/20
0 ± 0
0 ± 0
0 ± 0
0 ± 0
0 ± 0
8/13/20
0 ± 0
0 ± 0
0 ± 0
0 ± 0
0 ± 0
8/15/20
0 ± 0
0 ± 0
0 ± 0
0 ± 0
0 ± 0
9/4/20
0 ± 0
0 ± 0
0 ± 0
0 ± 0
0 ± 0
9/5/20
0 ± 0
0 ± 0
0 ± 0
91 ± 0
490
9/20/2020
0 ± 0
0 ± 0
0 ± 0
0 ± 0
No survey
Unidentified phalarope
South Tufa
Lee Vining Tufa
Old Marina
County Park
Black Point
7/16/20
0 ± 0
0 ± 0
0 ± 0
0 ± 0
No survey
7/18/20
0 ± 0
0 ± 0
0 ± 0
0 ± 0
0 ± 0
7/31/20
0 ± 0
0 ± 0
0 ± 0
0 ± 0
985 ± 21
8/2/20
8 ± 0
8 ± 0
0 ± 0
0 ± 0
418 ± 32
8/13/20
0 ± 0
0 ± 0
0 ± 0
0 ± 0
73 ± 4
8/15/20
0 ± 0
0 ± 0
0 ± 0
0 ± 0
37 ± 2
9/4/20
0 ± 0
0 ± 0
0 ± 0
0 ± 0
0 ± 0
9/5/20
0 ± 0
0 ± 0
0 ± 0
0 ± 0
0 ± 0
9/20/2020
0 ± 0
0 ± 0
0 ± 0
0 ± 0
No survey
Table A1.2. Mono Lake 2020 boat survey totals from both observers. Totals shown
are unadjusted for overlap with shore points.
Date
WIPH
Observer
1
WIPH
Observer 2
RNPH
Observer 1
RNPH
Observer 2
Unidentified
phalarope
Observer 1
Unidentified
phalarope
Observer 2
7/16/20
0
0
82
82
0
0
7/18/20
152
141
122
132
2
0
7/31/20
1037
1061
5683
5511
390
410
8/2/20
641
581
8314
8125
310
340
8/13/20
573
564
1350
1324
158
178
8/15/20
96
83
11301
10480
0
0
9/4/20
15
15
3949
3921
0
0
9/5/20
0
0
7715
7681
0
0
Table A1.3. Mono Lake 2020 combined totals of boat and shore surveys. Totals are
shown for all points that were surveyed in 2020 (left), and for only points that were also
surveyed in 2019 (right). Points surveyed only in 2020 were Black Point (added July 18th)
and three boat survey points added August 13th (1 point) and 15th (2 points).
2020 totals--all points surveyed in 2020
2020 totals--only points also surveyed in 2019
Date
WIPH
RNPH
Unidentified
phalarope
WIPH
RNPH
Unidentified
phalarope
7/16/20
658 ± 18
82 ± 0
0 ± 0
658 ± 18
82 ± 0
0 ± 0
7/18/20
2475 ± 535
129 ± 7
1 ± 1
2475 ± 535
129 ± 7
1 ± 1
7/31/20
2765 ± 96
5597 ± 122
1385 ± 25
2765 ± 96
5597 ± 122
400 ± 14
8/2/20
1128 ± 57
8220 ± 134
743 ± 38
1128 ± 57
8220 ± 134
325 ± 21
8/13/20
637 ± 8
1337 ± 18
241 ± 15
611 ± 6
1337 ± 18
168 ± 14
8/15/20
203 ± 12
10891 ± 581
37 ± 2
203 ± 12
10597 ± 567
0 ± 0
9/4/20
15 ± 0
3938 ± 11
0 ± 0
15 ± 0
3428 ± 17
0 ± 0
9/5/20
0 ± 0
8251 ± 15
0 ± 0
0 ± 0
7032 ± 70
0 ± 0
Appendix 2: Shore survey methods
Standardized shore surveys were carried out on the same day as boat surveys. Shore surveys
were always in the morning. Two observers conducted the shore survey, driving between GPS-
marked points. Observers used a combination of spotting scopes and binoculars, but were
instructed to scan with spotting scopes on all surveys. See Figure 1 in the main text for shore
survey locations relative to the whole lake. At each site, observers used the maps below to
identify survey area boundaries (Fig A2.1, A, B, C, D, E). The County Park-Old Marina-Lee
Vining Tufa-South Tufa circuit was completed by one set of observers each survey day, and the
Black Point point was completed by a different pair of observers. All shore observers were
trained in-person in the field by project manager Ryan Carle.
Figure A2.1. Survey boundary maps used by shore observers during 2019.
A. County Park. County Park surveys were conducted at the viewing platform at
the end of the County Park boardwalk.
B. Lee Vining Tufa. Lee Vining Tufa surveys were conducted from Test Station
Road.
C. South Tufa. Note the “pirate ship” is a cluster of tufa that is currently an island but
used to be an isthmus near the “grotto” on the South Tufa interpretive trail. Shore
survey observers were familiar with this name for the geographical landmark. South
Tufa surveys were conducted from Test Station Road and the connector road
between South Tufa and Navy Beach.
D. Old Marina. Old Marina survey was conducted from the top of the paved road into
Old Marina off highway 395.
E. Black Point. The Black Point survey was conducted from a GPS point a short walk
east from the Black Point parking lot.
Calculation of shore survey areas and overlap with boat surveys
To calculate the area of shore surveys we used Google Earth Pro to draw polygons based on the
field maps of shore survey boundary areas (Fig. A2.2). We used the “measure” tool to calculate
the total area of each shore survey area polygon, based on a November, 2019 satellite photo of
Mono Lake. To calculate the area of overlap with boat points, we projected a 400 m radius circle
around the closest boat point to each shore point, then made polygons of the overlapping area
with the shore survey (Fig. A2.2). We used the “measure” tool to calculate the total area of
overlap at each shore survey/boat survey location (Fig. A2.2, Table A2.1). We also drew
polygons around the land area covered by each boat point that overlapped with a shore survey,
and subtracted land area from the total survey area to determine “total water survey area” (Table
A2.1). We divided the total overlap area with the shore survey by the total water area of each
boat survey point to determine the percentage of the boat survey area that overlapped with the
shore survey (Table A2.1). Finally, we subtracted the percentage of overlap between the shore
and boat survey from the boat survey total of each category of phalaropes (Wilson’s, red-necked,
and unidentified) for that point (Table A2.1). The entire shore survey result was included in the
whole-lake totals, as well as the adjusted boat survey sums.
Table A2.1. Calculations of area of overlap of shore surveys and nearby boat
survey points. To determine total proportion of overlap, the area of water overlap between
the shore and boat points was divided by the total water area of each boat point.
Boat
Point
Shore point
Area of
overlap
between
boat and
shore point
(m
2
)
Area of
boat
point
Land
area in
boat
point
Water area
of boat point
(total area
minus land)
Proportion of
boat point
water
overlapping
with shore
point
Point 38
South Tufa
(points 1 and 2
combined)
37,316
503,000
12,344
490,656
0.076
point 57
Lee Vining
Tufa (points 1
and 2
combined)
97,261
503,000
44,163
458,837
0.212
Point 2
Old Marina
215,119
503,000
0
503,000
0.428
Point 55
County Park
113,563
503,000
46,283
456,717
0.249
Figure A.2.2. Maps of overlap between boat and shore points at South Tufa, Lee
Vining Tufa, Old Marina, and County Park.
