Waterbird counts on large water bodies:
comparing ground and aerial methods
during different ice conditions
Dominik Marchowski1 , Łukasz Jankowiak2 , Łukasz Ławicki3 and
Dariusz Wysocki2
1
Ornithological Station, Museum and Institute of Zoology, Polish Academy of Sciences, Gdańsk, Poland
Department of Vertebrate Zoology and Anthropology, Faculty of Biology, Szczecin University, Szczecin,
Poland
3
West Pomeranian Nature Society, Szczecin, Poland
2
ABSTRACT
Submitted 17 March 2018
Accepted 18 June 2018
Published 17 July 2018
Corresponding author
Dominik Marchowski,
dominikm@miiz.waw.pl
Academic editor
Gregory Verutes
Additional Information and
Declarations can be found on
page 14
DOI 10.7717/peerj.5195
Copyright
2018 Marchowski et al.
The aerial and ground methods of counting birds in a coastal area during different
ice conditions were compared. Ice coverage of water was an important factor affecting
the results of the two methods. When the water was ice-free, more birds were counted
from the ground, whereas during ice conditions, higher numbers were obtained from
the air. The first group of waterbirds with the smallest difference between the two
methods (average 6%) contained seven species: Mute Swan Cygnus olor, Whooper
Swan Cygnus cygnus, Greater Scaup Aythya marila, Tufted Duck Aythya fuligula, Common Goldeneye Bucephala clangula, Smew Mergellus albellus and Goosander Mergus
merganser; these were treated as the core group. The second group with a moderate
difference (average 20%) included another six species: Mallard Anas platyrhynchos,
Eurasian Wigeon Mareca penelope, Common Pochard Aythya ferina, Great Crested
Grebe Podiceps cristatus and Eurasian Coot Fulica atra. The third group with a large
difference (average 85%) included five species, all of the Anatini tribe: Gadwall Mareca
strepera, Northern Pintail Anas acuta, Northern Shoveler Spatula clypeata, Eurasian
Teal Anas crecca and Garganey Spatula querquedula. During ice conditions, smaller
numbers of most species were counted from the ground. The exception here was
Mallard, more of which were counted from the ground, but the difference between
two methods was relatively small in this species (7.5%). Under ice-free conditions,
both methods can be used interchangeably for the most numerous birds occupying
open water (core group) without any significant impact on the results. When water
areas are frozen over, air counts are preferable as the results are more reliable. The
cost analysis shows that a survey carried out by volunteer observers (reimbursement of
travel expenses only) from the land is 58% cheaper, but if the observers are paid, then
an aerial survey is 40% more economical.
Subjects Ecology, Ecosystem Science, Zoology
Keywords Wintering, Costal lagoons, Baltic Sea, Ducks, Waterfowl, Accuracy of population
estimates
Distributed under
Creative Commons CC-BY 4.0
OPEN ACCESS
How to cite this article Marchowski et al. (2018), Waterbird counts on large water bodies: comparing ground and aerial methods during
different ice conditions. PeerJ 6:e5195; DOI 10.7717/peerj.5195
INTRODUCTION
Waterbirds are well-known indicators of the quality of aquatic environments. If a particular
site holds 1% or more of the flyway population of a given species, this area is said
to be important for this population. A flyway is a flight path used in bird migration
(Boere & Stroud, 2006) and a flyway population is the number of individuals of a certain
species included in a given flyway area. The 1% criterion is used to qualify an area as
a wetland of international importance under the Ramsar Convention on Wetlands and
by the European Union to identify Special Protection Areas (SPAs) under the Birds
Directive. It is also used by BirdLife International for identifying Important Bird Areas
(IBAs) on wetlands worldwide (BirdLife International, 2004; BirdLife International, 2015;
Wetlands International, 2010). Counting waterbirds on large, open water areas, like marine
areas, coastal lagoons and large lakes, is challenging, but accurate counts are critical for
estimating population sizes. Different methods have been used to conduct censuses of
birds in these open-water environments. Depending on local conditions, there are three
main census methods: counting from the ground, aircraft or boats (Komdeur, Bertelsen
& Cracknell, 1992; Wetlands International, 2010). The results of censuses carried out by
different methods are widely used in species population estimates over larger areas like the
Baltic Sea (e.g., Skov et al., 2011; Aunins et al., 2013) or for the whole flyway population
of species (Wetlands International, 2018). They are the basis for determining trends in
species’ abundances, which in turn affect conservation activities (e.g., Jensen, Perennou
& Lutz, 2009). Some studies made the assumption that aircraft counts detected 85% of
birds (Johnson, Pollock & Montalbano, 1989). The accuracies of different field protocols
for counting birds were tested at different locations (e.g., Briggs, Tyler & Lewis, 1985;
Smith, 1995; Frederick et al., 1996; Kingsford, 1999; Frederick et al., 2003; Green et al., 2008).
Some papers on non-breeding populations compared the results of air and ground
counts in Australia (Kingsford & Porter, 2009), on tidal sea coasts on the Wadden Sea in
Denmark (Laursen, Frikke & Kahlet, 2008) and Germany (Scheiffarth & Becker, 2008),
and in the Poyang Basin in China (Fawen, Changhao & Hongxing, 2011), but they did not
take ice coverage into account. This is a particularly important factor, considering that
a large proportion of waterbird species overwinter in areas around the mid-winter 0 ◦ C
isotherm (Van Erden & De Leeuw, 2010). Thus, relatively small variations in temperature
significantly affect the conditions in which counting is undertaken. Like many other
important Baltic bird wintering sites, our study area lies on the mid-winter 0 ◦ C isotherm.
The Baltic Sea as a whole is the most important wintering area for waterfowl anywhere
in the Western Palearctic (Durinck et al., 1996). Although birds originating from breeding
grounds situated in the vast expanses of northern Europe and Asia congregate on the Baltic
Sea during the winter, they are not evenly spread: there are few or no birds at all in some
areas, but huge numbers of them in others (Skov et al., 2011). These latter ‘hot spots’ are in
shallows on the open sea or in the estuaries of rivers where food, mainly mussels and fish,
is plentiful (Ławicki, Guentzel & Wysocki, 2012; Marchowski et al., 2015). It happens that
a significant percentage of the entire flyway population of a species gathers in a few such
optimal places: for example, 14% of the entire Greater Scaup Aythya marila population
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regularly overwinters in the Odra estuary (Marchowski et al., 2017). In the context of
climate warming and the related northward and eastward shifts in the wintering range of
waterbirds (Lehikoinen et al., 2013; Marchowski et al., 2017), the importance of the Baltic
Sea as a wintering area for this group of birds is now far greater than just a few decades ago
(Skov et al., 2011).
The investigation of such dynamic ecological processes requires precise research
methods. Here, we compare two standard methods of counting birds (from an aircraft and
from the ground) under different weather conditions in parts of the south-western Baltic Sea
where very large numbers of waterbirds congregate. The specific aim is to test the accuracy
of air counts vs. ground counts of waterbirds. Our study area is a key staging and wintering
site for significant numbers of a few species of waterbirds from the NW Europe—W Siberia
flyway, principally Greater Scaup Aythya marila, Smew Mergellus albellus and Goosander
Mergus merganser. Other species, such as Common Pochard Aythya ferina, Tufted Duck
Aythya fuligula, Common Goldeneye Bucephala clangula, Eurasian Coot Fulica atra and
Great Crested Grebe Podiceps cristatus are also present in significant numbers (Ławicki et
al., 2008; Marchowski & Ławicki, 2011; Marchowski & Ławicki, 2012; Marchowski, Ławicki
& Guentzel, 2013).
Although methodological publications mention the high cost of aerial surveys (e.g.,
Wetlands International, 2010; Meissner, 2011), they do not make any specific calculations.
Very few analyses compare the cost of air and ground counts; those that have been
performed concern other geographical regions and are out of date (e.g., Kingsford, 1999).
Bird counts used for large-scale population estimates often rely on the work of volunteers
(Wetlands International, 2010). This significantly reduces costs, which are limited to the
reimbursement of travel costs to the surveyed area. In this article we carry out a cost-benefit
comparison of the count method (air, ground) and the payment method (volunteering,
paid service). This analysis relates to Poland: the financial outlay in other countries will
obviously vary, depending on local labour and fuel costs, but the proportions may well be
similar and thus more universal.
We pose the following research questions: (1) which of the tested methods gives
higher/lower results of counts, and does this depend on ice cover and species? (2) Which
method is the more effective and methodologically correct in the context of the financial
outlay and ice conditions? Our hypotheses are that: (1) the overall result of a bird count in
ice-free conditions is higher from the ground than from the air; (2) the number of birds
detected during counts during ice conditions is higher from the air than from the ground;
(3) the difference in the counts between the two methods is greater during conditions when
ice is present; (4) some species are more sensitive to different census methodologies than
others; (5) there are differences in the quality of species identification depending on the
method (some species groups are better identified from the ground, others from the air).
MATERIAL AND METHODS
Study area
The study area lies in the south-western part of the Baltic Sea and forms the Polish part
of the Odra River Estuary system. It covers a total area of 530 km2 and includes the Great
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Lagoon (the Polish part of the Szczecin Lagoon), Świna Backward Delta, Kamień Lagoon,
Dziwna Strait and Lake Dąbie (Fig. 1). The average and maximum depths of the Lagoon are
3.8 and 8.5 m, respectively (the dredged shipping lane cutting across the Lagoon from Baltic
Sea to the port of Szczecin is 10.5 m deep). The waters of the Szczecin Lagoon, Kamień
Lagoon and Lake Dąbie are brackish. The salinity in the central part of the Lagoon varies
from 0.3 psu to 4.5 psu (mean = 1.4 psu) and declines with increasing distance from the sea.
Periodic inflows of water from the Pomeranian Bay (salinity ∼7 psu) take place through the
Świna Strait and, to a lesser extent, through the Dziwna and Peene Straits (the latter in the
German part of the estuary). The Odra estuary is subject to strong anthropogenic pressure,
which is manifested by a high level of eutrophication (Radziejewska & Schernewski, 2008).
Counts
We conducted ten aerial counts in parallel with ten ground counts in the non-breeding
period during 2009–2014 (see Table S3 for the raw data). Here we consider the following
taxa: Great Crested Grebe, Eurasian Coot and Anatidae (ducks, geese and swans). For each
taxon there were 10 aerial counts and 10 counts on the ground (in the case of Great Crested
Grebe only seven aerial and seven ground counts took place; a total of 394 records for
all species). The nomenclature of species and the systematic order used in the article are
according to the latest version of the HBW and BirdLife Taxonomic Checklist (Handbook of
the Birds of the World (HBW)/Birdlife International Working Group, 2017). The following
way of listing species in the text (e.g., Marchowski et al., 2015; Marchowski et al., 2017),
standard in ornithological publications, has been adopted: when the species name appears
for the first time, the full English name is given along with the full scientific name (genus
and species—in italics); whenever a species is referred to again in the text, the English name
only is used.
No observations were made during extreme weather conditions (heavy rain, wind, strong
wave action). When referring to the ‘census method’ we use the alternative terms ‘platform’
and ‘count method’. All count results were raw data: numbers were not processed by any
calculations, such as distance analysis. We used ‘total count’ methods with both platforms
and compared the results obtained with both. Air and ground counts took place on the
same day. This ‘total count’ method was also used in other studies (Joasen, 1968; Savard,
1982; Kingsford, 1999; Voslamber & Van Turnhout, 1999; Laursen, Frikke & Kahlet, 2008).
The same team of 17 trained and experienced observers was involved in all the counts. The
research involved observing birds from a distance so as not to disturb them. In Poland,
such studies do not require a special permit, as the whole area where we conducted the
survey is freely accessible to the public.
A slow-flying, high-wing aeroplane was used for the aerial counts. Two observers
identified and counted birds on both sides of the aircraft. The average flight speed was
about 100 km/h and the average flying height was about 80 m above the water. This gave
a roughly 1,500 m wide band within which birds could be recorded. The altitude of the
aircraft flight was tested before the study so that birds could be identified and at the same
time not scared off the water. During the study, however, there were a few cases when the
birds took off from the water surface at the sight of the aircraft. However, the frightened
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Figure 1 The study area—the Odra River Estuary, NW Poland.
Full-size
DOI: 10.7717/peerj.5195/fig-1
birds flew off just a small distance away, so they could be tracked from the aircraft and
not counted twice. The flight route was designed to cover as much of the water surface
as possible; we estimated that coverage was thus approximately 95% of the area surveyed.
Only the birds in a very small part of the middle of the Szczecin Lagoon (the largest water
body in the survey area—see Fig. 1—‘not covered area’) were not counted: we knew from
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previous field experience that birds rarely used that area, if at all. The flight route is shown
on Fig. 1: the aircraft took off from the Szczecin Aeroklub airfield in Szczecin Dąbie, then
flew over Lake Dąbie, the Szczecin Lagoon, Kamień Pomorski and Świnoujście. We used
the same flight route for all the aerial surveys. The detailed methodology is adopted from
Komdeur, Bertelsen & Cracknell (1992).
Ground counts were usually done on foot, although cars were also involved. Each
observer was equipped with 10 × 40 or 10 × 50 binoculars and tripod-mounted spotting
scopes with variable magnification, usually 20–60×. During the counts, observers walked
along the same routes, stopping every few hundred metres to scan the area with binoculars
and/or spotting scope and then count the birds. Alternatively, counts were conducted
from vantage points accessible by car (Fig. 1). We used the best vantage points and routes,
dividing the study area up into areas that were visible from such points or routes so that
no counted areas overlapped and no parts of the study area were overlooked. Of course,
some birds hidden in the rushes may not be detected, which results in an underestimation
of the result. On the other hand, it was still possible that the same birds were counted twice
by two observers, especially at the points of contact between adjoining sections, at places
where the shoreline is strongly indented, or when the same flock of birds swimming far
from the shore was being counted by two observers from two different points. Being aware
of this problem, we devised methods to reduce the risk of double counting. The observers
are in touch by phone and report to each other the species composition and number of
birds present at the intersections; they then decide who will include this group to his/her
section. In order to eliminate the double counting of flocks of birds away from the shore,
we use maps. The observer draws on the map the approximate area and place occupied
by the flock. If the other observer marks the flock in a similar place, we only add one
number to the result. Which result we add to the general results depends on several factors:
which observer saw the flock from a closer distance, the observer’s position in relation to
the sun, his/her position in relation to the structure of the landscape, etc. Birds on the
water and birds flying (along with the flight direction) are recorded separately. The present
analysis took only birds on the water into consideration. All the counts were carried out
from the same routes and observation points. The methodology of the ground counts is
consistent with generally accepted standards in this field (Komdeur, Bertelsen & Cracknell,
1992; Wetlands International, 2010). The two count methods cannot be compared to a
completely unbiased estimator, only with each other.
Species identification
To compare the effectiveness of the method in species identification, the number of
unidentified individuals (identified to generic level only) was compared. For the calculations
we used the following groups of birds: (1) Anatini (Anas sp., Spatula sp., Mareca sp.) (2)
Aythya sp. and (3) Cygnus sp. Then, the numbers of these groups were compared and the
statistical significances of these differences evaluated (see the ‘Statistical analysis’ below).
Cost accounting
In the case of aircraft counts where the observers are volunteers, only the cost of hiring
the aircraft is given. The calculation of volunteer labour costs involves multiplying the
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rate per kilometre by the number of kilometres that the observer covered, route to
the counting site and during the counting. The rate per kilometre covers the costs of
fuel and vehicle wear and tear, and is standard in Poland: these rates are set out in the
Regulation of the Minister of Infrastructure of 25 March 2002 on the conditions for
determining the means of reimbursing the costs of using, for business purposes, cars,
motorcycles and mopeds not owned by the employer (Dz.U. 2002 nr 27 poz. 271 available
at: http://prawo.sejm.gov.pl/isap.nsf/DocDetails.xsp?id=WDU20020270271). The total
number of kilometres driven by all observers using cars during counting (the average
of 10 counts) was used in the calculations. The calculation of paid services covers the
reimbursement of fuel costs, as well as those of vehicle wear and tear and an expert’s
work in Poland (28 ¤/h). The calculation of the commercial aircraft service covers the
costs of hiring the aircraft and the remuneration for highly qualified specialists (43 ¤/h).
The authors and observers are familiar with the hourly rates for the work from their own
experience. The costs in Polish zlotys (PLN) were converted into Euro at the average
exchange rate on 26 February 2018, i.e., 1 ¤ = 4.2 PLN.
Statistical analysis
The descriptive statistics were performed using R software (R Development Core Team,
2014) using simple bootstrap to estimate means, standard error and confidence intervals.
To check the statistical significance of the differences between groups of unidentified bird
species (identified only to the genus level) we used a permutation test based on resampling
without replacement (see Script S1).
Waterbirds are characterized by different features such as shape, colour and behaviour,
which determine their detectability during counting. Accordingly, we divided the birds
into three groups:
Group 1—‘‘core group’’: large birds, readily visible on the water from a large distance
(swans: Mute Swan Cygnus olor, Whooper Swan Cygnus cygnus); medium-sized species
that usually form large flocks, numerous in the study area, occurring on the open water
(Greater Scaup, Tufted Duck, Smew Mergellus albellus, Goosander Mergus merganser and
Common Goldeneye Bucephala clangula).
Group 2—medium-sized birds, fairly numerous in the study area, forming large and
medium-sized flocks, usually occurring close to the coast and aquatic vegetation, where
they can hide (Mallard, Eurasian Wigeon, Common Pochard and Eurasian Coot) or on
the open water, well away from coastal vegetation, singly or in small groups (Great Crested
Grebe).
Group 3—medium-sized, fairly or not numerous species, forming small flocks or
occurring singly, close to the coastal vegetation zone, where they can hide (Gadwall,
Eurasian Teal, Northern Shoveler, Garganey).
We used the generalized linear mixed-effect model (GLMM) to analyse the relationship
between the results of the aerial and ground counts. To account for the paired nature of the
counts (i.e., ground and aerial counts were done on the same day), we added the count date
as a random effect; this therefore factors in within-day variation. We also added species as
a random factor to account for between-species variation. The number of birds of a target
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Table 1 Mean number of waterbirds during the non-breeding period in the Odra River Estuary (NW
Poland); standard error and confidence intervals, taking into account the method and weather conditions (ice = 0 − no ice, ice = 1 − ice cover over 70%).
Method
Aircraft
Land
Ice
Mean
Standard error
Confidence intervals 95%
Lower limit
Upper limit
0
3,907.064
653.361
2,622.483
5,191.645
1
1,848.523
480.258
904.283
2,792.764
0
4,323.867
706.905
2,934.012
5,713.722
1
944.410
341.824
272.346
1,616.475
species was the dependent variable. The occurrence of ice (1: over 70% of the water surface
covered by ice, 0: no ice observed—see Table S3 for details), count type (Aircraft/Ground)
and group of species (levels: Group 1, Group 2, Group 3) were treated as categorical
fixed effects. To check how the different counts of particular species and the changes in
these numbers were affected by the two counting methods and the presence of ice in
relation to the different groups of species, we applied the three-way fixed effect interaction
ice*method*group. Because of the high overdispersion of the dependent variable, we
used the negative binomial distribution with log-link function. The mixed model was fitted
using maximum likelihood. The statistics were performed using R software (R Development
Core Team, 2014) with the ‘‘lme4’’ package (Bates et al., 2015). The results were considered
statistically significant for P < 0.05 and marginally significant at P < 0.06.
RESULTS
All birds together
There were more birds during ice-free conditions (all species combined and the aggregate
number of all individuals, Table 1). The numbers obtained from ground counts in such
conditions were higher than from aerial ones (10%, see Table 1). During ice conditions, the
overall number of birds was lower than when the water was free of ice, and the difference
between the two census methods was much higher (50%, see Table 1).
Species and groups of species
We found a significant three-way interaction between the effect of count method and ice
occurrence and groups (Table S2). This indicates that for species from group one (core
species) when ice coverage was high, significantly more birds were counted during aerial
surveys than ground surveys, whereas when the water was ice-free, the same numbers of
birds were counted from the ground and the aircraft (Fig. 2A). For Group 2 species we
found that when ice coverage was high, marginally significantly more birds were counted
during surveys from the air than from the ground, whereas when the water was ice-free,
more birds were counted from the ground (Fig. 2B). In the case of Group 3 species, more
birds were counted from the ground when the water was ice-free (Fig. 2C). We did not
compare aerial vs ice in relation to Group 3, because when the ice cover was substantial,
no birds from this group were counted (Fig. 2D).
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Figure 2 Predicted values of the fitted generalized mixed model. This shows differences between the results of waterbird counts during the non-breeding period in the Odra River Estuary carried out with two
research platforms, i.e., from the ground and from the air in relation to different species groups. (A, B, C)
show different groups of target species according to different ice conditions and count methods; whiskers
indicate standard errors; the asterisk shows a statistically significant difference between Aircraft and Land
counts performed as post-hoc tests adjusted by the Tukey method; **, p < 0.001; *, p < 0.06.
Full-size DOI: 10.7717/peerj.5195/fig-2
The method-related difference during ice conditions was considerable (Table 1), except
in the case of Mallard. When waters were free of ice, ground counts were generally higher.
In only two cases were the results slightly higher from the air: the differences relating to
Greater Scaup and Smew were 0.2% and 1.5% respectively (Table S1). Greater Scaup must
be considered in the broader context of the whole Aythya genus. A higher air count result is
a consequence of the greater efficiency of species identification using this method. Hence,
if we consider all the Aythya species together, i.e., Aythya sp. + A. marila + A. ferina + A.
fuligula, the difference is slightly greater (2.8%), but the numbers are still higher from the
ground than from the air, as they are for most species (Table S1).
The general range of differences for ice-free waters varied from 0.2% (Greater Scaup) to
93.6% (Northern Pintail) (Table S1).
The group of species with low average levels of difference between the two counting
methods (6%) were Mute Swan, Whooper Swan, Greater Scaup, Tufted Duck, Goldeneye,
Smew and Goosander—all from Group 1. Species with a moderate average difference
level (20%) were Common Pochard, Mallard, Eurasian Wigeon, Great Crested Grebe
and Eurasian Coot—Group 2 species. Dabbling ducks (Gadwall, Eurasian Teal, Northern
Shoveler, Northern Pintail and Garganey) were the only group with a high level of average
difference (85%)—these were species from Group 3.
Under ice conditions, only one species (Mallard) displayed a moderately small difference
between the aerial and ground results (7.5%). The other species in such conditions exhibited
moderate to high differences—from 34.4% to 582.9%; there was a very wide disparity in
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Table 2 Comparison of the number of unidentified species using two counting methods (ground and
aircraft) (1); mean ± standard error of ground counts (2); 95% confidence intervals of ground counts
(3); mean ± standard error of aircraft counts (4); 95% confidence intervals of aircraft counts (5); P
value (6).
Genus (1)
Ground
Aircraft
P value (6)
Mean ± SE (2)
95% CI (3)
Mean ± SE (4)
95% CI (5)
All species
1,194 ± 466.11
409–2,215
517 ± 151.83
256–846
0.192
Anatini
80 ± 30.97
26–146
277 ± 56.77
163–385
0.012
Aythya sp.
3,392 ± 1,110.03
1,452–5,759
1,075 ± 387.28
387–1,890
0.064
Cygnus sp.
111 ± 40.17
38–195
198 ± 85.76
59–387
0.454
differences between species from the same ecological guild (e.g., Tufted Duck—64.6% and
Greater Scaup—582.9%; see Fig. 2).
Species identification
With regard to all recorded birds that could not be identified to species level, there was no
statistically significant difference between the methods (P = 0.192), although the number
of unidentified species was smaller from the aircraft (Table 2).
Looking at each of the three groups separately, we see important differences between
them. The group with the most significant differences is Anatini (Anas, Spatula and
Mareca): the number of unidentified birds was significantly higher from the aircraft. In the
case of the Aythya group, more birds counted on the ground were unidentified (this result
is marginally significant, P = 0.064). Differences in the number of unidentified Cygnus
swans were non-statistically significant, although more unidentified swans were observed
from the aircraft (Table 2).
Cost estimate
All the counts for this study were carried out by volunteers; some persons even waived
the reimbursement of travel costs to the counting site. The costs involved in this study
were low, being limited to the hiring fee for the aircraft and part of the fuel costs for
ground observers’ cars. They were even lower than the following calculations in relation to
volunteers. However, if we include the fuel cost for all observers and the cost of aircraft hire,
we obtain the real overall cost of voluntary counts. Reimbursing the 12 ground observers
involved in the counting for their fuel outlay amounts to around 300 ¤ and the aircraft
hire fee is 720 ¤. The study area covers 530 km2 and the coastline is 340 km long, so the
cost of an air count is 136 ¤ for 100 km2 of a water body and 180 ¤ for 100 km of flight
route if the count is carried out by volunteers. The corresponding ground count costs are
57 ¤/100 km2 and 88 ¤/100 km of coastline. If the observers are paid for their services,
the costs increase to 1,400 ¤ for an air count and 2,300 ¤ for a ground count (see Table
3 for details). These figures cover only the labour costs in the field and do not include the
costs of subsequent data processing and analysis.
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Table 3 Waterbird counts in the non-breeding season − calculation of costs. Calculation of labour costs in the field; payment methods and
study methods are distinguished.
Form of
payment
Form of
counting
Cost of one count in the study area
(530 km2 and 340 km of coastline)
in Euros
Cost of one count over a
100 km2 water body
in Euros
Cost of one count along a 100 km
coastline (ground count) and 100 km
of flight route (air count) in Euros
Voluntary
Aircraft
720
136
180
Voluntary
Ground
300
57
88
Paid service
Aircraft
1,400
264
350
Paid service
Ground
2,300
434
677
DISCUSSION
The major factor affecting the census results was ice cover. In ice-free conditions, ground
counts yielded higher numbers of birds than aerial ones. When ice was present, more birds
were counted from the air, and the difference between the two methods was much greater
than in ice-free conditions. This discrepancy highlights the importance of ice coverage on
the water in impacting survey results in relation to the survey method. With respect to the
core species combined, it does not really matter which censusing method is used during
ice-free conditions as the counts are not specially affected by this (Fig. 2): the two methods
can be used interchangeably. Similar conclusions were reached by Kingsford, Brandis &
Porter (2008) in Australia, where the correlation of results from the land and from the air
was highly significant. The results of air and ground counts also differed little in the Poyang
Basin in China (Fawen, Changhao & Hongxing, 2011).
In contrast, once there is significant ice cover of the waters (above 70%), the survey
method does become important; this has not been demonstrated before. Our aerial census
results under such conditions gave higher numbers relative to ground counts. During ice
conditions, there were a number of occasions when significant numbers of birds were
overlooked by ground observers but were recorded from the aircraft on their sections.
This is why greater numbers of birds are counted from the air in ice conditions; hence,
the aerial method is more effective under such conditions. Wetlands International (2010)
recommends the aerial method in areas covered (incompletely) by ice but does not underpin
this assertion with any concrete results; our work supports it. Again, in Australia, there
are similarities, such as limited visibility from the land on lakes such as Lake Illawarra and
Norring Lake, where aerial counts yielded much higher numbers of birds than ground ones
(50.1% and 101.5% respectively; Kingsford, Brandis & Porter, 2008). The similarity lies in
the limited visibility from the land of sites where significant numbers of birds congregate.
The differences in the results varied over a very wide range—from nearly identical, i.e.,
0.2% for Greater Scaup under ice-free conditions, to 582.9%, also for Greater Scaup but
in ice conditions. This very considerable difference under ice conditions in the case of
Greater Scaup emerges from this species’ preference to concentrate in a few places, i.e.,
in ice-free areas usually far from the shore (Johnsgard, 1978; Mendel et al., 2008). In ice
conditions, several thousand Greater Scaup were recorded from the aircraft in ice-free
patches of water not visible to ground observers. Visibility from the land in ice conditions
is often difficult because piles of ice protrude above the waterline, a problem that ceases
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to exist when counting from the air. The much lower difference with regard to the Tufted
Duck (sympatric with Greater Scaup) is due to the tendency of this species to occupy
anthropogenic sites like ports and harbours when ice covers more open sea areas (Jakubas,
2003), as does Mallard (Meissner et al., 2015); they are thus more easily detected by ground
observers. In ice conditions the numbers of most species were higher when counted from
the air, Mallard being the exception. We recommend aerial surveys when waters are frozen
over. Even if, as seen from the land, the entire water body appears to be frozen, from the
air we can still find unfrozen patches, which are occupied by many birds.
The opposite situation prevails when waters are free of ice: ground count numbers
are then generally higher. This corresponds with most papers on this topic, in which
ice conditions were either not analysed or did not exist (e.g., Pollock & Kendall, 1987;
Kingsford, 1999; Laursen, Frikke & Kahlet, 2008). If we take into account particular species
of birds, comparable results under ice-free conditions can be obtained by both methods
with respect to the following species: Greater Scaup (difference 0.2%), Smew (1.5%), Mute
Swan (3.9%), Goosander (4.9%), Common Goldeneye (5.3%) and Tufted Duck (5.5%)
(see Table S1). The most numerous species of waterbirds in the study area, they make up
the core of the waterbird community here (79–85% of all waterbirds present in the area).
The observed difference (6% in core species group) between methods may be acceptable
in some situations for the two methods to be used interchangeably. However, researchers
must consider the influence these small differences between methods might have on their
results and consider developing correction factors where these differences are deemed
unacceptable.
In ice-free conditions, comparable results are obtained for numerous birds occupying
the open water. We can generalize that diving ducks (Aythya, Mergus, Mergellus, Bucephala)
and swans (Cygnus) can be counted from the air without significant differences between the
methods. The differences between count numbers are higher for the less numerous birds
occupying abundantly vegetated near-shore areas, so we do not recommend surveying
these species from the air. Generally speaking, this applies to dabbling ducks (Anatini):
here there are significant differences between the methods, with aerial counts being lower
relative to ground counts (see Table S1 for more details).
In most cases the ground observer identifies species more accurately than his/her
counterpart from the air. The exception of Greater Scaup and Tufted Duck must be
explained here. Birds often swim on the water far away from the shore—perhaps as much
as a few kilometres. In such a situation, it is difficult to identify the species even with a
spotting scope from the shore. In addition, at the level of a single flock, Greater Scaup
prefers deeper areas (further from the shore) and Tufted Duck shallower waters (closer to
the shore), so an observer from the shore watching such a mixed flock sees Tufted Duck in
the foreground and Greater Scaup farther away. As this makes it difficult or even impossible
to separate these two species accurately, observers sometimes decide to treat them at the
generic level (Aythya sp. = Greater Scaup/ Tufted Duck). The situation is different from an
aircraft. Such a flock is seen from a much shorter distance and the spatial distribution is
much better visible from the air. Species giving the impression of a mixed flock from the
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shore, can be seen from the aircraft that they are actually occupying slightly different areas
(within one flock), which makes it easier to count them separately.
As we stated in the Methods section, the two count methods cannot be compared to a
completely unbiased estimator, only with each other. However, the method yielding higher
total counts can be regarded as superior. Such a conclusion is based on experience from the
area; for example, one or two species are deliberately overlooked from the air as they can
be equally well counted from the ground (during ice-free conditions). On the other hand,
during ice conditions, ground observers may not report birds of a given species at all on
their sections, whereas significant numbers of this species are observed (a few cases) from
the aircraft on these sections. Of course, double counting is still possible: especially at the
points of contact between adjoining sections, or where the shoreline is strongly indented,
two observers may count the same flock of birds situated far offshore from two different
points. Being aware of this problem, we have developed specific methods to reduce this
risk (described in Methods).
Any broader application of our results is limited by the fact that our survey covered just
one area. But they can be of use more generally in other similar areas in the same climatic
zone, where wintering waterbirds congregate. These include the estuaries of large rivers
where there are extensive shallow lagoons, such as the German part of the Odra estuary,
the Vistula Lagoon, the Curonian Lagoon, or the indented coastline of the south-western
Baltic Sea.
The most economical form of counting is to use volunteer observers on the ground:
this is the method most commonly used in our study area and generally during winter
waterbird counts worldwide (Wetlands International, 2010). The disadvantage of this
approach, however, is that one needs a large group of qualified observers equipped with
good optical equipment who will not get paid for their services. This condition cannot
always be met. Counting from an aircraft requires only two people and, assuming that they
will not get paid for their services, the costs are also not high, but still 58% higher than
for a ground count. If the observers are paid, then the aircraft method will be the most
cost-effective: only two qualified people are needed and the cost is 40% less than for all
the persons involved in a ground count (Table 3). In addition, aerial surveys can be used
in both ice-free and ice conditions. The present calculation of costs relates to conditions
in Poland; in other countries, costs will vary depending on local labour and fuel costs, but
the proportions should be similar. The undisputable disadvantage of the aircraft method
is its riskiness (Sasse, 2003); in addition, surveys may need to be conducted in places where
there are airspace restrictions, stricter limitations regarding weather conditions suitable
for counting, difficulties in identifying taxa similar in appearance, and, in some situations,
high levels of disturbance to wildlife.
CONCLUSIONS
Overall, more birds are counted from the ground than from the air in ice-free conditions,
but in ice conditions, the overall results of bird counts are higher from the air than from the
ground. The differences in counts between the two methods are higher in ice conditions. In
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ice-free conditions, the results from both platforms for the core group of birds occupying
open water (diving ducks and swans) are comparable. In the same conditions there are
significant differences between the methods regarding two other groups of birds—aerial
counts yield lower numbers.
ACKNOWLEDGEMENTS
We thank all the people who took part in the fieldwork—mainly members of the WestPomeranian Nature Society—but especially those who were the most active during the
entire study period: Michał Barcz, Sebastian Guentzel, Michał Jasiński, Zbigniew Kajzer,
Jacek Kaliciuk, Krzysztof Kordowski, Andrzej Kostkiewicz, Aneta Kozł owska, Wojciech
Mrugowski, Bartosz Racławski, Tomasz Rek, Artur Staszewski, Marcin Sołowiej, Paweł
Stańczak and Mirosław Żarek. We also would like to thank Włodzimierz Meissner, Tomasz
Mazgajski, Virginia Kowal and two anonymous reviewers for their helpful comments on
earlier versions of the manuscript. We also would like to thank Peter Senn for kindly
improving our English.
ADDITIONAL INFORMATION AND DECLARATIONS
Funding
The study was funded by West Pomeranian Nature Society (ZTP) and Polish Society for
the Protection of Birds (OTOP). The funders had no role in study design, data collection
and analysis, decision to publish, or preparation of the manuscript.
Grant Disclosures
The following grant information was disclosed by the authors:
West Pomeranian Nature Society (ZTP).
Polish Society for the Protection of Birds (OTOP).
Competing Interests
The authors declare there are no competing interests.
Author Contributions
• Dominik Marchowski conceived and designed the experiments, performed the
experiments, analyzed the data, contributed reagents/materials/analysis tools, prepared
figures and/or tables, authored or reviewed drafts of the paper, approved the final draft.
• Łukasz Jankowiak analyzed the data, contributed reagents/materials/analysis tools,
prepared figures and/or tables, authored or reviewed drafts of the paper, approved the
final draft.
• Łukasz Ławicki and Dariusz Wysocki performed the experiments, analyzed the data,
contributed reagents/materials/analysis tools, authored or reviewed drafts of the paper,
approved the final draft.
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Animal Ethics
The following information was supplied relating to ethical approvals (i.e., approving body
and any reference numbers):
The research involved observing birds from a distance, which did not cause any
disturbance of birds. In Poland, such studies do not require a special permit.
Data Availability
The following information was supplied regarding data availability:
The raw data are provided in a Supplemental File.
Supplemental Information
Supplemental information for this article can be found online at http://dx.doi.org/10.7717/
peerj.5195#supplemental-information.
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