Post on 10-May-2023
Foraging guild membership explains variation in waterbirdresponses to the hydrological regime of an arid-regionflood-pulse river in Namibia
GRAEME S. CUMMING*, MARK PAXTON †, JACKIE KING ‡ AND HANS BEUSTER §
*Percy FitzPatrick Institute, DST ⁄NRF Centre of Excellence, University of Cape Town, Cape Town, South Africa†Shamvura Camp, Rundu, Namibia‡Institute for Water Studies, University of the Western Cape, Cape Town, South Africa§Beuster Clarke and Associates, Noordhoek, South Africa
SUMMARY
1. Little is known about hydrological influences on tropical waterbird communities. We used a
16-year data set (1991–2007) of waterbird censuses, together with a classification of observed
species into foraging guilds, to explore the relationships between natural variations in flow
regime, foraging guild and the community composition of waterbirds at the Okavango River in the
Caprivi Strip of north-eastern Namibia, southern Africa.
2. We addressed three hypotheses to explain variation in waterbird community composition:
(i) exploitation (birds move towards resource-rich patches to exploit periods of high food
abundance); (ii) escapism (declines in regional habitat quality force birds to aggregate in perennial
waterbodies); and (iii) interaction (bird assemblages are dominated by intra- and interspecific
interactions, such as flock formation for breeding or moulting, that can be explained better by life
history demands or competition than by resource availability).
3. Waterbirds in different foraging guilds responded strongly but at different periods to changes in
the hydrological environment, creating a complex but predictable successional pattern in
community composition through time. Deep-water feeders responded fastest (abundance peaking
2 months post-flood), followed by shallow-water feeders (4 months) and emergent vegetation
feeders (7 months). Species that forage on short vegetation or in mud showed a bimodal response
with peaks in abundance at 3 and 8 months post-flood.
4. Our results indicated a strong effect of the local flow regime and hence supported the
exploitation hypothesis. The foraging guild approach allowed us to identify clear patterns in a
highly complex ecosystem and shows considerable promise as an analytical tool for similar data
sets. Our results further suggest that while the entire bird community will be affected by
hydrological alterations such as impoundments, water extraction and climate change, deep-water
feeders may be one of the most vulnerable groups.
Keywords: dispersal, flow regime, functional group, hydrology, Okavango
Introduction
Most waterbirds are highly mobile, aggregating and
dispersing seasonally in response to fluctuations in
resources and the demands of their different life history
requirements. Although ecologists have paid considerable
attention to long-distance waterbird migrations (e.g.
Alerstam, Hedenstrom & Akesson, 2003; Dingle, 2008;
Gilg & Yoccoz, 2010), shorter and less predictable avian
movements (e.g. nomadic, semi-migratory or partially
migratory) are less well studied globally. Such move-
ments must nonetheless be understood before effective
Correspondence: Graeme S. Cumming, Percy FitzPatrick Institute, DST ⁄ NRF Centre of Excellence, University of Cape Town, Rondebosch, Cape
Town 7701, South Africa. E-mail: graeme.cumming@uct.ac.za
Freshwater Biology (2012) 57, 1202–1213 doi:10.1111/j.1365-2427.2012.02789.x
1202 � 2012 Blackwell Publishing Ltd
conservation and management of waterbird populations
can occur (Haig, Mehlman & Oring, 1998; King & Brown,
2010).
In the Northern Hemisphere, long-term data sets from
temperate freshwater ecosystems (e.g. Carpenter et al.,
2001; Likens, 2004) and intensive studies of waterbird
responses to hydrological variation (Desgranges et al.,
2006; Timmermans, Badzinski & Ingram, 2008; Frederick
et al., 2009) have created a solid scientific basis to guide
conservation and management (e.g. Smit et al., 1997;
Schindler et al., 2008). There is less knowledge of influ-
ences on waterbird communities in tropical environments
(Traill, Whitehead & Brook, 2009) with the possible
exceptions of some communities in Australia (e.g. Kings-
ford, Curtin & Porter, 1999; Tockner & Stanford, 2002;
Kingsford, Jenkins & Porter, 2004) and the Niger Delta of
West Africa (Cappelle et al., 2010). Relatively little is
known about the waterbird communities of arid-region
Southern Hemisphere river ecosystems, despite the
importance of these species in ecosystem functioning
(Sekercioglu, Daily & Ehrlich, 2004; Sekercioglu, 2006), for
tourism (Sekercioglu, 2002), as protein sources for rural
communities and as vectors of pathogens (Kilpatrick et al.,
2006; Cumming et al., 2011).
We explored the relationships between natural varia-
tions in river flow regime and the community composi-
tion of waterbirds in the reaches of the Okavango River
that run through the Caprivi Strip of north-eastern
Namibia, southern Africa. Flow changes have important
consequences for birds because they drive changes in
available hydraulic habitat and hence in foraging and
breeding sites. The Okavango has its headwaters in the
Angolan highlands and receives negligible contributions
of water within Namibia. Thus, it can take several weeks
for Angolan rains to bring floodwater into the Namibian
and Botswanan sections of the river where it spreads out
over huge floodplains. Dry and flood season flows at our
study site, Mahango, are out of synchrony with local
rainfall. This makes the Okavango an ideal test case for
exploring the relationships between waterbirds and flow
regime.
We used a 16-year data set (1991–2007) to test the
hypothesis that the changing intra- and inter-annual
community composition of waterbirds at Mahango is
driven primarily by seasonal variation in the river’s flow
regime. We label this the ‘exploitation hypothesis’. It is a
‘pull’ hypothesis in that it suggests that birds move into
the Okavango basin to exploit seasonal pulses of nutrients
and ⁄or favourable breeding habitat. Alternative hypoth-
eses that might explain variations in community compo-
sition include (i) escapism (i.e. birds may move into the
Okavango to escape drying down events in other, more
arid locations) and (ii) ecological interactions (i.e. com-
munity composition may be dominated by inter- or
intraspecific interactions, such as breeding aggregation,
predation or competition).
The exploitation hypothesis predicts a strong effect of
the local environment (the flow regime) on the composi-
tion of the bird community. If this hypothesis is correct,
we would predict that waterbird species in different
foraging guilds will respond predictably and differen-
tially to variations in the local flow regime. For example,
the number of diving birds should tend to peak while
water levels are high, while the number of substratum-
probing waders should peak towards the middle or end of
a drying down period when there is more shoreline.
Alternatively, waterbirds residing in the area may be
responding most strongly to regional changes in food
availability, breeding opportunities, predation or some
other larger-scale dynamic. If this were so, we would
predict little correlation between local hydrology and
variations in waterbird foraging guild composition. We
would instead predict that species would move to the
larger rivers during low flow periods when the surround-
ing landscape is drier, resulting in an increase in the
abundance of species for which the local habitat is not
ideal and obscuring successional patterns. The escapism
hypothesis thus predicts a strong effect of the regional
environment on local avian abundance but relatively
weak local effects and little difference in the abundance of
species by foraging guild in relation to local flow regime.
For example, the arrival of significant numbers of diving
birds in the late dry season would be strongly supportive
of the escapism hypothesis.
The interaction hypothesis predicts that any influence
of flow regime on waterbird abundance will be masked by
interactions with dominant competitors (both avian and
non-avian), breeders (e.g. dominance of roosting sites by
single-species breeding colonies or by species like Egyp-
tian geese, Alopochen aegyptiaca, which actively exclude
other birds during breeding) or predators, again resulting
in weak differences between the abundance of species in
different foraging guilds in relation to flow regime.
Compensatory, co-varying trends within foraging guilds
are one of the most likely signatures of competitive
interactions (Houlahan et al., 2007). If a shared resource is
limiting, decreases in the population size of one species
within a guild should be compensated for by increases in
the population of another, resulting in no detectable
guild-wide response. Predation, conversely, may be
expected to act against more vulnerable species regardless
of their foraging guild, again obscuring any guild-related
Community dynamics of waterbirds in Namibia 1203
� 2012 Blackwell Publishing Ltd, Freshwater Biology, 57, 1202–1213
responses to hydrology. Although there has again been
relatively little research on predation impacts on water-
bird communities in southern Africa, evidence from other
locations suggests that predators can have a major impact
on ground- and reed-nesting waterbirds (Erwin, Truitt &
Jimenez, 2001; Elmberg, Dessborn & Englund, 2010) and
crocodiles, monitor lizards, mongooses, snakes and large
fish are common in our study site. There is also good
evidence from other locations that non-piscivorous water-
birds may compete with fish for invertebrate prey (Haas
et al., 2007) and that density dependence can regulate
breeding success in ducks (Elmberg et al., 2005).
The first step in distinguishing between these three
hypotheses was to quantify the relative strength of the
influence of the local flow regime on waterbird commu-
nity composition. This was the primary goal of this paper.
Secondarily, we were also interested in the question of
whether, and how, changes in the flow regime of the
Okavango River (for example, following climate change
and ⁄or the construction of proposed impoundments)
might impact the waterbird community.
Methods
Field sites and study species
Bird censuses were undertaken in the Caprivi Strip
section of the Okavango River at Mahango in the
Bwabwata National Park (18�7¢60¢¢S; 21�40¢60¢¢E; Fig. 1).
The wetlands that were surveyed during the bird counts
include c. 28 km of the main channel of the Okavango
River, which runs through Bwabwata National Park to the
Botswana border where it becomes the Okavango Pan-
handle and eventually the Okavango Delta. In the
Bwabwata National Park, the river flows into marshy
wetlands, some sections of which are up to 5 km wide and
are connected to the main river by numerous small
channels that allow access by boat to the expansive, and
otherwise impenetrable, wetlands themselves. In addi-
tion, there are many large pans or low-lying areas close to
the river that are not directly connected to it but are filled
by rains in the wet season. These pans offer seasonally
attractive habitat for wetland birds. The main river also
has numerous sandbanks and vegetated islands that are
ideal breeding sites for specialised wetland bird species.
There is some illegal fishing and poaching and some
tourist activity, but the area is a proclaimed conservation
area and remains relatively undisturbed.
The highest flow periods at the study site occur in
January–March, following the rains in Angola, and the
lowest flows are in September–October. The water is
coolest during the June–August period, which is the
austral winter. Flows are largely contained within the
river channel from July to November, but waters start to
flood onto floodplains in December with the greatest
inundation in March and April.
The community of waterbirds using Mahango is
diverse, including over 100 different species (Appen-
dix 1). Many southern African waterbirds are highly
adapted to cope with environmental stochasticity, dis-
persing widely to take advantage of favourable conditions
or escape dry periods (e.g. Petrie & Rogers, 1997; Okes,
Hockey & Cumming, 2008). Species such as red-billed
teal (Anas erythrorhyncha) and comb duck (Sarkidiornis
melanotos) are known for their ability to appear at seem-
ingly isolated locations following rainfall events. Other
species, such as blacksmith lapwing (Vanellus armatus), are
thought to be consistently resident within a single locality
through most of the year. Many supposed residents
nonetheless appear to be capable of substantial move-
ments (Underhill et al., 1999).
Fig. 1 Map of part of southern Africa, showing the Okavango basin
and surrounding countries (below) and a Google Earth image of the
study area near Mahango (above). The grey-shaded area in the
Caprivi Strip in the lower map is Bwabwata National Park.
1204 G. S. Cumming et al.
� 2012 Blackwell Publishing Ltd, Freshwater Biology, 57, 1202–1213
Bird count methods
The data set comprised 26 bird censuses conducted
over the period 1991–2007. Censuses were generally
undertaken twice a year, roughly 6 months apart and
approximately in midsummer (January) and midwinter
(July), as part of the African Waterbird Census (AWC) and
following AWC guidelines (see http://www.
wetlands.org for more details). Censuses were occasion-
ally missed, there was some variation in sampling month
and some years had just a single census, but the site was
visited at least once every year. The timing of censuses was
as follows (month-year): 8-91, 1-92, 7-92, 1-93, 4-93, 4-94,
1-95, 5-95, 3-96, 7-97, 5-98, 11-98, 1-99, 7-00, 1-01, 1-02, 7-02,
1-03, 8-03, 2-04, 8-04, 1-05, 7-05, 2-06, 7-06 and 2-07. We
used the actual month for all monthly analyses, rather than
assigning observations to a January or June category.
Each census was a total count undertaken over a full
day of observations by an expert observer, assisted by
volunteers. The same route was followed each year,
subject to changes in channel morphology, flow regime
and total inundated area. The main river, with its
sandbanks and islands, was accessed primarily by boat.
Some sections were accessed on foot where necessary. The
small channels leading into the adjoining marshy wet-
lands were penetrated by boat, and the separate seasonal
pans (i.e. inundated areas outside the river channel) were
surveyed on foot. In all cases, as near as possible to
complete coverage of the river, side channels and nearby
seasonal pans was achieved.
Birds were counted using binoculars, and the numbers
of each species (see Appendix 1 for species counted and
their specific names) were recorded verbally on tape, with
data subsequently transcribed and captured digitally. The
primary observer, Mark Paxton, was present at all
censuses. Although the original data included all species
encountered, some water-associated species (terrestrial
passerines such as weavers and doves, kingfishers and
bee-eaters) were not consistently transcribed across all
years and were excluded from the analysis.
Hydrological data
River levels (and derived average daily flows) in the Caprivi
StripsectionoftheOkavangoRiverwererecordedatMukwe
by the Namibian Department of Water Affairs, and at
Mohembo just downstream of the Namibia-Botswana bor-
der by the Botswana Department of Water Affairs.
The Mukwe measuring station (Station number
2521M04; 18�02¢09¢¢S; 21�25¢40¢¢E) is near Andara Mission,
about 42 km upstream of Mahango, while the Mohembo
flow measuring station is 15 km downstream of Mahango,
in Botswana (Fig. 1). Because of backwater effects (i.e.
water backed up or slowed down in its course by
comparison to its normal condition of flow) in the
Panhandle section of the Okavango River, flow measure-
ments at Mohembo were less reliable than those recorded
at Mukwe. We therefore used daily flow data from
Mukwe to characterise the hydrological regime in the
Mahango section of the Okavango River.
The Mukwe flow measuring station was opened in 1965,
but the Namibian Department of Water Affairs has
extended the record as far back as 1947 by making use of
earlier gauge post observations. Comparison of annual
discharge over a long period (October 1959–September
2007; 304 m3 s)1) and the period for which bird censuses
were available (October 1991–September 2007; 268 m3 s)1)
showed that the census period was drier than average.
This dry spell ended recently, with the highest-in-record
flood events occurring in the wet seasons of 2009 and 2010.
The characteristics of the flow regime in the Mahango
section of the Okavango River were summarised by
ecologically relevant flow variables, here called annual
flow indicators, for the two periods (Table 1). Compared
to the long-term (1959–2007) data, median values for the
flow regime during the bird census period show longer
dry seasons and shorter flood seasons, lower dry-season
minimum flows and lower wet season flood peaks and
volumes. In some analyses, we also used the mean
monthly discharge (m3 s)1). Although the data run from
October 1959 to September 2007, we used the standard
calendar year (January–December) for all analyses. This
meant that n = 25 rather than 26 for some analyses
because a full set of annual hydrological data was not
available for 2007.
Foraging guilds
We developed a simple classification of the waterbirds
found at our study sites, based on extensive natural history
knowledge of the southern African avifauna and published
summaries for each species (Hockey, Dean & Ryan, 2005).
We divided waterbirds into five different feeding guilds
basedon their use of foraging habitat (Table 2). Detailsof the
assignments of individual species to guilds are presented in
Appendix 1. We excluded species that were not primarily
aquatic foragers (e.g. marsh owl Asio capensis, black-headed
heron Ardea melanocephala) or were considered near-impos-
sible to census accurately (e.g. white-backed night heron
Gorsachius leuconotus).
Community dynamics of waterbirds in Namibia 1205
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Data analysis
Assessment of the relevance of the flow regime to
waterbird community composition was undertaken in a
series of steps. The first step was to test for bivariate
correlations between the annual values of the flow
indicators (Table 1) and three standard metrics describing
the bird community: the Shannon diversity index, Shan-
non’s evenness index and species richness (Shannon,
1948; Macdonald, 2001). Shannon’s indices were calcu-
lated in Excel, and bivariate Spearman’s correlation
coefficients between different variables were calculated
in SPSSSPSS (IBM Corporation, Armonk, NY, USA).
Collapsing the matrix of bird abundances by census
into species diversity, richness and evenness indices loses
a considerable amount of information. A more informa-
tion-rich approach to analysing the census data is to work
with entire matrices of observations. The second step was
thus to test for an overall multivariate response of the bird
community to the annual values for each annual flow
indicator at Mahango. This involved converting a census-
by-species matrix of bird abundances (i.e. rows are
censuses, columns are bird species, and cell entries are
observed abundance) to a dissimilarity matrix, using
Bray-Curtis distances. The dissimilarity matrix captures
the differences in species abundance and species compo-
sition between each pair of censuses. We generated a
comparable matrix by census for the values of the annual
flow indicators (i.e. 10 different flow indicators with a
value for each census, converted to a census-by-census
difference matrix) and then compared the two difference
matrices using a Mantel test.
Following the guidelines of Legendre & Legendre (1998),
we used a partial Mantel test to separate the influence of the
year and month of sampling from the direct influence of the
flow regime. The Mantel test is a form of multivariate
correlation that yields a correlation coefficient, r. We
estimated the statistical significance of Mantel’s r using the
randomised form of the test with 1000 iterations. Multivar-
iate analyses were run in PC-ORD version 5 (McCune &
Table 1 Ecologically relevant annual flow indicators, with their median values over two periods, at Mukwe, near the study section of the
Okavango River (King, Brown & Barnes, 2010). Flood type, FT, is derived by dividing data from a plot of FSV against FSP into five equal
categories (very low, low, medium, high, very high). This was used in place of FSV because of the high covariance between these two data sets
Flow
indicator Acronym Unit
Median values
(unless otherwise stated)
Ecological significance
October 1959–
September 2007
October 1991–
September 2007
Mean annual runoff MAR m3 s)1 304 268 Total amount of water influences
available foraging habitat. More is better
Dry-season onset DSO Hydro week 44 44 Earlier or longer dry season implies
habitat and food scarcity towards end
of dry season; DSMF measures
water scarcity directly
Dry-season duration DSD Days 119 136
Dry-season minimum
flow
DSMF m3 s)1 128 103
Inundated area at
panhandle
IAPH km2 803 774 IAPH quantifies extent of nearby
flooded wetland, an important habitat
for many birds
Flood season onset FSO Hydro week 18 19 The same volume of water may be
distributed in time and space in
different ways. These different flood
characteristics capture the
nature of flood dynamics. Large
floods imply habitat alteration,
sediment deposition and
extensive floodplain inundation.
Smaller floods and a more
consistent flow imply less perturbation
but possibly higher breeding success,
depending on flood numbers
and timing. Each alternative
flooding scenario has
winners and losers among
the bird community
Flood season peak FSP m3 s)1 625 575
Flood season volume FSV mm3 5269 4921
Flood type FT Ordinal 3 3
Flood season duration FSD Days 153 125
Slope of hydrograph
from flood to dry season
SHFD m3 s)1 day)1 )1.8 )2.1
1206 G. S. Cumming et al.
� 2012 Blackwell Publishing Ltd, Freshwater Biology, 57, 1202–1213
Grace, 2002) and the vegan package for RR (R Development
Core Team, 2010; Oksanen et al., 2011).
We next explored the influence of finer-grained hydro-
logical variation by explicitly testing for a time-lagged
effect of hydrology on the waterbird community at a
monthly time scale, repeating Mantel tests on Bray-Curtis
difference matrices for all censused birds and mean
monthly discharge at lags ranging from 0 back to
12 months prior to the bird count. We used only this
variable because (i) its ecological significance is readily
interpretable and (ii) the other flow indicators do not have
monthly values and hence cannot explain differences
between months within the same year. Lastly, to better
interpret the ecological significance of the results of the
monthly analyses, we divided the bird community into
five different guilds according to their foraging strategy
(Table 2) and repeated the analysis for each guild.
Results
Bivariate Spearman’s correlations between the hydrolog-
ical data for Mahango (i.e. calendar year and the 11
variables in Table 1) and three standard community-level
metrics (species richness S, Shannon’s diversity index H
and Shannon’s evenness index E) yielded only two
significant correlations. There was a weak positive corre-
lation between species richness and year (Fig. 2; r = 0.44,
P < 0.026), indicating a trend of increasing bird species
richness over the 16-year sampling period. With the point
for November 1998 removed, this correlation improved to
r = 0.52.
There was also a negative correlation between the date
of dry-season onset (DSO, measured in hydrological
weeks) and Shannon’s evenness index E (r = )0.440,
P < 0.027, n = 25). Comparing the mean numbers of birds
counted in mid-year censuses from the 2 years with the
latest DSO (2000 and 2003, DSO in week 51) to the 2 years
with the earliest DSO (1993 and 1995, DSO in week 38) as
the ratio of numbers seen in early DSO years to those seen
in late DSO years, a late DSO appears to particularly
favour spur-winged goose Plectropterus gambensis
(early : late abundances 6 : 146), squacco heron Ardeola
ralloides Scopoli (4 : 76), African sacred ibis Threskiornis
aethiopicus (3 : 36) and white-faced duck Dendrocygna
viduata (50 : 456). An early DSO appears to favour fewer
or no birds, with no large increases in absolute numbers in
early DSO years and only two species having early:late
ratios >2 [African pygmy goose Nettapus auritus (18 : 2)
and red-billed teal A. erythrorhyncha (5 : 2); both involve
inconsequentially small numbers of birds].
A Mantel test on difference matrices by census, for birds
and annual flow indicators respectively (n = 25, 1000
iterations), indicated a strong relationship between annual
values for flow indicators and abundances by species in
waterbird communities (Mantel’s r = 0.79, P = 0.001).
Principal components analysis suggested that about 76%
of the variation in annual flow indicators could be
explained by two components, which corresponded to a
volume component (first component) and a timing com-
ponent (second component).
65
55
60
45
50
35
40
25
30
20Num
ber o
f wat
erbi
rd s
peci
es
151990 1992 1994 1996 1998 2000 2002 2004 2006
Year of census
Fig. 2 The gradual increase in bird species richness observed at
Mahango over the 16-year sampling period. Note that many years are
represented by two censuses. The black-filled diamonds indicate
summer counts (November–March), and the white-filled diamonds
indicate winter counts (April–August).
Table 2 Description of the different foraging guilds into which
waterbird species were placed. See Appendix 1 for a full list of the
species in each guild and their Latin names
Guild code Feeding habitat
Number of
species
in guild Examples
0 Exclude from
analysis
10 Marsh owl
1 Short vegetation
or mud,
including grass
31 Blacksmith lapwing,
Egyptian goose,
three-banded plover
2 Emergent
vegetation,
including reeds
and lilies
15 African jacana,
black crake
3 Shallow water 35 White-faced duck,
red-billed teal,
grey heron
4 In or over
deep water
8 Reed cormorant,
southern pochard,
osprey
5 Aerial feeder, roosting
on sandbanks,
rocky outcrops, or
grassy verges
3 Rock pratincole
Community dynamics of waterbirds in Namibia 1207
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To distinguish between the influences of time and
individual flow indicators, we ran a partial Mantel test in
which three variables (year, month and months since start
of sampling) were placed in a separate matrix of covari-
ates and used to constrain the correlation between a
matrix of bird numbers (by census and species) and a
matrix of all annual flow indicators (Table 1). This test did
not yield a significant result, suggesting that the greatest
single overall influence on the recorded nature of the
waterbird community was the time (year and month) at
which sampling occurred, rather than the specific prop-
erties of the flow regime. When we repeated the partial
analysis by foraging guild, results were insignificant for
Foraging Guilds 1, 3, 4 and 5, but there was a strong
significant response for Guild 2 (i.e. foragers in emergent
vegetation) to the flow regime (P < 0.028, Mantel’s
r = 0.248, n = 14), independent of year and month. A
canonical correspondence-based ordination of Guild 2 in
relation to the flow indicators (Fig. 3) followed by a
cluster analysis (Fig. 4) suggested that there were five
distinct groups in Guild 2, each with slightly different
responses to hydrological variation.
These analyses do not adequately test the potential for
time-lagged, successional effects in the months following
a flood. We used the data on mean monthly discharge to
test for an intra-annual influence of the flow regime on the
waterbird community. We first tested for an overall
community-level response and then examined the
responses of different components of the bird community
using the same five different functional groups (Table 2).
The full community responded most strongly to
variations in discharge with a time lag of 2–4 months
(Fig. 5; at 3 months, Mantel’s r = 0.29; P < 0.001; n = 26).
Lesser.Moorhen
0.08
African.CrakeStriped.Crake
3MARFSP
0.04
0.06
2
0.02C
A2 African.Jacana
African.Marsh.Harrier
DSOFSO
IAPH
African.Rail
Baillons.CrakeBlack.Crake
Spotted.CrakeDSMF
FTFSD
4
5
0.00
–0.0
4–0
.02
African.Purple.Swamphen
Green.backed.HeronLesser.Jacana
Little.Bittern
Rufous.bellied.Heron
1
DSDSHFD
–0.06 –0.04 –0.02 0.00 0.02 0.04 0.06CA1
Fig. 3 Plot of Guild 2 species on the first two canonical axes, with
hydrological variables overlaid (acronyms are provided in Table 1).
The length of each hydrological axis indicates its strength, and the
distance of each species from the hydrological axes and from the
centre of the ordination indicates the strength of its relationship to a
given environmental gradient. For example, Striped Crake appears to
be particularly responsive to mean annual runoff (MAR) and flood
season peak (FSP). The bold numbers indicate group membership as
displayed in Fig. 4 and discussed in the text.
0.25
0.30
0.10
0.15
0.20
0.00
0.05
Hei
ght
Afri
can.
Rai
l
Bla
ck.C
rake
Littl
e.B
itter
n
Afri
can.
Jaca
na
Less
er.M
oorh
en
Afri
can.
Cra
ke
Stri
ped.
Cra
ke
Spo
tted.
Cra
ke
Less
er.J
acan
a
Bai
llons
.Cra
ke
Gre
en.b
acke
d.H
eron
Afri
can.
Mar
sh.H
arrie
r
Afri
can.
Pur
ple.
Sw
amph
en
Ruf
ous.
belli
ed.H
eron
Group: 1 2 3 4 5
Fig. 4 Dendrogram showing the five main clusters of birds within
Guild 2. The bold numbers (‘Group’) indicate group membership as
displayed in Fig. 3 and discussed in the text.
0.3
0.35
0.2
0.25
0.15
Man
tel's
R
P<0.05
P>0.05
0.05
0.1
0–0 –2 –4 –6 –8 –10 –12
Lag (months)
Fig. 5 Mantel’s r statistic calculated on difference matrices at dif-
ferent lags using the bird data from the counts and the monthly flow
indicators volume and discharge. The value of the lag indicates the
number of months difference between hydrological data and the bird
community. For example, a peak lag of )3 indicates that the bird
community response correlates most strongly with the hydrological
data from 3 months earlier.
1208 G. S. Cumming et al.
� 2012 Blackwell Publishing Ltd, Freshwater Biology, 57, 1202–1213
The appearance of an 11- to 12-month response is
probably a statistical artefact resulting from the periodic-
ity of the hydrological data.
Although the overall response of the community is a
consequence of the responses of individual species, the
breakdown by foraging guild yielded some interesting
results (Fig. 6). Guild 4 (diving or deep-water birds)
responded most rapidly to changes in discharge, followed
by Guild 1 (short vegetation or mud), then Guild 3
(shallow water) and finally Guild 2 (emergent vegetation).
Guild 1 exhibited a bimodal response with significant
peaks at lags of both 3 and 8 months post-flood.
Discussion
Waterbird communities at Mahango exhibited a strong,
community-level response to hydrological variation.
Analysis of Shannon’s diversity index suggested that in
years when the dry season starts later, the avian commu-
nity will be numerically dominated by a subset of species.
The increase in species richness that we observed over
time was based on a more consistent representation of
species that were patchily or sequentially present in the
earlier censuses, implying increases in populations of
mobile species rather than an influx of new species. It may
also have resulted from improvements in survey tech-
niques, although the sampling methods were consistently
applied throughout and the primary observer had exten-
sive bird-watching and counting experience prior to
commencing this project.
The responses of waterbirds to changes in water depth
differed by foraging guild, suggesting that they were
attracted to suitable foraging habitats when such habitats
were present and moved elsewhere when such habitats
were absent. In relation to our initial hypotheses, our data
support the exploitation hypothesis (i.e. waterbirds enter
the area or remain in it as a response to suitable foraging
conditions) over either the escapism or interaction
hypotheses as the dominant structuring mechanism for
this community. Time did emerge as a more significant
influence than hydrology for four of the five guilds in the
Mantel analysis of annual variation, which might be
interpreted as implying that regional rather than local
influences dominate bird numbers and hence as support
for the escapism hypothesis. This did not hold for Guild 2
birds, however, and analysis of the finer-grained monthly
data identified strong hydrological influences that were
more consistent with the exploitation hypothesis. We
found no evidence for peaks in bird numbers that were
independent of periods of local habitat suitability, which
would have provided the strongest support for the
escapism hypothesis. The influence of hydrology on the
bird community also appeared to be strong enough that
we can safely conclude that the relative magnitude of
interaction effects was low and had little impact on overall
bird numbers.
The flood peak is one of the dominant signals in
hydrological data. Bird species that forage in deep or open
water responded most rapidly to changes in hydrology,
with peak response occurring 2 months after hydrological
events. In contrast, shallow-water foragers responded
more slowly, with highest numbers 4 months after
hydrological events. Given that diving birds (Guild 4)
typically prefer slow-moving, deeper water, and that
shallow-water birds (Guild 3) can only forage once flood
waters have receded, these responses were much as we
would have predicted from the exploitation hypothesis.
Additionally, more food tends to be available during the
flood recession periods because receding waters concen-
trate prey.
Interpreting the response times of the other two guilds
with significant responses to hydrology (i.e. Guilds 1 and
2) is less simple. The responses of birds that rely on
emergent vegetation (Guild 2) peaked at a lag of 7 months
after the August flood peak, which implies (given the
positive correlation) that they did best when the water
level was low. There may be a gradual increase in suitable
habitat that relates to the time taken for regrowth and
recruitment of reedbeds and water lilies following flood-
ing. It is also possible that a change in detectability affects
these data, with some of the less detectable species,
0.4
0.3
0.1
0.2
0
–0.1
Man
tel's
R s
tatis
tic
–0.2–12–10–8–6–4–20
Lag (months)
Guild 1 Guild 2 Guild 3 Guild 4 Guild 5
Fig. 6 Variation in Mantel’s r statistic for different foraging guild-
discharge rate correlations, with lags from 1 to 12 months. Peak lags
and their associated Mantel’s r and P values are: Guild 1, )3 (r = 0.32,
P < 0.001) and )8 (r = 0.19, P < 0.012); Guild 2, )7 (r = 0.23,
P < 0.021); Guild 3, )4 (r = 0.4, P < 0.001); Guild 4, )2 (r = 0.26,
P < 0.005); Guild 5, relationship not significant.
Community dynamics of waterbirds in Namibia 1209
� 2012 Blackwell Publishing Ltd, Freshwater Biology, 57, 1202–1213
particularly those that prefer reeds in water, being forced
more into the open as water levels dropped below the
bases of reedbeds. The Rallidae (crakes and rails) in Guild
2 do, however, have similar diets, habitat requirements
and foraging ecology (Hockey et al., 2005), making it
challenging to explain differences in their responses to
hydrology from the little we know of their ecology. The
most obvious distinctions between these species are in
their movement behaviours. In Figs 3 & 4, in Group 1,
black crake (Amaurornis flavirostris) and African rail
(Rallus caerulescens) are thought to be sedentary; in Group
3, African crake (Crecopsis egregia), striped crake (Anig-
matolimnas marginalis) and lesser moorhen (Gallinula
angulata) are intra-African migrants; in Group 4, spotted
crake (Porzana porzana) is a palearctic breeding migrant;
and in Group 5, baillon’s crake (Porzana pusilla) and
African purple swamphen (Porphyrio madagascariensis) are
suspected residents with some local movements (Hockey
et al., 2005). For the non-rallid species in Group 2, the
African jacana (Actophilornis africanus) in Group 2 is
described as ‘highly mobile and nomadic’ (Hockey et al.,
2005), supporting its placement near to the intra-African
migrant species in Group 3; green-backed heron (Butorides
striata) is sedentary, like other Group 1 members; and
rufous-bellied heron (Areola rufiventris) is described as
‘sedentary within the Okavango’ (Hockey et al., 2005),
fitting well with other Group 5 members. The three
exceptions to these generalisations include lesser jacana
(Microparra capensis), a vagrant in a resident group; little
bittern (Ixobrychus minutus), a palearctic migrant in a
sedentary group; and African marsh harrier (Circus
ranivorus), a largely sedentary raptor grouped with a
palearctic migrant (although according to Hockey et al.
(2005), African marsh harriers do undergo seasonal
changes in visibility that may mirror those of palearctic
migrants). The consistency of broad-scale movement
behaviours within groups implies that differences in their
movement behaviours may explain the observed patterns
in Guild 2 species’ responses to hydrological variation.
The bimodal response of the mud and short vegetation
foragers (Guild 1) was the least intuitively obvious result
from this analysis. The first peak in guild response
occurred at a lag of 3 months after the flood peak, which
would align with an increase in bird numbers during the
period when flood waters were receding, grass beginning
to grow on the previously flooded areas, and mudflats
and sandbanks increasingly becoming exposed. It is less
obvious why numbers of mud and short vegetation
foragers should then drop further and recover. The
pattern may be partially explained by the broader-scale
influence of global climate patterns. Nearly 50% of Guild
1 species are palearctic migrants (such as ruff Philomachus
pugnax and sandpipers, Tringa spp.), and the trough at a
lag of 7 and 8 months after flooding (August) probably
results from a combination of the return of these migrants
to Europe in March ⁄April and movement by local species
away from drying floodplains in October. If flooding
typically occurs in March or April at Mahango, receding
flood waters may attract residents and nomads as well as
the tail end of the departing migrants. The second peak in
avian abundance would occur around November as the
first migrants start to return. November and December
may also bring local rainfall, which (although smaller in
magnitude than the volumes of water received later in the
year from Angola) may contribute to some local habitat
replenishment.
Our analysis thus revealed a complex pattern of
responses by arid-region waterbirds to the flow regime
of the Okavango River, largely supporting the exploitation
hypothesis. Despite the complexity of fluctuations over
time in the numbers of individual bird species, these
responses can be interpreted and predicted by foraging
style. This predictability carries some interesting implica-
tions for the conservation and management of waterbirds.
For example, diving birds such as cormorants (Phalacroc-
orax spp.), African darter Anhinga rufa, maccoa duck
Oxyura maccoa and southern pochard Netta erythrophth-
alma, which depend on deep open water, would probably
be the most directly affected by reductions in water flow
and flooding in the system and one of the first guilds to
respond (see also Okes et al., 2008). The amount of
available foraging habitat as flood waters recede will
strongly influence short vegetation and mud feeders;
larger, longer floods presumably create more habitat for
these species and boost late-season nutrient availability.
Reductions in late dry-season flows, as might follow from
the construction of impoundments upstream, would
probably have a significant impact on those riverine bird
species that require late-successional riverine vegetation
and most notably on Guild 2 (emergent vegetation forag-
ers). These species rely heavily on reedbeds and waterl-
ilies, and these resources may be impacted heavily by
hydrological alterations, particularly those that change the
degree to which existing vegetation is scoured out during
floods and the persistence of long-standing pools of water
in which lily leaf cover can grow. Dry-season flow
reductions are likely to reduce food availability and
breeding locations for Guild 2 species, as well as increasing
their vulnerability to the predators (e.g. jackals, genets,
mongooses, crocodiles, monitor lizards and eagles) that
concentrate around rivers during the driest time of the
year. Cessation of late dry-season flows would also impact
1210 G. S. Cumming et al.
� 2012 Blackwell Publishing Ltd, Freshwater Biology, 57, 1202–1213
palearctic migrants, such as ruffs P. pugnax and terns
(Sterna spp.) that arrive at the start of the austral summer
and have historically depended on the Okavango system
as a stopover point in the middle of an arid region (data for
migrants using Lake Ngami are presented in Cumming
et al., 2011). Finally, although shallow-water feeders
should in many ways be the group that is best able to
cope with hydrological variability (as evidenced by the
cosmopolitan nature of such species as red-billed teal
A. erythrorhyncha, yellow-billed duck Anas undulata, grey
heron Ardea cinerea and little egret Egretta garzetta), many
shallow-water feeders are dependent on other components
of the food web such as frogs and dragonfly larvae that
will be directly impacted by alterations to the flow regime.
The complex, flow-related community dynamics
observed in this study system are likely to be typical of
waterbird communities in pulsed riverine systems. Riv-
erine waterbird communities will also be influenced by
other aspects of the system that we have not addressed
directly during this study. These include human impacts
on feeding and breeding habitat and on flow and
sediment regimes through water-resource developments,
the effects of fire and vegetational succession on move-
ment patterns of birds on and off the floodplain and
broad-scale environmental change. Given the many var-
iable influences on waterbird numbers, it is interesting
that such clear patterns emerge when bird communities
are stratified by foraging guild.
Our results primarily supported the exploitation
hypothesis, suggesting that birds are drawn to the area
to feed on seasonal resource peaks in the Okavango River.
These movements must, however, be interpreted within a
complex mosaic of changing habitats across the southern
African subregion. Our data suggest that even within
foraging guilds, subtle differences in movement behav-
iours (as detailed for Guild 2) may create both spatial and
temporal variation in community composition. Under-
standing the regional inter-relationships between
waterbird movements and the availability of shifting
resources remains an important goal for future research in
this field.
Acknowledgments
We are grateful to Linda Sheehan for indispensable field
assistance; to Charlene Paxton for help with logistics and
support; and to numerous other observers who gave
generously of their private time and expenses to assist
with the counts. Five MET Park Wardens (Patrick Lane,
Anton Esterhuizen, Willibald Mukena, Leverty Muyoba
and Helmut Tjikurunda) made the counts possible from
within Mahango Park and are thanked for their enthu-
siastic co-operation. We thank Phil Hockey for his
assistance with checking and finalising the foraging
guild classification. The hydrological data were provided
by the Namibian Department of Water Affairs through
the Okavango Basin River Commission’s EPSMO project.
Funding support for writing this paper was provided by
the DST ⁄NRF Centre of Excellence at the Percy FitzPa-
trick Institute.
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Appendix 1
Foraging guilds to which waterbirds were assigned for this study. Species with Guild ‘0’ were excluded from the analysis
for various reasons, including rarity, difficulties with detectability, or their use of a primarily terrestrial food base. Guilds
1–5 are described in Table 2.
Guild Species name
0 Abdim’s stork (Ciconia abdimii Lichtenstein)0 Black-headed heron (Ardea melanocephala Anon)
0 Dwarf bittern (Ixobrychus sturmii Wagler)
0 Grey-headed gull (Larus cirrocephalus Vieillot)
0 Marabou stork (Leptoptilos crumeniferus Lesson)
0 Marsh owl (Asio capensis Smith)
0 Pallid harrier (Circus macrourus Gmelin)
0 Western marsh harrier (Circus aeruginosus Linnaeus)
0 White stork (Ciconia ciconia Linnaeus)
0 White-backed night-heron (Gorsachius leuconotus Wagler)
1 African snipe (Gallinago nigripennis Bonaparte)
1 African wattled lapwing (Vanellus senegallus Linnaeus)
1 Blacksmith lapwing (Vanellus armatus Burchell)
1 Blackwinged stilt (Himantopus himantopus Linnaeus)
1 Caspian plover (Charadrius asiaticus Pallas)
1 Cattle egret (Bubulcus ibis Linnaeus)
1 Common greenshank (Tringa nebularia Gunnerus)
1 Common sandpiper (Actitis hypoleucos Linnaeus)
1 Common whimbrel (Numenius phaeopus Linnaeus)
1 Crowned lapwing (Vanellus coronatus Boddaert)
1 Curlew sandpiper (Calidris ferruginea Pontoppidan)
1 Egyptian goose (Alopochen aegyptiaca Linnaeus)
1 Glossy ibis (Plegadis falcinellus Linnaeus)
1 Great snipe (Gallinago media Latham)
1 Greater painted-snipe (Rostratula benghalensisLinnaeus)
1 Hadeda ibis (Bostrychia hagedash Latham)
1 Kittlitz’s plover (Charadrius pecuarius Temminck)
1 little stint (Calidris minuta Leisler)
1 Long-toed lapwing (Vanellus crassirostris ‘de Philippi’
Hartlaub)
1 Marsh sandpiper (Tringa stagnatilis Bechstein)
1 Ruddy turnstone (Arenaria interpres Linnaeus)
1 Ruff (Philomachus pugnax Linnaeus)
1 African sacred ibis (Threskiornis aethiopicus Latham)
1 Spur-winged goose (Plectropterus gambensis Linnaeus)
1 Terek sandpiper (Xenus cinereus Guldenstaedt)
1 Three-banded plover (Charadrius tricollaris Vieillot)
1 Water thick-knee (Burhinus vermiculatus Cabanis)
1 Wattled plover (V. senegallus Linnaeus)
1 White-crowned lapwing (Vanellus albiceps Gould)
1 White-fronted plover (Charadrius marginatus Vieillot)
1 Wood sandpiper (Tringa glareola Linnaeus)
2 African crake (Crex egregia Peters)
2 African jacana (Actophilornis africanus Gmelin)
2 African marsh harrier (Circus ranivorus Daudin)
2 African purple swamphen (Porphyrio madagascarensisLatham)
2 African rail (Rallus caerulescens Gmelin)
2 Allen’s gallinule (Porphyrio alleni Thomson)
2 Baillon’s crake (Porzana pusilla Pallas)
2 Black crake (Amaurornis flavirostris Swainson)
2 Green-backed heron (Butorides striatus Linnaeus)
Guild Species name
2 Lesser jacana (Microparra capensis Smith)
2 Lesser moorhen (Gallinula angulate Sundevall)
2 Little bittern (Ixobrychus minutus Linnaeus)
2 Rufous-bellied heron (Butorides rufiventris Sundevall)
2 Spotted crake (Porzana porzana Linnaeus)
2 Striped crake (Aenigmatolimnas marginalis Hartlaub)
3 African openbill (Anastomus lamelligerus Temminck)
3 African pygmy goose (Nettapus auritus Boddaert)
3 African skimmer (Rynchops flavirostris Vieillot)
3 African spoonbill (Platalea alba Scopoli)
3 Black heron (Egretta ardesiaca Wagler)
3 Black-crowned night-heron (Nycticorax nycticorax Linnaeus)
3 Cape shoveler (Anas smithii Hartert)
3 Cape teal (Anas capensis Gmelin)
3 Comb duck (Sarkidiornis melanotos Pennant)
3 Common moorhen (Gallinula chloropus Linnaeus)
3 Fulvous duck (Dendrocygna bicolor Vieillot)
3 Goliath heron (Ardea goliath Cretzchmar)
3 Great egret (Egretta alba Linnaeus)
3 Great white pelican (Pelecanus onocrotalus Linnaeus)
3 Grey heron (Ardea cinerea Linnaeus)
3 Hamerkop (Scopus umbretta Gmelin)
3 Hottentot teal (Anas hottentota Eyton)
3 Little egret (Egretta garzetta Linnaeus)
3 Little grebe (Tachybaptus ruficollis Pallas)
3 Pink-backed pelican (Pelecanus rufescens Gmelin)
3 Purple heron (Ardea purpurea Linnaeus)
3 Redbilled teal (Anas erythrorhyncha Gmelin)
3 Red-knobbed coot (Fulica cristata Gmelin)
3 Saddlebilled stork (Ephippiorhynchus senegalensis Shaw)
3 Slaty egret (Egretta vinaceigula Sharpe)
3 Squacco heron (Ardeola ralloides Scopoli)
3 Wattled crane (Bugeranus carunculatus Gmelin)
3 Whiskered tern (Chlidonias hybridus Pallas)
3 White-backed duck (Thalassornis leuconotus Eyton)
3 White-faced duck (Dendrocygna viduata Linnaeus)
3 White-winged tern (Chlidonias leucopterus Temminck)
3 Woolly-necked stork (Ciconia episcopus Boddaert)
3 Yellow-billed duck (Anas undulate Dubois)
3 Yellow-billed egret (Egretta intermedia Wagler)
3 Yellow-billed stork (Mycteria ibis Linnaeus)
4 African darter (Anhinga rufa Daudin)
4 African fish eagle (Haliaeetus vocifer Daudin)
4 Black-necked grebe (Podiceps nigricollis Brehm)
4 Maccoa duck (Oxyura maccoa Eyton)
4 Osprey (Pandion haliaetus Linnaeus)
4 Reed cormorant (Phalacrocorax africanus Gmelin)
4 Southern pochard (Netta erythropthalma Weid)
4 White-breasted cormorant (Phalacrocorax carboLichtenstein)
5 Blackwinged pratincole (Glareola nordmanni Fischer)5 Collared pratincole (Glareola pratincola Linnaeus)
5 Rock pratincole (Glareola nuchalis Gray)
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