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)

Community dynamics of waterbirds in Namibia 1213

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