Comparative foraging behavior of 3 heron species in small standing-water ecosystems in the arid zone of Oman

Abstract

The composition of assemblages, diet and behavior of waterbird species with similar ecological features are important aspects in the functioning of aquatic ecosystems. Closely related animal species often share resources such as space and food in ways that reduce competition, but if the diets of different species strongly overlap, interspecific competition may intensify. This analysis examined behavioral data relating to Great Egret, Little Egret, and Squacco Heron to explore their foraging efficiencies in rich aquatic habitats in an arid zone during post-breeding movements. The fieldwork was carried out in small estuaries of a Wadi on the southern coast of Oman. The frequency of interactions was the highest in Squacco Heron and the lowest in Great Egret. However, the differences in the numbers of inter- and intra-specific interactions between the 3 species were significant. Activity indices calculated for a 5-min period, i.e., the number of interactions (interaction index) and times of flying and walking (movement index) differed among the 3 species. The number of successful attacks was the highest in Squacco Heron, while the foraging effectiveness of Great Egret and Little Egret was similar. GLM analysis showed that only the movement index was significant, as it had a positive impact on individual foraging success. Great Egret caught more fish than the other 2 heron species which, in turn, caught a greater number of smaller prey items, mostly invertebrates. This study shows that heron species in the same rich habitat employ different foraging tactics. In comparison to the other two heron species, the foraging tactics of Squacco Heron seem to be the most susceptible to competition. However, its greater mobility and using different foraging tactics, enhance its foraging success.

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Current Zoology, 2024, 70, 780–785
https://doi.org/10.1093/cz/zoae011
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Received 22 November 2023; accepted 25 March 2024
Comparative foraging behavior of 3 heron species in small
standing-water ecosystems in the arid zone of Oman
Zbigniew Kasprzykowski*, and Artur Golawski
Faculty of Sciences, University of Siedlce, Prusa 14, 08-110 Siedlce, Poland
*Address correspondence to Zbigniew Kasprzykowski. E-mail: zbigniew.kasprzykowski@uws.edu.pl
Handling editor: Fumin Lei
Abstract
The composition of assemblages, diet and behavior of waterbird species with similar ecological features are important aspects in the functioning
of aquatic ecosystems. Closely related animal species often share resources such as space and food in ways that reduce competition, but if the
diets of different species strongly overlap, interspecific competition may intensify. This analysis examined behavioral data relating to Great Egret,
Little Egret, and Squacco Heron to explore their foraging efficiencies in rich aquatic habitats in an arid zone during post-breeding movements.
The fieldwork was carried out in small estuaries of a Wadi on the southern coast of Oman. The frequency of interactions was the highest in
Squacco Heron and the lowest in Great Egret. However, the differences in the numbers of inter- and intra-specific interactions between the 3
species were significant. Activity indices calculated for a 5-min period, i.e., the number of interactions (interaction index) and times of flying and
walking (movement index) differed among the 3 species. The number of successful attacks was the highest in Squacco Heron, while the foraging
effectiveness of Great Egret and Little Egret was similar. GLM analysis showed that only the movement index was significant, as it had a positive
impact on individual foraging success. Great Egret caught more fish than the other 2 heron species which, in turn, caught a greater number
of smaller prey items, mostly invertebrates. This study shows that heron species in the same rich habitat employ different foraging tactics. In
comparison to the other two heron species, the foraging tactics of Squacco Heron seem to be the most susceptible to competition. However,
its greater mobility and using different foraging tactics, enhance its foraging success.
Key words: competition, food niche, foraging success, hunting tactics, interactions.
Wetlands are endangered ecosystems that face both climate
and land-use change pressures (McKenna et al. 2021). Global
climate change is anticipated to exacerbate damage to and
deprivation of many wetlands, thus reducing numbers of
individuals and species in wetlands (Ostad-Ali-Askari 2022).
Land reclamation, intensive resource exploitation, hydrolog-
ical changes, conversion to cropland, and pollution threaten
wetlands on all continents (Junk et al. 2013). Localized threats
to wetlands may derive from areas given over to tourism and
recreation, from garbage and solid waste dumping, and from
roads and railway lines (Battisti et al. 2008). Every type of
freshwater ecosystem provides critical habitats for many taxa
across all trophic levels (Dudgeon et al. 2006; Almeida et al.
2020). However, compared with their larger counterparts,
small standing-water ecosystems possess wider ecotones,
sometimes as large as their whole surface area, which max-
imizes structural heterogeneity and supports an exception-
ally high biodiversity (Bolpagni et al. 2019; Cantonati et al.
2020). As a result, small standing-water ecosystems are gen-
erally acknowledged to be among the most productive ecosys-
tems at the global scale (Schiemer et al. 1995).
To understand the functions of wetlands that are to be pre-
served or restored, it is important to study the composition,
diet, and behavior of the assemblages of ecologically similar
aquatic organisms inhabiting them (see Gonzalez-Solis et al.
1997; Brzorad and Maccarone 2013; Nea and Nouira 2016;
Grabowska et al. 2019; Cecala et al. 2020). Closely related
species of animals often partition resources such as space and
food in ways that reduce competition, and they are expected
to develop strategies of niche differentiation (Chesson and
Huntly 1997). On the other hand, it also happens that indi-
viduals of one species use successfully settled individuals of
another species as sources of information regarding the loca-
tion of high-quality habitat (Hromada et al. 2008; Sridhar
et al. 2009). Abundance of food can lead to a high degree
of dietary overlap between species, but in conditions of
restricted access to food, such overlap is expected to decrease
and interspecic competition to intensify (Wiens 1989; Rizzo
and Battisti 2009). Should the restrictions in the food supply
persist, however, the trophic niches may once again overlap
(Wiens 1989; Bell and Ford 1990).
Herons hunt a variety of aquatic prey, such as small sh and
invertebrates, using visual cues to do so (Voisin 1991; Fasola
1994; Kushlan and Hancock 2005; Wood and Stillman 2014;
Navarro-Ramos at al. 2021). To forage effectively, they also
employ various feeding techniques. The food preferences of
individual species are basically related to the structure of the
bill, particularly its size and shape, which is used to seize prey,
and to the lengths of the neck and legs, which respectively
govern the bird’s reach and the depth of water in which it
can wade. Within these constraints, each species has evolved
a repertoire for obtaining food (Kushlan and Hancock 2005).

Kasprzykowski and Golawski · Comparative foraging behavior of 3 heron species 781
Heron feeding behavior has been studied in different habitats,
mainly during the breeding season (Campos and Lekuona
1997; Lekuona 1999; Takaki and Eguchi 2008; Regos 2011).
In this study, behavioral data on Great Egret, Little Egret,
and Squacco Heron were used to explore their foraging
efciency and also intra- and interspecic interactions dur-
ing post-breeding movements in rich aquatic habitats of the
arid zone. All three species occurred in the same small, rich,
standing-water ecosystems, so it was likely that competi-
tion between them would be strong in a multi-species com-
munity. Body mass is a good predictor of dominance rank
across species (Francis et al. 2018). It was therefore antici-
pated that Great Egret, the largest species, would be involved
in the fewest interactions, that Squacco Heron, the smallest
species, would participate in the most interactions, and Little
Egret, a medium-sized species, in an intermediate number.
The exploitation of food sources in a competitive situation
compels the use of different hunting tactics (see Hafner et al.
1982; Takaki and Eguchi 2008) to prevent the food niches
of the competing species from overlapping (Golawski et al.
2020; Kent et al. 2022). The hypothesis examined assumed
that these 3 heron species would differ in the number of inter-
actions they were involved in, their movement frequencies,
and the sizes and categories of the prey items they caught. All
these parameters contribute to determining foraging success,
which may vary between species.
Materials and Methods
Study area
The study areas lie on the south coast of Oman, and the
data were gathered in the outskirts of the city of Salalah
in the Sultanate of Oman located in Dhofar Governorate
(17°00ʹ10″N and 54°04ʹ41 ″E, Figure 1). A map of the study
area was made in QGIS 3.33.4 software using a Google satel-
lite map. The area consists of sparsely vegetated desert steppe
with pools of water persisting in the Wadi beds. From June
until mid-September, the coasts are blanketed in moisture-
laden clouds (El-Sheikh 2013). The mean annual tempera-
ture is c. 27 °C and the total annual precipitation is 95 mm
(Al-Habsi et al. 2014) and the total rainfall in this month does
not exceed 10 mm (https://en.tutiempo.net/climate).
The habitat consists of natural wetlands—rather small estu-
aries of a Wadi, locally known as Khawr—which are covered
with dense oating vegetation near the shores and have an
area of open, fresh water in the center. Because of their depth
and size, they are classied as small standing-water ecosys-
tems (SWEs) (Cantonati et al. 2020). The features they have
in common are the natural succession of vegetation, proxim-
ity to the sea and a low level of human pressure. This type of
habitat offers good foraging conditions as illustrated by the
presence of eight heron species: Grey Heron Ardea cinerea,
Purple Heron Ardea purpurea, Great Egret Ardea alba, Reef
Heron Egretta gularis, Little Egret Egretta garzetta, Squacco
Heron Ardeola ralloides, Cattle Egret Bubulcus ibis, and
Black-crowned Night Heron Nycticorax nycticorax. This is
also a consequence of the scarcity of similar freshwater bod-
ies on the Arabian Peninsula. The respective mean densities
of the 3 most numerous heron species, namely, Great Egret,
Little Egret, and Squacco Heron, were similar throughout the
study: 0.8-0.9 ind./1 ha, 2.0-2.3 ind./1 ha, and 6.6-7.1 ind./1.
Data collection
The eldwork was carried out in September 2019, during the
herons’ post-breeding period. At this time, herons take prey
for their own use, as opposed to the breeding season, when the
feeding of chicks requires a greater frequency of catches. The
observations focused on feeding birds and were made only
Figure 1. Location of the study site in southern Oman (A). Observation points are marked with dots at two small estuaries of a Wadi (B and C).

782 Current Zoology, 2024, 70, 780–785
on sunny days, between 09:00 and 17:30 h local time, when
conditions were favorable for observing foraging herons (see
Hafner at al. 1982; Papakostas et al. 2005; Nea and Nouira
2016). The weather conditions were similar on all the obser-
vation days, with temperatures hovering around 30 °C and
no precipitation or strong winds that might affect the birds’
behavior. Observation methods were developed based on pre-
vious behavioral studies of herons with modication due to
the specic foraging characteristics of the studied species and
their foraging sites (see Dimalexis et al. 1997; Papakostas et al.
2005; Takaki and Eguchi 2008). The observers used 10 × 42
binoculars and 40–60× spotting scopes to watch the birds and
to determine the size of prey, and a dictaphone to describe
the herons’ behavior. Camouage clothing was worn so as
to minimize the observer’s inuence on the birds’ behavior,
and observation points were selected in shaded areas. In addi-
tion, with spotting scopes, the birds could be observed from a
distance without disturbing them. The following parameters
were recorded: 1) duration of each activity, 2) hunting suc-
cess, i.e., successful attack or unsuccessful, 3) category and
size of prey, 4) other interacting bird species, and 5) time of
day (before noon/ after noon). Six activity categories were
assessed: standing, walking, ying, preening, interactions,
and others (attack and manipulation of prey). An attack was
deemed successful if the bird caught a prey item in its bill.
Prey size was assessed relative to bill length (Bayer 1985). The
following bill lengths were adopted: 121 mm for Great Egret
(Bayer 1985), 90 mm for Little Egret (Cardarelli et al. 2017),
and 64 mm for Squacco Heron (Hafner et al. 1982). Three
classes of prey size were thus established: small (<
30 mm),
medium (30–60 mm) and large (> 60 mm). The size index
was calculated according to the formula: prey size * number
of prey. Observations conducted through a telescope from a
distance allowed the prey to be divided into 2 groups: inverte-
brates and sh. The frequency with which these 2 prey catego-
ries were caught was expressed as the number of items caught
per hour. A particular bird was observed for a maximum of
30 min (Golawski and Kasprzykowski 2018; Golawski and
Kasprzykowski 2021), unless it disappeared from view ear-
lier, but for at least 5 min (av. = 21.1 min, SE = 0.80, n = 125
birds). The mean durations of the observation sessions for
the 3 species were: Great Egret—21.7 min (SE = 1.55, n = 37
birds), Little Egret—22.7 min (SE = 1.34, n = 40 birds),
Squacco Heron—19.3 min (SE = 1.28, n = 48 birds). The total
time of the observations was 2641 min. As the study did not
take into account individuals with juvenile features, the effect
of age on foraging behavior was not analyzed. The birds were
not individually marked, so multiple observations of the same
individual were unavoidable. Nevertheless, all records were
treated equally, i.e., each recorded individual was counted
separately (see also Peck et al. 2014; Tryjanowski et al. 2016;
Rothery et al. 2017; Golawski and Sytykiewcz 2021).
Statistical analyses
The differences in the numbers of inter- and intraspecic inter-
actions, in the numbers of successful and unsuccessful attacks
among the 3 species, in the size index of the 3 prey categories
and the frequency index of sh and invertebrates among the her-
ons were evaluated using the chi2 test. Interspecic differences in
the interaction index and movement index were assessed using
ANOVA and Tukey’s post hoc test. A model analyzing the inu-
ence of the interaction index, movement index, time of day, and
heron species on the success index was set up using a general
linear model (GLM) with identity link function and Gaussian
distribution error. The index of success (percentage of successful
attacks in relation to all attacks), interaction index (number of
interactions) and movement index (times of ying and walking)
were calculated for a 5-min period. The models were selected
using the information-theoretic approach (AIC) (Burnham and
Anderson 2002) and performed using the glm function in the
lme4 package for R (Bates et al. 2015). All possible combinations
of the global model were analyzed using the dredge function in
the MuMln package for R. Only the models with ΔAIC ≤ 2 are
discussed, because they are treated as being equally supported
(Burnham and Anderson 2002). Multiple competing models
were assessed with regard to their t to the data using AIC as
the leading criterion, and those with the lowest AIC value were
selected as the best tting ones. All the data were analyzed in the
R environment (R Core Team 2021). The reported values are the
mean ± 1 SE. Only those results with a probability of α ≤ 0.05
were assumed to be statistically signicant.
Results
Foraging behavior
Squacco Heron was involved in the most interactions (68%
of all interactions), Little Egret in fewer (24%), and Great
Egret in the fewest (only 8%). The number of species par-
ticipating in the interactions was similar for all 3 herons: 5
for Great Egret, 6 for Little Egret, and 4 for Squacco Heron
(Supplementary Table S1). However, Grey Heron and Purple
Heron were the only aggressors toward Great Egret, while
Whiskered Tern Chlidonias hybrida and Gull-billed Tern
Gelochelidon nilotica were the only victims of Little Egret.
The dominant type of interaction was intraspecic: 92%
of cases for Squacco Heron, 59% for Little Egret, and 8%
for Great Egret. The differences in the numbers of inter- and
intraspecic interactions among the three species were sig-
nicant for all comparisons: Little Egret and Squacco Heron
(chi2 = 37.22, P < 0.001, df = 1), Great Egret and Squacco
Heron (chi2 = 96.87, P < 0.001, df = 1), and Little Egret and
Great Egret (chi2 = 16.21, P < 0.001, df = 1).
There were interspecic differences with regard to both
the interaction index (ANOVA F2,121 = 3.77 P = 0.026) and
movement index (ANOVA F2,121 = 3.30 P = 0.040). The
interaction index for Great Egret was lower than for Little
Egret and Squacco Heron (Tukey’s post hoc test, P = 0.011
and P = 0.035, respectively; Figure 2). On the other hand, the
movement index differentiated the 2 smaller heron species, its
value for Squacco Heron being higher than for Little Egret
(Tukey’s post hoc test, P = 0.040; Figure 3). The other 2 com-
parisons—between Great Egret and Squacco Heron as well
as between Great Egret and Little Egret—were not signicant
(Tukey’s post hoc test, P = 0.312 and P = 0.650, respectively).
Foraging success and prey items
Squacco Heron had a higher mean percentage of successful
attacks (62.7%, N
=
118 attacks) than Little Egret (48.4%,
N = 93 attacks) and Great Egret (47.9%, N = 117 attacks).
The differences in the numbers of successful and unsuccess-
ful attacks were signicant between Great Egret and Squacco
Heron (chi2 = 5.24, P = 0.021, df = 1) and between Little Egret
and Squacco Heron (chi2 = 4.34, P = 0.037, df = 1), but not
between Little Egret and Great Egret (chi2 = 0.01, P = 0.940,
df = 1). The models set up on the basis of Akaike’s informa-
tion criterion (AIC) included only 2 of the 4 variables that

Kasprzykowski and Golawski · Comparative foraging behavior of 3 heron species 783
could be important in an analysis of foraging success (Table
1). However, the best model showed that only the movement
index was signicant, as this had a positive impact on individ-
ual foraging success (Table 2).
In contrast to the 2 smaller heron species, Great Egret
caught more sh than invertebrates (Supplementary Figure
S1), and these differences were signicant compared with
Little Egret (chi2 = 9.65, P = 0.004, df = 1) and Squacco
Heron (chi2 = 11.50, P < 0.001, df = 1). No differences were
found in this respect between Little Egret and Squacco
Heron (chi2 = 0.01, P = 0.943, df = 1). The prey size category
was different in the case of Great Egret, and the size indices
of 3 items were signicant between Little Egret and Great
Egret (chi2 = 34.54, P < 0.001, df = 2) and between Great
Egret and Squacco Heron (chi2 = 51.96, P < 0.001, df = 2,
Supplementary Figure S2). Little Egret and Squacco Heron
caught a greater number of smaller prey items, but the dif-
ferences in the size index was also signicant (chi2 = 16.03,
P < 0.001, df = 2).
Discussion
This study revealed a considerable variety of interactions
both among the 3 heron species themselves, and between
them and other bird species foraging in the same places. The
results suggest that the very abundant food supply could have
underpinned a different approach to food access so as to
reduce competition. Rich foraging resources increased both
inter- and intraspecic interactions, with stronger interactions
observed among closely related species (Fasola 1986; Bolton
et al. 2019). In this study, the frequency of interaction was
the lowest in the largest heron (Great Egret) and successively
higher in the other 2, smaller species (Little Egret and Squacco
Heron). Squacco Heron, in particular, was more frequently
involved in intraspecic interactions than the other 2 heron
species. In consequence, this led to more movements and
changes of feeding site.
All 3 heron species behaved aggressively toward each other
and toward 3 other species. Negative interactions may have
been due to density-dependent factors or to the fact that
herons prey on species inhabiting shallow waters (see Amat
1990). However, as the densities of foraging herons were
similar, foraging behavior was not density-dependent. When
resources are limited in dispersed, rich patches, interactions
between predators can be very intense. This may also indi-
cate strong competition in mixed-species aggregations, result-
ing from food usurpation, either directly as kleptoparasitism
or indirectly through copying and supplanting (Amat 1990;
Phillips et al. 1996; Wood et al. 2015). Direct kleptoparasit-
ism was only observed in Little Egret, all their acts of aggres-
sion against terns being provoked by their attempts to steal
the terns’ prey. This foraging tactic involves a higher energy
expenditure: as it relies on the victim being pursued in ight,
the aerobatic skills of the pursuer must be similar (Iyengar
2008).
In herons, changes in foraging strategy may depend on
biological features like body size (Nota 2003; Papakostas
et al. 2005). They also modify their tactics to suit local con-
ditions (Nea and Nouira 2016). According to the optimal
Figure 2. Mean values (dots) and SE (whiskers) of the interaction index
(per 5-min observation) for 3 heron species. Arrows indicate comparisons
between species: * - P < 0.005, ns—non-significant (Tukey’s post hoc
test).
Figure 3. Mean values (dots) and SE (whiskers) of the movement index
(per 5-min observation) for 3 heron species. Arrows indicate comparisons
between species: * - P < 0.005, ns—non-significant (Tukey’s post hoc
test).
Table 1. Results of the model describing the influence of activity
parameters on the foraging success of three heron species in southern
Oman. Degrees of freedom (df), model log-likelihood (LL), corrected AIC
(AIC), difference between the model and the best model in the data set
(Δ AIC), and weight for the model (AICwt) are shown
Model (xed effects) df LL AIC ΔAIC AICwt
Intercept + interaction
+ movement
4 –108.29 224.9 0.00 0.351
Intercept + movement 3 –109.84 225.9 0.96 0.217
Intercept + interaction
+ movement + time
5 –107.91 226.3 1.41 0.173
Table 2. Results of the best model describing the influence of activity
parameters on foraging success in three heron species
Fixed effects Estimate SE t-value P-value
Intercept –0.087 0.094 –0.931 0.354
Interaction –0.180 0.103 –1.750 0.083
Movement 0.130 0.019 6.838 <0.001

784 Current Zoology, 2024, 70, 780–785
foraging theory, foragers search for prey that will provide
the highest energetic benet (Stephens and Krebs 2019).
Squacco Herons exhibited the highest movement frequency
among the 3 heron species. This could have been due to the
high percentage of interactions that force displacements.
On the other hand, Squacco Herons may have employed a
more active strategy of searching for better feeding grounds
and, especially in comparison with Little Egret, were more
exible in their foraging repertoire (see Dimalexis et al.
1997). The highest mean percentage of successful attacks
lends greater plausibility to the latter explanation. The lev-
els of this parameter in Great Egret and Little Egret were
similar but lower than in Squacco Heron. This analysis
has shown that only the movement index is signicant as
regards foraging success.
Not only the energy spent on catching prey and attack
effectiveness is important; prey size is, too (Gawlik 2002;
Morelli et al. 2015; Golawski and Kasprzykowski 2021). In
this study, Great Egrets caught more sh than invertebrates
than the other 2 herons, probably because its prey items were
larger. This nding is in line with other studies showing that
heron species with a greater body mass achieve higher bio-
mass intakes per unit effort than smaller ones (Dimalexis
et al. 1997). Little Egret and Squacco Heron caught more
invertebrates, i.e., smaller prey items. In light of Little Egret’s
higher energy requirements resulting from its only slightly
greater body mass, its use of more energy-costly tactics and
its poorer foraging success, Squacco Heron seems to be the
more effective forager.
Conclusions
The present study has shown that heron species inhabiting the
same rich habitat use different foraging tactics. These differences
relate to inter- and intraspecic interactions, movement fre-
quency and prey items. The smallest species—Squacco Heron—
seems to be the most susceptible to competition. On the other
hand, its greater mobility, possibly the effect of compensating for
pressure from other heron species, and using different foraging
tactics, enhance its foraging success. Therefore, one may argue
that the parameters of foraging behavior in multi-species com-
munities found in rich aquatic habitats, such as the number of
interactions and movement frequency, may provide meaningful
results regarding the energy costs and benets of foraging only
when used in combination with information on hunting success
and the type of prey.
Acknowledgments
We would like to thank to Maia Sarrouf Willson from the
Environmental Society of Oman and Thuraya Said Al-Sariri
from the Ministry of Environment and Climate Affairs of
Oman for supporting the study. We are grateful to Peter Senn
for the English language editing, Przemysław Obłoza for map
editing as well as the reviewers for their valuable work, which
enabled us to improve the manuscript.
Funding
The research was carried out during an internship in Oman
and was supported by the University of Siedlce, Poland
(Theme No. 151/23/B and 152/23/B nanced from a science
grant by the Ministry of Education and Science, Poland).
Conflict of Interest statement
The authors have no relevant nancial or non-nancial inter-
ests to disclose.
Ethics Statement
All eld survey procedures complied with the relevant regula-
tions pertaining to Oman.
Authors’ Contributions
Z.K. and A.G. conceived the study and undertook the eld-
work; ZK wrote the manuscript.
Supplementary Material
Supplementary material can be found at https://academic.
oup.com/cz.
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