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. 2024 Mar 30;70(6):780–785. doi: 10.1093/cz/zoae011

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

Zbigniew Kasprzykowski 1,✉, Artur Golawski 2
Editor: Fumin Lei
PMCID: PMC11634682  PMID: 39678822

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.

Keywords: 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, hydrological 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 maximizes structural heterogeneity and supports an exceptionally high biodiversity (Bolpagni et al. 2019; Cantonati et al. 2020). As a result, small standing-water ecosystems are generally acknowledged to be among the most productive ecosystems at the global scale (Schiemer et al. 1995).

To understand the functions of wetlands that are to be preserved 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; Nefla 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 individuals of one species use successfully settled individuals of another species as sources of information regarding the location 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 interspecific 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 fish 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). 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 efficiency and also intra- and interspecific interactions during 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 competition between them would be strong in a multi-species community. Body mass is a good predictor of dominance rank across species (Francis et al. 2018). It was therefore anticipated 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 interactions 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 satellite 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 temperature 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).

Figure 1.

Figure 1.

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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).

The habitat consists of natural wetlands—rather small estuaries of a Wadi, locally known as Khawr—which are covered with dense floating vegetation near the shores and have an area of open, fresh water in the center. Because of their depth and size, they are classified as small standing-water ecosystems (SWEs) (Cantonati et al. 2020). The features they have in common are the natural succession of vegetation, proximity 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 bodies 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 fieldwork 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 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; Nefla and Nouira 2016). The weather conditions were similar on all the observation 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 previous behavioral studies of herons with modification due to the specific 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. Camouflage clothing was worn so as to minimize the observer’s influence on the birds’ behavior, and observation points were selected in shaded areas. In addition, 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 success, 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, flying, 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: invertebrates and fish. The frequency with which these 2 prey categories 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 earlier, 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 intraspecific interactions, 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 fish and invertebrates among the herons were evaluated using the chi2 test. Interspecific differences in the interaction index and movement index were assessed using ANOVA and Tukey’s post hoc test. A model analyzing the influence 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 flying 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 fit to the data using AIC as the leading criterion, and those with the lowest AIC value were selected as the best fitting 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 significant.

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 participating 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 intraspecific: 92% of cases for Squacco Heron, 59% for Little Egret, and 8% for Great Egret. The differences in the numbers of inter- and intraspecific interactions among the three species were significant 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 interspecific 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 comparisons—between Great Egret and Squacco Heron as well as between Great Egret and Little Egret—were not significant (Tukey’s post hoc test, P = 0.312 and P = 0.650, respectively).

Figure 2.

Figure 2.

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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.

Figure 3.

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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).

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 unsuccessful attacks were significant 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 information criterion (AIC) included only 2 of the 4 variables that could be important in an analysis of foraging success (Table 1). However, the best model showed that only the movement index was significant, as this had a positive impact on individual foraging success (Table 2).

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 (fixed 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
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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
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In contrast to the 2 smaller heron species, Great Egret caught more fish than invertebrates (Supplementary Figure S1), and these differences were significant 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 significant 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 differences in the size index was also significant (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 intraspecific 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 intraspecific 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 indicate strong competition in mixed-species aggregations, resulting from food usurpation, either directly as kleptoparasitism or indirectly through copying and supplanting (Amat 1990; Phillips et al. 1996; Wood et al. 2015). Direct kleptoparasitism was only observed in Little Egret, all their acts of aggression 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 flight, 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 conditions (Nefla and Nouira 2016). According to the optimal foraging theory, foragers search for prey that will provide the highest energetic benefit (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 flexible in their foraging repertoire (see Dimalexis et al. 1997). The highest mean percentage of successful attacks lends greater plausibility to the latter explanation. The levels 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 significant 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 fish than invertebrates than the other 2 herons, probably because its prey items were larger. This finding is in line with other studies showing that heron species with a greater body mass achieve higher biomass 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 intraspecific interactions, movement frequency 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 communities found in rich aquatic habitats, such as the number of interactions and movement frequency, may provide meaningful results regarding the energy costs and benefits of foraging only when used in combination with information on hunting success and the type of prey.

Supplementary Material

Supplementary material can be found at https://academic.oup.com/cz.

zoae011_suppl_Supplementary_Table_S1_Figures_S1-S2
zoae011_suppl_supplementary_table_s1_figures_s1-s2.docx (22.5KB, docx)

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.

Contributor Information

Zbigniew Kasprzykowski, Faculty of Sciences, University of Siedlce, Prusa 14, 08-110 Siedlce, Poland.

Artur Golawski, Faculty of Sciences, University of Siedlce, Prusa 14, 08-110 Siedlce, Poland.

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 financed from a science grant by the Ministry of Education and Science, Poland).

Conflict of Interest statement

The authors have no relevant financial or non-financial interests to disclose.

Ethics Statement

All field survey procedures complied with the relevant regulations pertaining to Oman.

Authors’ Contributions

Z.K. and A.G. conceived the study and undertook the fieldwork; ZK wrote the manuscript.

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