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Background: Interspecific competition is known to be strongest between those species that are both closely related and sympatric. Egrets are colonially nesting wetland birds that often overlap and can therefore be expected to compete in roosting and nesting habitat as well as in diet. According to the niche partitioning hypothesis, it is to be expected that these similar species would show differentiation in at least one of the main niche dimensions to reduce competition. We tested niche partitioning between the colonially nesting Little Egret (Egretta garzetta) and Cattle Egret (Bubulcus ibis) in temporal, spatial and trophic dimensions. Methods: Field study was conducted in three mixed egret colonies in Yangxian County, southwest Shaanxi Prov- ince, central China. For each nest colony we recorded its spatial location, the height of nesting trees and of nests, the height of roosting trees and of roosting individuals within the trees. We determined the first egg-laying and first hatching dates of the two species. Craw dissection of storm-killed egret nestlings was used to measure the diet. Six transects were surveyed to study foraging habitat selection. Results: We found that hatching time of Little Egrets peaked earlier (by about 1 month) than that of Cattle Egrets. Cattle Egrets nested and roosted higher than Little Egrets. The foraging habitats used by Little Egrets were dominated by river banks (73.49%), followed by paddy fields (13.25%) and reservoirs (10.84%), whereas Cattle Egret foraging sites were characterized by grasslands (44.44%), paddy fields (33.33%) and river banks (22.22%). Little Egrets consumed more fishes (65.66%) and Odonata larvae (13.69%) than Cattle Egrets, while Cattle Egrets were found feeding mainly on Coleoptera (29.69%) and Orthoptera (23.29%). Little Egrets preyed on larger mean biomasses of food items than Cattle Egrets. Conclusions: Our results confirm the niche partitioning hypothesis as a mechanism for coexistence among ecologi- cally similar species. In two coexisting egret species, niche partitioning is multidimensional, such that the two coexist- ent species occupy differing ecological space based on all three temporal, spatial and trophic niche dimensions. Keywords: Bubulcus ibis, Egretta garzetta, Interspecific competition, Niche partition, Sympatric species Background How closely related sympatric species can coex- ist has long been a major issue in ecology (Hutchin- son 1959; Pianka 2000; Beaulieu and Sockman 2012) and is essential for understanding the maintenance of *Correspondence: cqding@bjfu.edu.cn 1 diversity (Chesson 2000; Levine and HilleRisLambers School of Ecology and Nature Conservation, Beijing Forestry University, Beijing 100083, China 2009; Kent and Sherry 2020). Gause’s theorem states Full list of author information is available at the end of the article that two species cannot occur in the same place if the © The Author(s) 2021. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/. The Creative Commons Public Domain Dedication waiver (http:// creat iveco mmons. org/ publi cdoma in/ zero/1. 0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data. Ye et al. Avian Res (2021) 12:33 Page 2 of 8 niche characteristics they occupy are extremely similar Methods (Gause 1934). Niche theory predicts that similar spe- Study area cies when in coexistence should show differentiation in The study was conducted in Yangxian County (33° 06′– at least one of the main niche dimensions (food, space 33° 36′ N, 107° 17′–108° 02′ E), southwest Shaanxi and/or time) in order to escape the negative impacts Province, central China (Fig. 1). The study area is of interspecific competition (MacArthur and Levins approximately 600 km . The mean annual temperature is 1967; Schoener 1974; Hutchinson 1991; Julliard et al. between 12 and 14 °C, with maximum temperatures that 2006; Cameron et al. 2007). In mixed-species heron- can exceed 38.7 °C during summer. Average annual rain- ries, where hundreds or thousands of breeding birds of fall ranges from 900 to 1000 mm. The annual mean frost - more than one species can coexist (Parsons 1995), nest- less season is 238 days. Local vegetation is dominated by ing and foraging resources are critical for ardeid spe- Masson’s Pine (Pinus massoniana), Chinese Arborvitae cies during the breeding season (McCrimmon 1978; (Platycladus orientalis), Pagoda Tree (Sophora japonica), Fasola 1994; Kazantzidis et al. 1997). Competitive Siberian Elm (Ulmus pumila), Sawtooth Oak (Quercus interactions are known to be stronger between closely acutissima), Oriental Oak (Quercus variabilis), Black related sympatric ardeid species (Bolton et al. 2019). Locust (Robinia pseudoacacia), Chinese Chestnut (Cas- There have been numerous studies on niche partition - tanea mollissima) and Ring-cupped Oak (Cyclobalano- ing between ardeid species, but most of them have only psis glauca) (Ding 2004). In our study area, some of the investigated the diets of various herons (e.g., Trexler Little Egrets are permanent residents but most are migra- et al. 1994; Salazar et al. 2005; Boyle et al. 2012; Choi tory and arrived in Yangxian County in late March. The et al. 2016; Ashoori et al. 2017). And there were a few entire Cattle Egret population is migratory and arrived in studies on nest site selection (Jenni 1969; McCrimmon Yangxian County in late April. The breeding season of the 1978; Burger 1979; Parejo et al. 1999; Metallaoui et al. two species occurs within the period from early April to 2020) and temporal separation between species within late August. There were approximately 450 pairs of Little heron colonies (Weber 1975; Burger 1978; Ashkenazi Egret and 500 pairs of Cattle Egret in three nesting colo- and Yom-Tov 1997). Despite those separate studies of nies (Leichaogou Reservoir 180 pairs of Little Egret and diet, nest site selection and nesting phenology, we still 150 pairs of Cattle Egret; Donglian Village: 200 pairs of do not have a comprehensive understanding of niche Little Egret and 240 pairs of Cattle Egret; Caoba Village: partitioning that integrates the temporal, spatial and 70 pairs of Little Egret and 110 pairs of Cattle Egret). The trophic dimensions of sympatric ardeid species. three colonies were similar in structural appearance and The Little Egret (Egretta garzetta) is widespread vegetation (e.g., Oriental Oak, Sawtooth Oak, Siberian throughout Europe and Asia (Wong et al. 2000). It is Elm were found within our sampled area). The elevation closely related to the Cattle Egret (Bubulcus ibis) which of the colonies ranged from 459 to 593 m above sea level. was once native only to Africa, Europe and Asia, but dra- matically expanded its geographic range to America and Sampling design Australia during the last century (Arendt 1988; Massa Data were collected from March to September 2012 in all et al. 2014). These two ardeid species often form mixed three colonies (Fig. 1). Dates of first egg-laying and dates nesting colonies that may contain hundreds and thou- of first nestlings in openly visible nests were checked in sands of breeding pairs (Kushlan and Hafner 2000), and each colony three or four times per week from March to may compete in terms of roosting and nesting habitat as late June. A total of 108 nests were monitored including well as diet (Snow et al. 1998). However, little study has 54 of Cattle Egrets and 54 of Little Egrets. We randomly yet tested potential niche partitioning between these chose one nest per tree to minimise the impact of abso- species along the three main ecological dimensions lute tree height measurements on the nest height data. If simultaneously. there were nests of both species on a single tree, we ran- In this study, we combined reproductive time, habi- domly selected one nest of each species. In total, 27 Lit- tat selection and diet to test the mechanism facilitating tle Egret nests and 34 Cattle Egret nests were sampled. coexistence of Little and Cattle Egret during the breed- For each nest, we recorded its spatial location (eTrex20, ing season. Specifically, the objectives of this study were Garmin, USA), measured the height of nesting trees, and to examine whether the two species partitioned their (1) the height of the nests above the ground (Diastimeter onset of laying the first egg, (2) use of nesting sites, roost - 202342, Bushnell, USA). Roost site characteristics of the ing trees and foraging habitats, and (3) dietary composi- two egret species were investigated in August: on locating tion and biomass. We hypothesized that Little Egrets and a roosting tree containing both species, we firstly meas - Cattle Egrets would differ in these temporal, spatial and ured the height of the roosting trees (Diastimeter 202342, trophic niche dimensions. Bushnell, USA) and within each roosting tree, the height Ye et al. Avian Res (2021) 12:33 Page 3 of 8 Fig. 1 Map of the study location in Yangxian County, Shaanxi Province, China above ground of the roosting position of each species was wind are common causes of breeding failure for egret and measured by Diastimeter (Diastimeter 202342, Bushnell, heron species (Frederick and Collopy 1989; Baxter 1994; USA). We randomly chose one individual per tree, and Zhu and Zou 2001). In our study, during heavy monsoon randomly selected one nest of each species if both spe- weather, strong winds and rain sometimes upset nests, so cies on a single tree. A total of 46 roost positions of Little that some egret nestlings fell from nests and died natu- Egrets and 57 of Cattle Egrets were measured. rally. Dead birds were sought after any day of heavy rain Six transects were surveyed to study foraging habi- or strong winds, and dead egret nestlings were collected tat selection (Fig. 1). Each transect covered the major below the nests in order to avoid major disturbance to habitat types in the study area. Two investigators riding the colony. A total of 5 dead Little Egret nestlings and 17 motorcycles surveyed along the transects at a speed of Cattle Egret nestlings were collected during the breed- 10 km/h and censused the egrets using 10 × 42 binocu- ing season. Dead birds were immediately stored in freez- lars. The habitats were classified into four types: grass - ers until craw dissection. Prey items were identified and lands (level marshes and mudflats with a vegetation cover quantified by counting undigested pieces (Brown and of more than 30%), river banks (the river floodplain and Ewins 1996; Martínez 2004). To estimate the biomass of bank), paddy fields (the area that farmers planted with the prey items, measurements of total or partial length of rice and maintained water within the rice) and reser- prey were taken and compared with length-mass regres- voirs (the natural or artificial lakes including small ponds sion functions calculated for each taxon (Dorn et al. with mudflats along their edges). We recorded the forag - 2011). ing habitat where each bird was sighted. All the surveys were conducted from 7:00 to 11:00 a.m. The length of Data analysis each transect was between 8 and 20 km (total 85 km). We The one-sample Kolmogorov–Smirnov test was used to conducted the surveys once a month from April through determine whether data were normally distributed (Field June. Foraging habitat was recorded for 273 individuals 2009). We conducted a Mann–Whitney U test to test the including 196 Little Egrets and 77 Cattle Egrets. difference in breeding dates between the two species, The low reproductive success of colonial egrets and as data were not distributed normally even after being herons has often been reported (Teal 1965; Dusi and transformed. We made univariate comparisons using an Dusi 1968; Maxwell and Kale 1977). Rainfall and strong independent-sample t-test for significance of differences Ye et al. Avian Res (2021) 12:33 Page 4 of 8 in nest height of the two species as the data were nor- mally distributed, but we used a Mann–Whitney U test on nest tree height, roost tree height and roost height since these data were not normally distributed. Chi- squared test was used to examine whether the two spe- cies used different foraging habitats (Zar 1999). Prey items were classified by taxon (at family or order level) and mass (Martínez 2010). The frequencies of every prey class in the taxon categories over all the prey items were calculated as percentages. Niche overlap of the two species was estimated using: P P ij ik O = , ij 2 2 P P ij ik Fig. 2 Temporal distribution of the egg-laying date by Little Egrets where O denotes dietary overlap, P and P represent and Cattle Egrets ij ij ik proportions use of food type i by species j and k. This equation gives values between 0 and 1 which signify no overlap and complete overlap respectively (Pianka 1973; Liordos and Kontsiotis 2020). Chi-squared test was used to compare the frequencies of prey items (Zar 1999). An independent-sample t-test was used to test differ - ent mean biomass of the prey for data that met the con- ditions of independence and normality (Zar 1999). All probabilities are two-tailed, and the significance level was set at α = 0.05. Values are presented with mean ± SE. For all statistical calculations we used software SPSS 18.0 for Windows (SPSS 2009). Results Reproductive time Little Egrets started breeding on 7 April and ended by 4 July, and Cattle Egrets started breeding on 1 May and Fig. 3 Percentage of foraging habitats used by Little Egrets and ended by 2 August. The overlap in breeding period was Cattle Egrets 72.7% (64 of 88 d) for Little Egrets and 68.8% (64 of 94 d) for Cattle Egrets. The average egg-laying date of Little Egrets (19 April ± 8 days, n = 54) was significantly different from (12.60 ± 1.31 m, n = 57; Mann–Whitney U test, that of Cattle Egrets (16 May ± 8 days, n = 54; Mann– Z = − 2.410, P = 0.016). However, roost tree height Whitney U test, Z = − 8.827, P < 0.001) (Fig. 2). By the was not significantly different between Little egret time that Cattle Egrets began laying, 92.6% of Little Egret (13.08 ± 1.54 m, n = 46) and Cattle egret (13.27 ± 1.17 m, chicks had already hatched from the eggs. n = 57; Mann–Whitney U test, Z = − 0.440, P = 0.660). Nesting and roosting height Nesting tree height of Little Egrets (13.22 ± 1.18 m, Foraging habitats n = 27) were similar in altitude with Cattle Egrets A Chi-squared test showed a significant difference in for - (13.07 ± 1.28 m, n = 34; Mann–Whitney U test, aging habitats between the two egret species (χ = 85.422, Z = − 0.408, P = 0.683). Nest height of Little Egret df = 3, P < 0.001). The foraging habitats used by Little (10.87 ± 1.63 m, n = 27) differed significantly from that of Egrets were dominated by river banks (73.49%), followed Cattle Egret (11.79 ± 1.37 m, n = 34; Independent-sample by paddy fields (13.25%) and reservoirs (10.84%) (Fig. 3), t-test, t = − 2.473, P = 0.018). whereas Cattle Egret’s foraging habitats were dominated Cattle Egrets (11.71 ± 1.88 m, n = 46) also pre- by grasslands (44.44%), paddy fields (33.33%) and river ferred to roost significantly higher than Little Egrets banks (22.22%) (Fig. 3). Ye et al. Avian Res (2021) 12:33 Page 5 of 8 Nestling dietary selection land-based prey, whereas the prey fed to Little Egrets The trophic niche overlap between nestlings of the included more aquatic taxa. two egret species was 0.377 calculated by prey taxon abundance and 0.257 by prey individual biomass. The Discussion degree of overlap was related to significant differences As predicted, our results demonstrated significant differ - between the two species in their diet (Chi-squared test, ences in the timing of reproduction, habitat utilization, χ = 6.926, df = 15, P = 0.008), which in both species con- and diet fed to nestlings between two egret species. Thus sisted entirely of animal food provided by the parents. they segregated in the use of the temporal, spatial and The Little Egret nestlings received fishes (6.20 ± 2.22 trophic niche dimensions, resulting in reduced interspe- items/craw and average biomass 1.02 ± 0.32 g/item) cific competition during the breeding season. Our results and Odonata larvae (5.40 ± 3.34 items/craw and aver- are in agreement with the niche partition hypothesis, age biomass 0.24 ± 0.00 g/item) (Table 1). Cattle Egret whereby morphologically, ecologically and closely related nestlings did not receive any fish but were fed mainly sympatric species segregate in at least one of the niche Coleoptera (25.65 ± 4.72 items/craw and average biomass dimensions to allow coexistence. 0.20 ± 0.02 g/item) and Orthoptera (34.80 ± 9.81 items/ Along the time dimension, our results underlined a craw and average biomass 0.18 ± 0.04 g/item) (Table 1). clear pattern of niche partitioning in the mean clutch The mean biomasses of the prey fed to Little Egret initiation date between the two species. Egg laying and nestlings (0.40 ± 0.45 g/item) were significantly higher hatching of the Cattle Egrets peaked about 1 month than those fed to Cattle Egret nestlings (0.20 ± 0.54 g/ later than that of the Little Egrets. Such differentiation in item; Independent-sample t-test, t = − 4.116, df = 1639, reproductive timing can play an important role for coex- P < 0.001). Taxonomic diversity of prey was greater in isting species as it is related to the availability and choice Cattle Egrets (14 categories in Table 1) than in Little of nesting sites and food resources (Ye et al. 2019), which Egrets (8 categories). Cattle Egrets took predominantly could maximize their fitness (Sanz-Aguilar et al. 2015). Table 1 Prey of Little Egrets and Cattle Egrets, determined by abundance (%N) and biomass (%B) Prey types Little Egrets (n = 5) Cattle Egrets (n = 17) %N Mean %B Weight (g) %N Mean %B Weight (g) Insecta Coleoptera 5.22 1.20 ± 1.20 2.28 0.18 ± 0.00 28.57 25.65 ± 4.72 29.69 0.20 ± 0.02 Coleoptera larvae 17.39 4.00 ± 4.00 12.68 0.30 ± 0.00 9.11 8.18 ± 5.73 7.84 0.19 ± 0.01 Diptera 14.78 3.40 ± 2.93 2.64 0.09 ± 0.01 5.37 5.13 ± 3.66 3.25 0.12 ± 0.00 Diptera larvae 6.96 1.60 ± 1.60 1.69 0.10 ± 0.00 5.90 5.63 ± 4.33 5.19 0.17 ± 0.00 Hemiptera 1.74 0.40 ± 0.40 0.85 0.20 ± 0.00 0.85 0.81 ± 0.37 1.19 0.28 ± 0.02 Odonata 2.03 1.94 ± 1.81 0.36 0.26 ± 0.06 Odonata larvae 23.48 5.40 ± 3.34 13.69 0.24 ± 0.00 8.65 8.25 ± 3.28 9.90 0.23 ± 0.01 Orthoptera 34.21 34.80 ± 9.81 23.39 0.18 ± 0.04 Arachnida Araneae 3.48 1.66 ± 0.66 0.51 0.06 ± 0.00 2.62 2.50 ± 1.09 2.62 0.07 ± 0.03 Branchiopoda Triopsidae 1.38 1.31 ± 0.63 1.46 0.19 ± 0.01 Crustacea 0.13 0.13 ± 0.12 0.20 0.31 ± 0.00 Nematomorpha 0.52 0.50 ± 0.48 0.05 0.02 ± 0.00 Aves Tree sparrow 0.07 0.06 ± 0.06 5.10 15.52 ± 0.00 Amphibian Frog 0.59 0.53 ± 0.17 9.74 3.87 ± 0.94 Teleostei Fish 26.96 6.20 ± 2.22 65.66 1.02 ± 0.32 Total 115 prey items 0.40 ± 0.45 1526 prey items 0.20 ± 0.54 The dominant prey types are highlighted in italics n number of dead birds included in the collection Ye et al. Avian Res (2021) 12:33 Page 6 of 8 The late arrival of Cattle Egrets enabled them to take first nesters seem not to choose the theoretically best over some abandoned nests of Little Egrets, saving time positions. One possible reason is that the local vegeta- and energy in nest construction that would lead to low tion structure (i.e., thin twigs located higher in the tree interspecific competition (Burger 1978). Another way of crown) could influence nest-site selection of ardeid saving energy while building nests is probably by stealing birds (Zhu and Zou 2001). As Cattle Egrets are the nesting material, which was observed in this study (e.g., smaller species, they are able to utilize nest positions we observed 43 nest building events from May 14‒17, among higher and thinner twigs that are not available including 67.4% by stealing material and 32.6% events by to Little Egrets. Additionally, there can be aggressive picking twigs) mainly in Cattle Egrets and reported also contests between species (e.g., we observed 6 events by other studies (Burger 1978; Ashkenazi and Yom-Tov when Cattle Egrets acted aggressively towards Little 1997). Egrets from May 14‒17, but only 2 events when Little The reproductive period is a critical time in the annual Egrets attacked Cattle Egrets), and these can allow spe- life cycle, when a lot of energy is expended and trophic cies to overcome the competitive disadvantage of small resources may be limited and must be geared to meet size (Hino 2005; Martin and Ghalambor 2014). Burger the specific needs of the developing chick (Fasola 1994; (1982) found that Cattle Egrets had the highest aggres- Samraoui et al. 2012). By the time Cattle Egrets began sion rates and successfully defended higher perches nesting in Yangxian County, 92.6% of Little Egret chicks compared to other egret species, which might account had already hatched. Variations in the growth rate of Lit- for their ability to nest higher up. tle Egrets have been detected and at the age of 23 days, While the nestling diets of the two species showed par- young Little Egrets have normally reached 82% of adult tial niche overlap, there were striking dietary differences weight, beyond which growth is slow (Zhu et al. 2005). in exploiting food resources. There is general agreement The mean incubation period of Cattle Egrets is 24.2 days that insects make up the most important dietary com- (Zhu and Zou 2001), so that when Cattle Egrets in our ponent for Cattle Egret nestlings (Siegfried 1971; Foga- study area first hatched, the peak in energy demand for rty and Hetrick 1973; Ashoori et al. 2017). Most of the chick development of Little Egrets has passed. Temporal studies carried out to date show that the diet of Little partitioning may thus facilitate coexistence of these two Egret nestlings is mainly composed of fish (Zhou et al. species. Another factor related to the late arrival date of 2000, 2003; Ashoori et al. 2017). Our sample size of only Cattle Egrets might be a benefit from the abundance of 5 Little Egret nestlings would normally be considered too insects they consume. Cattle Egrets nestlings feed pri- small to give a stand-alone result—65.7% fish in the pre - marily on insects (Siegfried 1971; Si Bachir et al. 2001; sent study—but the close agreement with other studies Table 1), and insects are typically plentiful during June to quoted, and the fact that not a single fish was included August (e.g., Huang et al. 2005). Postponing reproduction in our sample of more than 1500 food items for Cattle might involve in the adaptation of Cattle Egret to local Egret nestlings, indicating that our results reflect a real environments (Lack 1968; Sanz-Aguilar et al. 2015). difference in nestling diet of the two species. One adap - Nests sites are important for rearing offspring and tive explanation for the dietary difference is that it allows nest height is a key characteristic that can influence resource partitioning between the two egrets, thereby nest survival and reproductive success (MacDon- increasing individual fitness through reducing inter - ald et al. 2016; Jara et al. 2020; Overduijn et al. 2020). species competition (Ashoori et al. 2017; Nicolaus et al. Competition for nesting space should be intense where 2019). In addition, Little Egrets examined in the present nesting birds congregate in dense colonies during the study preyed on larger mean biomasses of the food items breeding season (Kazantzidis et al. 1997). The advan - than Cattle Egrets. It has been tested that prey size or tages of nesting higher may relate to increased visibil- biomass serves as a partitioning mechanism among her- ity of predators, increased ability to take flight quickly ons (Britton and Moser 1982). Food type and size of a when predators approach, and decreased losses to species is often closely related to foraging site selection ground predators (Burger 1982). Although Cattle Egrets (Kasahara and Katoh 2008; Martínez 2010; Jensen et al. in our study area arrived and began nesting later than 2017). Using different feeding sites may serve to sub - Little Egrets, our results showed that Cattle Egret nests divide the resource spectrum (Cody 1968). Our results were located higher in trees by a small but statistically demonstrated that the egrets used different foraging significant amount. There are theoretical reasons to habitats contributing to their coexistence in an area of suppose that higher nest positions should be preferred, sympatry. Little Egrets foraged in shallow, open waters such as unobstructed flight paths and nest hygiene (e.g., using river banks whereas Cattle Egrets foraged mainly in McCrimmon 1978; Parejo et al. 1999; Metallaoui et al. grasslands and paddy fields. 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Avian Research – Springer Journals
Published: Jun 19, 2021
Keywords: Bubulcus ibis; Egretta garzetta; Interspecific competition; Niche partition; Sympatric species
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