INTRODUCTION

The Cattle Egret’s ecological plasticity, es-pecially for foraging, is certainly one of themain causes of its expansion on a planetaryscale. Numerous studies have shown its con-sumption of a wide range of prey. Several au-

thors, using data either from the analysis ofstomach contents of adult birds or from foodregurgitated by chicks, showed that this spe-cies feeds primarily on insects (e.g. Kadry-Bey,1942 in Egypt; Ikeda, 1956 in Japan; Burns &Chapin, 1969 in Louisiana, USA; Siegfried,1966, 1971 in South Africa; Fogarty & Hetrick,

Ardeola48(2), 2001, 217-223

DIET OF ADULT CATTLE EGRETS BUBULCUS IBISIN A NEW NORTH AFRICAN COLONY

(SOUMMAM, KABYLIE, ALGERIA): TAXONOMICCOMPOSITION AND SEASONAL VARIABILITY

Abdelkrim SI BACHIR*, Heinz HAFNER**, Jean-Noel TOURENQ***,Salaheddine DOUMANDJI**** & Sovan L EK***

SUMMARY .—Diet of adult Cattle Egrets Bubulcus ibisin a new North African colony (Soummam, Kaby-lie, Algeria): taxonomic composition and seasonal variability. We examined 100 pellets regurgitated by adultCattle Egrets Bubulcus ibisand collected monthly between May 1996 and February 1997 in an area ofSoummam Kabylie, Algeria. We analyzed some 11000 fragments and identified 3493 individual prey animalsof 73 species or genera, 35 families and 12 orders. The prey biomass per pellet was on average 62.25 g offresh weight, which corresponds to mean daily consumption. Insects were the most common prey (99.0 % innumber and 73.3 % in biomass). Orthoptera were the main insect prey (63.2% in numbers and 53.6% in bio-mass). Vertebrates (Rodentia, Muridae) accounted for only 0.8% of prey items and 26.2% of the total biomass.The diet was highly variable between seasons, both quantitatively and qualitatively. The prey diversity(Shannon index) was inversely related to its biomass. During the year cycle, the species showed a high adap-tability and ecological plasticity in a newly colonized area. To our knowledge, this is the first diet study ofadult Cattle Egrets based only on the examination of pellets.

Key words: Algeria, biodiversity, Bubulcus ibis, Cattle Egret, diet, Soummam Kabylie.

RESUMEN.—Dieta de adultos de Garcilla BueyeraBubulcus ibis en una nueva colonia del norte deÁfrica (Soummam, Cabilia, Argelia): composición taxonómica y variabilidad estacional. Examinamos 100egagrópilas regurgitadas por adultos de Garcilla Bueyera Bubulcus ibis recogidas en muestreos mensuales en-tre mayo de 1996 y febrero de 1997 en el área de Soummam Kabylie, Argelia. Analizamos unos 11000 frag-mentos e identificamos 3493 presas individuales de 73 especies o géneros, 35 familias y 12 órdenes. La bio-masa de las presas por egagrópila fue de media 62,25 g de peso fresco, correspondiente al consumo mediodiario. La presa más común fueron los insectos (99,0% en número y 73,3% en biomasa) y, dentro de ellos, elorden Orthoptera fue la principal presa (63,2% en número y 53,6% en biomasa). Los vertebrados (Rodentia,Muridae) supusieron únicamente el 0,8% de las presas y el 26,2% de la biomasa total. La dieta fue muy va-riable entre estaciones, cualitativa y cuantitativamente. La diversidad de presas (índice de Shannon) estuvo in-versamente relacionada con la biomasa. Durante el ciclo anual, la especie mostró una gran adaptación y plas-ticidad ecológica en un área recién colonizada. Que sepamos, este es el primer estudio de la dieta de adultosde Garcilla Bueyera basado únicamente en el examen de egagrópilas.

Palabras clave: Argelia, biodiversidad, Bubulculus ibis, dieta, Garcilla Bueyera, Soummam Kabylie.

* Laboratoire d’Ecologie et Environnement. Université de Béjaia. 06000 Algérie.** Station Biologique de la Tour du Valat. Le Sambuc, 13200, Arles, France.*** CESAC, UMR C5576, CNRS-Université Paul Sabatier. 118 route de Narbonne, 31062 Toulouse ce-

dex 4, France.**** Laboratoire de Zoologie. Ecole Nationale Supérieure d’Agronomie, 13200. El Harrach, Alger, Al-

gérie.

1973 in Florida, USA; Bredin, 1983 in the Ca-margue, France). Other authors, however, stres-sed the importance of Amphibia (Anura) in thediet (e.g. in Spain, Gil, 1945, Ruiz & Jover,1981, Ruiz, 1985; in Florida, USA, Jenni, 1969,1973; and in the Camargue, Hafner, 1977). Re-ports of feeding on fish, reptiles, birds andmammals are rare (Duxbury, 1963; Cunning-ham, 1965; Herrera, 1974; Taylor, 1979). InAlgeria, the Cattle Egret is mainly insectivo-rous (Doumandji et al., 1992, 1993; Fellag,1995; Bentamer, 1998). All of these studiesagree that the Cattle Egret is an opportunisticpredator feeding on any abundant and accessi-ble prey (Kushlan & Hafner, 2000).

This study was conducted in the SoummamKabylie region where the species has bred since1993 (Si Bachir et al.,2000). It also provides in-formation regarding feeding strategies in a newlycolonized environment from May 1996 to Fe-bruary 1997. Here we report on the taxonomiccomposition and monthly variation of the diet ofadult Cattle Egrets, based on the examination ofregurgitated pellets. According to Bredin (1983),adult Cattle Egrets produce a single pellet per 24h. Analysis of two trophic parameters (the Shan-non diversity index and the biomass of the preyingested) improved our knowledge on prey choi-ce over the whole biological cycle (breeding,postbreeding, wintering).

AREA OF STUDY AND METHODS

Around the fifteenth of each month, betweenMay 1996 and February 1997, we collected aminimum of ten regurgitated fresh pellets underthe perches of the nesting colony in the Soum-mam Valley (Béjaia, 36°43′ N, 05°04′ E). Thissite, near the village of El Kseur (20 km westof Béjaia), is also used for roosting outside thebreeding season. Grassland and fallow land,low crops, young orchards and rubbish dumpsare the main feeding localities in the area.

To ensure that whole fresh pellets were ob-tained, the site was cleared the day before pelletcollection. Adult pellets were recognized onthe basis of their stocky shape and slender tipon one side. The breadth, length and dry weightof the pellets were measured, before they wereuntangled in Petri dishes containing a smallquantity of water. Approximately 11000 frag-ments (cuticles, hair, bones, etc.) were collected

and identified under a binocular microscopewith the help of the collections in the ZoologyDepartment of the El Harrach National Collegeof Agronomy (Alger). In most cases, prey iden-tification was possible to species level. Thecost in terms of time and expertise of pelletstudies is rather high. The advantage, howe-ver, is valuable information on adult diet wi-thout having to kill birds.

For each prey category, the frequency of oc-currence is given as the proportion of pellets inwhich it appears. The frequency in numbersand in biomass of each prey taxon was calcula-ted from the number and the total biomass ofprey identified. The fresh mass of prey was es-timated from the mean wet weight of a freshsample of a minimum of 50 intact individualsof each group of prey captured in the area.Monthly variation in food intake was estimatedusing the Shannon diversity index as:

H′ = Σ (Ni/N) log

2(N

i/N),

where N represents the total number of indivi-duals and N

ithe number of individuals of spe-

cies i (Magurran, 1988). The relationship bet-ween the Shannon index (diversity) and theaverage prey biomass per pellet (grams of freshweight) were established by Pearson correla-tion test. The monthly variations of these twoparameters were established by the LOWESS

(Locally Weighted Regression ScatterplotSmoothing) non-parametric regression method,which allows to investigate relationships thatmay not be obvious using standard methods(Cleveland, 1979; Trexler & Travis, 1993;Dauba et al., 1997).

RESULTS

Characteristics of the pellets

The pellets contained a maximum of 79 preyand a minimum of 10 prey for an average of35.26 ± 10.32 prey per pellet (mean ± SD).They measured on average 30.4 ± 7.13 mm inlength, 22.49 ± 4.37 mm in breadth and 2.51 ±0.21 g in dry weight.

Some pellets contained whole fragments, suchas olive stones, pepper seeds and various other species of wild or cultivated plants andsmall pieces of wood. These various fragments

218 SI BACHIR, A, HAFNER, H., TOURENQ, J.-N., DOUMANDJI, S. & LEK, S.

Ardeola48(2), 2001, 217-223

probably were part of the stomach contents ofthe rodents eaten by the birds, or derived fromaccidental ingestion by the bird when feeding.The prey biomass per pellet reached on average62.25 g of fresh weight.

General dietary spectrum

Examination of the 100 pellets collected ledto the identification of 3493 animal prey of 73species or genera, 35 families and 12 orders(Table 1). Egrets consumed a wide range ofprey, both invertebrate and vertebrate. Verte-brates, however, occurred in low numbers(0.8%) and were represented by three speciesof Muridae: Mus musculus, M. spretusand Rat-tus norvegicus.

The invertebrate prey, composed entirely ofarthropods, were dominated by Insecta whichalone accounted for 99.0% of the number of

prey consumed. Arachnids represented only0.2%. Among the insects, Orthoptera (63.2%),Coleoptera (19.3%), Dermaptera (9.4%) andHymenoptera (5.2%) were the most frequentprey. Dictyoptera (1.8%), and Lepidoptera andHeteroptera (0.1% together), were only weaklyrepresented. In terms of biomass, it was alsoOrthoptera (53.6%) the group which accountedfor the majority of the Egret’s diet. Mammals(Muridae) occupied the second position butmade up only 26.2% of the consumed biomass.Coleoptera (14.2%) and Dermaptera (4.2%)also contributed significantly to biomass.

The frequency of occurrence was highest forOrthoptera (87.0% for Acrididae and 46.0%for Gryllidae) and for Coleoptera (76.0% forCarabidae and 33.0% for Scarabeidae). Der-maptera appeared in 38.0% of the pellets analy-sed. Mammals, although numerically not wellrepresented, showed a high frequency of occu-rrence (29.0%).

DIET OF ADULT CATTLE EGRETS BUBULCUS IBISIN A NEW NORTH AFRICAN COLONY 219

Ardeola48(2), 2001, 217-223

FIG. 1.—Monthly variation of the dietary spectrum of the Cattle Egret (May 1996-February 1997). Uppergraph: numbers; lower graph: biomass. Closed bars: Orthoptera; pointed bars: Coleoptera; open bars: Ro-dentia; striped bars: Other Arthropoda.[Variación mensual de la dieta de la Garcilla Bueyera (Mayo 1996-Febrero 1997). Gráfico superior: númerode presas; gráfico inferior: biomasa. Barras rellenas: ortópteros; barras punteadas: coleópteros; barras va-cías: roedores, barras rayadas: otros artrópodos.]

220 SI BACHIR, A, HAFNER, H., TOURENQ, J.-N., DOUMANDJI, S. & LEK, S.

TABLE 1

Taxonomic composition of the diet of the Cattle Egret in Algeria (contents of 100 pellets, +: value lower than 0.1).[Composición taxonómica de la dieta de la Garcilla Bueyera en Argelia (contenido de las egagrópilas; +: va-lores inferiores a 0,1.]

Frequency Proportion Frequency Maximum number ofPrey taxon of occurrence of biomass in numbers objects in one pellet

[Taxón de las presas] [Frecuencia [Proporción [Frecuencia [Número máximode aparición] de biomasa] en número] de presas en una

(%) (%) (%) egagrópila]

InsectaDictyoptera 1.3 1.8

Mantidae 5.0 1.8 11Orthoptera 53.6 63.2

Gryllidae 46.0 9.2 34Gryllotalpidae 3.0 0.1 1Tettigonidae 23.0 1.7 7Acrididae 87.0 51.9 79Pamphagidae 3.0 0.3 5

Dermaptera 4.2 9.4Forficulidae 5.0 0.4 2Labiduridae 38.0 9.0 70

Heteroptera + +Lygeidae 1.0 + 1

Lepidoptera 1.0 + + 1Coleoptera 14.2 19.3

Carabidae 76.0 9.2 23Cicindelidae 1.0 + 1Scarabeidae 33.0 3.5 17Cetonidae 1.0 + 1Elateridae 10.0 0.4 2Chrysomelidae 3.0 0.1 1Curculionidae 5.0 0.4 3Scolytidae 2.0 + 1Silphidae 14.0 1.0 6Dermestidae 5.0 0.2 2Carabidae 2.0 + 1Cerambicidae 2.0 + 1Dytiscidae 3.0 0.1 1Geotrupidae 5.0 0.2 2Harpalidae 26.0 1.4 6Licinidae 1.0 + 1Staphylinidae 24.0 1.4 8Indeterminate 33.0 0.9 5

Hymenoptera 0.3 5.2Formicidae 31.0 5.2 43Apidae 2.1 + 1

Arachnida 0.1 0.2Araneida 1.0 + 1Scorpionida 1.0 + 1Solifugida 2.0 + 1Phalangida 2.0 + 1

MammaliaRodentia 26.2

Muridae 29.0 0.8 1

The maximum number of individuals of thesame taxon found in one pellet was noted forAcrididae with 79 specimens, and for Labidu-ridae with 70. A maximum of 43 individualswas recorded for Formicidae, 34 for Gryllidaeand 23 for Carabidae (Table 1).

Monthly variation of the dietary spectrum

The composition of the Egret’s diet showedwide monthly variations in both numericalcomposition (number and diversity) and in pro-portions of biomass. Orthoptera were the mostcommon prey throughout the study period, withfrequencies ranging from 5.1% in May to91.0% in September. Coleoptera dominatedonly in May (82.9%) and in February (60.7%).Rodents showed their highest frequency (2.1%)in February (Fig. 1).

Although rodents had a high biomass, theywere relatively less important than insects be-cause of their lower numbers. Over the sevenmonths from June to December, Orthopteradominated with a biomass of 44.8 to 60.7%. InJanuary and February, rodents accounted for58.4% and 35.6% of the total biomass, whilebeetles were dominant with 67.9% in May(Fig. 1).

The relationship between the Shannon di-versity index and biomass presented a signifi-cant negative linear correlation (r = –0.716, P <0.05). Applied to the monthly variations of di-versity and biomass, LOWESSadjustment alsoshowed that the inverse relationship betweendiversity and biomass occurred over the Egret’swhole cycle. During May and February, theShannon diversity index (H′) gave maximumvalues of 4.26 and 4.54. This index, which un-derwent rather strong fluctuations, generallyshowed low values between September and Ja-nuary, when large amounts of biomass wereingested (Fig. 2).

DISCUSSION

Most of the studies carried out to date showthat the diet of the Cattle Egret is mainly com-posed of insects. This is also the case in theSoummam Kabylie area, where 99% of theprey were insects. Siegfried (1971) calculatedthat about 80% of the diet was composed ofinsects in South Africa. Fogarty & Hetrick

(1973) found 90% Insecta in Florida (USA),and Ruiz & Jover (1981) reported 68.4% in theEbro Delta (Spain). In Algeria, Doumandji etal. (1992, 1993) indicated 99.8% and 96.8%composition at Draa El Mizan and at Chlef,respectively; Fellag (1995), Bentamer (1998)and Boukhamza (2000, 2001) noted over95.0% in Sebaou Kabylie.

Moreover, there is general agreement thatOrthoptera make up the most important orderin the dietary spectrum of the Cattle Egret:63.2% in the present study, 79.8% in Egypt(Kadry-Bey, 1942), 77.0% in Louisiana, USA(Burns & Chapin, 1969), 96.8% in Florida,USA (Fogarty & Hetrick, 1973), 78.8% at DraaEl Mizan (Doumandji et al.,1992).

Consumption of Arachnids was very low inour study area (0.2%), whereas they made up asignificant proportion in several other regions:over 4% in South Africa (Siegfried, 1971),10.02% in the Ebro Delta, Spain (Ruiz & Jover,1981) and 12.2% in Veracruz, Mexico (Torres& Mayaudon, 1972).

Although rodents were regularly includedamong the vertebrates consumed in SoummamKabylie, reptiles and amphibians were absent.

DIET OF ADULT CATTLE EGRETS BUBULCUS IBISIN A NEW NORTH AFRICAN COLONY 221

Ardeola48(2), 2001, 217-223

FIG. 2.—Variation over the yearly cycle of the Shan-non diversity index (open squares, pointed line) andthe biomass (closed circles, line) of the prey consu-med by the Cattle Egret in Algeria, showing the ad-justment curves obtained by the LOWESS non-para-metric regression method. [Variación en el ciclo anual del índice de diversidadde Shannon (cuadrados vacíos, línea punteada) y dela biomasa (círculos rellenos, línea continua) de laspresas consumidas por la Garcilla Bueyera en Arge-lia. Se muestra el ajuste de las curvas obtenidas porel método LOWESS de regresión no paramétrica.]

Coleoptera and Dermaptera, however, consti-tuted a notable component of the diet (19.2 and9.4%, respectively). It should be noted that thepreponderance of insects in the diet of the Cat-tle Egret in Soummam Kabylie reflects notonly numbers but also biomass. In the reportsquoted above, the most important biomass in-put came from vertebrate prey. The daily con-sumption biomass (62.25 g of fresh weight perindividual) is almost the same as that describedby Siegfried (1969, he mentions 68 g) in SouthAfrica.

Cattle Egrets show a high degree of ecologi-cal adaptability. They develop feeding strate-gies adapted to changing ecological conditions,as is shown by the monthly (seasonal) variationin diet. This variability corresponds to seasonalclimatic changes, which influence food availa-bility.

The biomass of the prey consumed fromMay to August, the breeding season, was lowerthan during other months, whereas the diet di-versity was higher. This may be because duringthe summer period the nesting Egrets find suf-ficient quantities of varied food near the co-lony. The preference given to Coleoptera inMay both in terms of numbers (82.9%) and interms of biomass (67.9%) suggests that the nes-ting birds feed Orthoptera to the young, andtake their own food from among a large va-riety of other prey (Shannon Index = 4.25) witha low biomass, in particular Coleoptera (Fig. 1and 2). Between September and January bio-mass was higher, although the diversity of preydiminished (Fig. 2). With the drop in tempera-ture and the first autumn rains, the Cattle Egretsdispersed to the coastal plain, and began to useother habitats. The birds may have fed on largerprey because of the greater flight distances andthe decreased availability of invertebrates. Theresults in February are rather particular. Du-ring this month, rainfall was exceptionally lowfor the area (8 mm) and temperatures rose to anaverage of 15 °C. The lack of Orthoptera,mostly in hibernation, may have caused theCattle Egrets to choose alternative prey, inclu-ding Muridae and large Carabidae (Carabusmorbillosus).

In full breeding season (May-August), thebiomass of prey consumed by adults was lowand the prey diversity was high. The latter de-creased after the breeding season, whereas preybiomass increased. In November, when the

winter migrants joined the local population, thebirds dispersed widely when foraging. Then,they took a greater variety of prey, with a rela-tively high biomass (Fig. 1, 2).

The diet of the Cattle Egret in general is sub-ject to considerable variation, not only from oneregion to another but seasonally within a singlearea (this study). This variability appears to re-sult from the availability of sufficient quanti-ties of prey in space and time (Siegfried, 1971;Bredin, 1984 ; Doumandji et al., 1992). In thePetite Soummam, when the prey biomass in-creased in autum, small prey was uncommon.Therefore, the question of prey selection outsidethe breeding season remains open and providesopportunity for future research.

ACKNOWLEDGMENTS.—This study has been pos-sible thanks to assistance from the Laboratory ofEcology and Environment, Université AbderrahmaneMira (Béjaia, Algérie); Biological Station Tour duValat (France) and Laboratory of Hydrobiology, Uni-versity Paul Sabatier (Toulouse, France), and the co-llaboration of colleagues and students, too numerousto be listed here, but all of whom we remember withgratitude. We thank Robert Bennetts for useful com-ments on a draft of this paper.

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[Recibido: 9-2-01][Aceptado: 4-7-01]

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