Consumption Rates of Olives by Choughs in Central Spain: Variations and Importance

.]. Field Ornithol., 65(4):482-489

CONSUMPTION RATES OF OLIVES BY CHOUGHS IN CENTRAL SPAIN: VARIATIONS AND IMPORTANCE

GUILLERMOBLANCO, JUAN A. FARGALLOAND JESf;S A. CUEVAS Departamentode Biolo•a Animal Universidadde Alcal• de HenaresAlcald de Henares.28871 Madrid, Spain

Abstract.--Oliveshave been recorded as food of many speciesof corvidsand other birds. The winter energetic importance and the seasonalvariation of the consumptionof olivesby Choughs(Pyrrhocorax pyrrhocorax) in an area of centralSpain were studied.Choughs roosted in a large vertical crevicefrom which olive stoneswere collected to estimate an index of monthly consumption.Choughs fed on olivesyear round with a major peak in winter. In mid-Decemberan estimatedminimum of 13% of the daily energetic demand for each Choughwas supplied by olives.Results are discussedin the contextof the availabilityof olives and the role of olivesas a valuableresource for the winter survivalof Choughs.

CONSUMO DE ACEITUNAS POR PARTE DE PYRRttOCORAX PYRRHOCORAX EN ESPAI•A CENTRAL: VARIACIONES E IMPORTANCIA Sinopsis.--Lasaceitunas forman parte de la dieta de muchas especiesde avesincluyendo c6rvidos.La variaci0nestacional en el consumode aceitunaspor parte de la Chovapiquirroja (Pyrrhocoraxpyrrhocorax) y su importancia energ6tica invernal se estudiaronen el surestede Madrid, Espafia.Las chovasutilizaron como dormidero una grieta verticalsituada en corm- dos fluviales,en cuyointerior se recogieron,regularmente, las semiliesde aceitunaregur- gitadasdurante la noche pare estimar un indice de consumomensual. Las chovasconsu- mieron aceitunasdurante todo el afio, especialmentedurante el invierno. A mediadosde diciembre se estim6 que un mfnimo del 13% de los requerimientosenerg6ticos dierios de cada ave fueron proporcionadospor las aceitunas.Se discutenlos resultadosen el contexto de la disponibilidadde aceitunasy su papel como valiosorecurso pare la supervivencia invernal.

Olive orchards are one of the most extensive cultivations in the Iberian peninsula;with severalhundred million treesthey constitutea major proportion of the woodedarea of Spain (Mufioz-Cobo1987). Although olive crop production variesamong yearsdepending on weather,a large quantity of olivesis alwaysavailable to be eaten by birds every winter (Mufioz- Cobo 1987, Rodriguezde los Santoset el. 1986). The importanceof olive orchards in maintaining a rich, winter avifauna has been repeatedly em- phasized (Jordano 1985, Mufioz-Cobo and Purroy 1980, Santos and Tel- lerie 1985, Suarez and Mufioz-Cobo 1984), and the accessibility,abundance and nutritive value of olives allow the existence of high winter densitiesof frugivorous birds. Olives form an important fraction of the diet of many species(Mufioz-Cobo 1987, Tutman 1969), with somespe- cies feeding almost exclusivelyon them (Bernis 1960; Manzanares 1983; Soler et el. 1986, 1988; Tejero et el. 1983). The adaptable and opportunistic feeding behavior of crows makes it possiblefor these birds to utilize a wide variety of food (Goodwin 1986) including fruit (Snow and Snow 1988). Despite this, corvids are not es- peciallyabundant in southernSpanish olive groves(Mufioz-Cobo 1987, Suarez and Mufioz-Cobo 1984). Nevertheless, olives have been recorded as a small part of the diet of Garrulus glandarius, Cyanopicacyaneus, Pica

482 Vol.65, •o. 4 Consumptionof Olives Oy Choughs [483 pica, Corvusmonedula, Corvus corone and Corvuscorax (Alvarez and Agui- lera 1988, Soler et al. 1990, Soler and Soler 1991, Tutman 1969). Unlike any other Corvidae, the Chough (Pyrrhocoraxpyrrhocorax) is compara- tively rare and patchily distributed in Europe (Bignal and Curtis 1988). In someSpanish localities Choughs coincide with olive cultivarsbut olives have never been mentioned as food of this species. In this paper, we report on the exploitation of olives by Choughs in central Spain. We analyzeseasonal variations in consumptionrates in re- lation to fruit availabilityand discussthe importance of the energeticrole played by olivesduring winter.

MATERLA_L AND METHODS BetweenAugust 1990 and August 1991, we examined the variation in the consumptionrates of cultivated olives (fruits of the olive tree Olea europaeavar europaea)by Choughs in an area to the southeastof Madrid, central Spain. The study area and the Chough population have been describedelsewhere (Blanco et al. 1991). Cultivation of olivesin the study area is predominantly in small patchily distributed orchards.Fruiting phenologyand availabilityof fruit wasmeasured on 39 olive treescorrespond- ing to two medium sizedolive grovesin which the cropsare not harvested. In the exploited orchardsthere were ripe olivesfrom November to April, and the crop washarvested in December (for general detailsof fruiting and availabilitytimes in harvestedgroves see Mufioz-Cobo1987). Each tree in the sample was visited monthly and the state of maturity of the fruit recorded accordingto a five-pointscale: (0) no fruit, (1) all unripe fruit, (2) 25% ripe fruit, (3) 50% ripe fruit, (4) 75% ripe fruit and (5) all ripe fruit. Mean percentagesof monthly ripening stateswere calculated, suchthat 100% indicatedthat all the treeshad only ripe fruit. Average monthly availabilitywas checked by counting the fruit (only ripe fruit) beneath each tree, recording abundance in the following categories:(0) no fruit, (1) 1-5 fruit, (2) 6-15 fruit and (3) •15 fruit. Consumption rates of olives by Choughs were assessedby collecting olive stones (en- docarp and kernel, "seeds"hereafter) falling on a 1.5 m• sheet of plastic placed inside a vertical crevice used as a roosting site. The plastic sheet was alwaysplaced in the samelocation once or twice every month for 5- 6 d. The lack of a significantdifference (X• = 6.7 P • 0.10 df = 4) in the number of seeds collected on the plastic or on other similar-sized sheetsplaced side by side and 2 m away,indicated the lack of any important changesin the Chough roosting positions.We carried out counts of Choughsattending the roost during the periods in which the plasticwas in position. A simple consumption index (CI) was used to compare the number of regurgitatedseeds by Choughs: NS/ID CI- x 100, NCH where NS is the number of seedson the plastic sheet, ID is the number of interval daysand NCH is the averagenumber of Choughs in the roost 484] G. Blancoet al. J.Field Ornithol. Autumn 1994

a .o b

% eo loot 40

o N D J F M A M J J A 8 O N D J F M A M J J A 8 O Month Month

FIGURE1. Mean percentageof monthly ripening statefor the sampledtrees (a) and average monthly availabilityof olives (b), expressedas the sum of scoresfor fruit abundance beneath the sampledtrees (seeMethods).

during each samplingperiod. During one night in July and August 1991, a 17m-longplastic sheet was also positioned to coverall the crevicefloor area. Lack of any difference in the number of seedscollected on the two plasticsheets on thesenights (X'• = 0.02 P = 0.90 df = 1) further supports the assumptionof no important changesin the roostingpositions. Daily energetic requirements (DER) of Choughs were calculated accordingto the equationsproposed by Kendeighet al. (1977), and were estimatedusing the mean massof 67 Choughscaptured with nets at the roosting sitesin this area. Bromatologiccomposition and energy-efficien- cy data of ripe oliveswere taken from Mufioz-Cobo (1987) and Herrera (1983, 1987).

RESULTS Fruiting phenologyand availability offruits.mOlive trees flower in May and bear green unripe fruits from June to November,thereafter acquiring mature black coloration. Mean percentage of monthly ripening is shown in Figure la. Olives ripen graduallyfrom November onwards.The peak period for treesbearing ripe fruits is in January;the number of treeswith ripe fruits begins to decreasefrom February onwards as the ripe fruit begins to fall. In April-May a very small number of trees containing des- iccated oliveswas observed,and by June not a single ripe olive could be found on the trees. Olive availabilityon the ground extends throughout the year with a peak in March (Fig. lb). The quantity of availablefruit decreasesfrom April onwards,reaching the lowestvalues in October,just before a new crop is produced. If it is assumedthat Choughs only ingest oliveswhen they fall to the ground, then Figure lb showsthe seasonalvariation in olive availabilityto the birds. The difference in the availabilityperiod between groveswas highly significant (X2 = 66.0 P < 0.001 df = 11) when considering the number of trees with some olives beneath, and not significant (X'• = 6.8 P > 0.30 df = 7) when taking into account the trees below which there was a lot of fruit (>15). Fruit abundance reached remarkably high levelsin both olive groves,and there were no significant Vol.65, No. 4 Consumptionof Olives by Choughs [485

c o 25 N S U 2O M P T 15 I O N 10

I N D 5 E X 0 i i i i i i i [ i i i i i i i • i i A $ O N D D J J F F M A M J J A A A I 2 1 2 I 2 1 2 3 Month

FIGURE2. Consumptionindex of olivesby Choughsin Central Spain.Numbers below the monthsindicate consecutivesamplings in thosemonths. April valueswere not estimated (see text). differencesin the fruit supplyfor each site (Mann-Whitney /5test U = 15.5 P > 0.05 n = 14). Consumptionrates and regurgitationpatterns.mChoughs ingest olives throughout the year. Consumption-index(CI) variation (Fig. 2) showsa strong peak halfwaythrough December,quickly increasingand decreas- ing just before and afterwards.CI showslowest values in spring and summer, except for Augustwhen a high and similar peak occurred in both years.We lack CI scoresin September 1991, but CI in October gave similar values(4.08 and 4.37) in both years.In April, the roostwas occupied by only 2-3 breeding pairs of Choughs,but these too consumedolives. In this month, however,we did not estimate the consumption index be- causebreeding Choughsusually roost near their nestsites, and therefore, roostingpositions were not comparable.For the rest of the year the number of Choughsattending the roostranged from 100 to 367. The consumptionindex, grouping samplesinto the months except for April, was not significantlycorrelated with fruit availabilityin the olive grovesstudied (r, = 0.21 P > 0.05 n = 11). This lack of correlation was due to a very different spatio-temporalavailability among unexploited and harvested groves (see methods), the latter being those where Choughsmainly forage (pets. obs.). Olive seed regurgitation occurred both in a simple, individual-seed manner and together with other indigestibleremains grouped in pellets. The majority of seeds(n = 2018) were regurgitated in the first manner; 486] G. Blancoet al. J.Field Ornithol. Autumn 1994

although pellets were scarcethey sometimescontained up to five seeds. The number of seedsregurgitated per Chough each night at the roost rangedfrom 0.06 to 1.87 (Juneto December,respectively). Energeticimportance.--DER estimation for Choughs in winter (photo- period: 10:14 L:D; ambient temperature: 0 C) gave a value of 90 Kcal/ bird-day.The number of olivesrequired to meet DER on a diet consisting only of olivesin winter wasapproximately 15. The energy supplied by the olives calculatedfrom the number of seedsregurgitated per Chough at roost ranged between 0.37 Kcal (June) and 11.4 (December). The percentageof DER suppliedby olivesin winter ranged from 3.3 to 12.7.

DISCUSSION Data from previousstudies (Cowdy 1973, Holyoak 1967, Meyer 1990, Roberts 1982) suggestthat although the Chough is a specialistinverte- brate feeder,vegetable matter, especiallyweed seed and cereal grain, occurs in the diet at certain times of the year (Goodwin 1986, McCracken et al. 1992, Warnes and Stroud 1988). The presenceof fruit as part of the Chough'sdiet is rarely mentioned in the literature, and would therefore suggestthat it is of little importance to the birds. Observationsby Spanishauthors (Garcia-Dory1983, Martin and Cardona 1989) together with those of our own, however,show that Choughs can consumethe fruit of at least 20 speciesof trees and shrubs.This study demonstrates that Choughsin central Spain feed on olivesyear round with a major peak in winter. In the two groveswe sampled,the different fruiting phenologyallowed the existence of a lengthy period of fruit availability.In both these orchards, however, the olives were not harvested, and our field observations suggestthat Choughsforage mainly in the few large exploited groves, where the fruit is a very punctualyet patchilydistributed resource because of the time and methods of harvesting.The quantity of ingested olives was in agreement with the peak in availabilityon the ground (mid-De- cember) for harvestedolive groves (see also Mufioz-Cobo 1987). In ad- dition, Choughs consumedolives even in spring-summer,when our studies show that the fruit is very scanton the ground and only present in a desiccatedstate. Olives do not occur on the ground in harvestedgroves except in August,when a natural fruit-fall occurs,and this is reflected in a high CI for this month in both years. Furthermore, a large (though unquantified) number of olive seeds has been observed inside and around Chough nestingcavities during severalyears of studyingbreeding Choughs.The presenceof such seedsin nestssuggests to us that olives perhaps also form a small part of nestlingsdiet. One explanation for the fact that Choughs consumeolives during the long periods when fruit is unavailablein harvestedgroves, could be that the birds hoard olivesduring the winter months. Alternatively,Choughs could occasionallyforage in the small, non-harvestedplots. Food caching has been extensivelyre- ported in corvids (Vander Wall 1990) and the Chough is no exception (Holyoak 1972, Turner 1959). Vol.6•5, No. 4 ConsumptionofOlives by Choughs [487

At dawn during winter, Choughsfly directlyevery day from the roosting sitesto olive groves.Probably the greatestpart of the olive ingestion-seed regurgitation processoccurs during the morning becausefood passes through the digestivesystem in 90-120 min (Bullock 1980) and this tran- sit is alwaysshorter than the interval for seedsthat passthrough the bird by regurgitation (Herrera 1981, 1984). Our data have revealed that by eating olives,Choughs obtain an important fraction of the DER in winter; in mid-December almost 13% of daily energetic demands for each Chough attending the roost (mean December countsof 284 Choughs) were supplied by olives. These results could be representativeof olive consumption by the entire population in southeasternMadrid (about 1000 individuals) becausewe have found great numbers of olive seedsin all the communal roosting sites in this area. These figures are almost certainly underestimated, however,because: (1) the percentage of DER is calculated only from seedsregurgitated at night, and the seedscorre- spondingto oliveseaten during the morning would be regurgitatedalso in the morning (see above); (2) a proportion of seedsremains on the crevice walls, and many are removed by rodents (unpubl. data); (3) Choughsseem to select the largestolives (pers. obs.) and (4) Choughs possiblyalso fed on abortive, seedlessfruit. Assumingthat pellets regurgitated at night are the indigestibleremains ingested during the day,some Choughsmight obtain up to 34% of DER from olives. As is the casefor many bird specieswintering in olive groves(Herrera 1983), feeding on olives could play an important role by increasing sur- vival rates of Choughsduring winter in the whole population of south- easternMadrid. Juvenile Choughsmay gain experience from older birds about this valuable, easy-to-locateand abundant food resource,so reduc- ing mortality due to foraging inexperience (Bignal et al. 1988, Roberts 1985); in other studies,winter mortality has been partially attributed to the Chough's specializedfeeding requirements (Bullock et al. 1983, Hol- yoak 1971). By foragingin flocks,Choughs could profit from the better location and exploitation of this patchily distributed,seasonally abundant and nutritive resource.In fact, the function of communal roosting aggre- gationshas been interpreted as information centers,where daily activity is organized to take maximum advantageof food resources(Blanco et al. 1994; Still et al. 1987).

ACKNOWLEDGMENTS

We thank FernandoG6mez for help with the field work, and Eric Bignal,JaimePotti, Davy McCrackenand an anonymousreferee for commentson the manuscriptand for correcting the English.We alsothank Aurelio Capilia for advisingus on the olive tree cycle.

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