J Raptor Res. 28(4):242-245 ¸ 1994 The Raptor ResearchFoundation, Inc.

SEASONAL ABUNDANCE OF BLACK KITES ASSOCIATED WITH THE RUBBISH DUMP OF MADRID, SPAIN

GUILLERMO BLANCO Departamentode BiologfaAnimal, Universidadde Alcaldde Henares, Alcaldde Henares,28877 Madrid, Spain

ABSTI•CT.--This study describesthe seasonalvariations in number of black kites ( migrans) foragingat the rubbishdump of Madrid, Spain. Seasonalchanges in the abundanceof kites usingthe dump showedtwo peakscorresponding to both pre- and post-breedingmigratory passages.The autumn passagewas more lengthy and producedthe highestnumber of kites (847). During the breedingseason the populationof kitesforaging in the dump showedno seasonaltrends, suggesting that it consistedmainly of nonbreedingbirds, both adults and subadults.Age-related differences were observedin the use of the dumpby blackkites during post-breeding which may simplyreflect the existenceof age-relateddifferences in migration.A possibleadvantage for kitesto foragein the dump is accessto an abundantand predictable food supply. On the other hand, kites may facesome dangers from entanglementin plasticmaterials and other rubbish in the dump. KEY WORDS: blackkite; Madrid; Milvus migrans;rubbish dump; seasonal abundance; Spain.

Abundanciaestacional de Milanos Negros(Milvus migrans) asociados al basurerode Madrid, Espafia RESUMEN.--Esteestudio describe las variacionesnfimgricas estacionales de Milanos Negros (Milvus migrans)en relaci6na superiodo de permanencia y paso migratorio sobre el basurerode Madrid, Espafia. Los cambiosestacionales en la abundanciade milanosreflejaron dos mlximos correspondientesa los pasosmigratorios. E1 pasomigratorio postreproductor fue milsprolongado yen el sealcanz6 el nfimero mas alto de milanos(847). Durante la gpocade crla la poblaci6nde milanosasociada al basurerono mostr6variaciones numgricas, sugiriendo que estuvocompuesta principalmente por individuosno repro- ductores,tanto adultoscomo subadultos.La proporci6nde cada clasede edad vari6 durante el paso postreproductoren el basurero,lo cual podrla estarrelacionado con diferenciasmigratorias entre las distintasclases de edad.Se discutenlas posiblesventajas y desventajasde alimentarseen basurerospara los Milanos Negros. [Traducci6n Autor]

Black kites (Milvus migrans)often gather in large Despite the widespreadrange and abundanceof communalroosts and breedingcolonies. Large ag- the black (Brown and Amadon 1968), limited gregationsalso occurwhere food is abundantand information is available concerningthe proportion during migration (Bernis 1980, Cramp and Sim- of age classesin residentgroups and in migration mons 1980). Their highly gregariousand opportu- (Bernis 1980, Cramp and Simmons 1980). Knowl- nlstic foraging behaviorleads them to eat the most edgeabout the agedistribution of kitesusing rubbish abundantand availableprey, especially slow-moving dumps, and the timing of residencyand migration and injured as well as food obtainedby associated with these sites is almost nonexistent scavenging(Valverde 1967, Arroyo 1978, Cramp (Pomeroy1975, G6mez-Tejedorand De Lope 1993). and Simmons1980, Shiraishi et al. 1990). As scav- The objectiveof this study was to determinethe engers,black kites have been recorded in large num- seasonal variations in numbers of black kites at the bers on waste accumulationsgenerated by human Madrid, Spain rubbish dump. I also presentdata activities,including rubbish dumps,markets, - concerningthe age distribution of black kites asso- ing-portsand abattoirsall over their world range ciatedwith the dump throughthe summer,and dis- (e.g., Bernis 1973, Pomeroy 1975, Roberts 1991, cussthe possibleadvantages and disadvantagesfor Shiraishi et al. 1990). black kites of foragingon refuse.

242 DECEMBER 1994 BLACK KITES IN THE MADRID DUMP 243

STUDY AREA AND METHODS NUMBER OF BLACK KITES 9OO The domesticrefuse generated by Madrid, a city of 800 about 4 000 000 people,is depositeddaily at the Valdem- ingomezmunicipal dump, about 12 km southeastof Ma- 700 drid. Several specieshave been recordedfeeding on 600 the more than 1 000 000 tons of refuse depositedthere annually (Cantos and Asensio1990). The major scaven- 500 gers recorded at this dump were white storks (Ciconia c•conia),black kites, black-backed gulls (Larusfuscus),black- 400

headedgulls (Larus ridibundus),magpies (Pica pica) and 300 - jackdaws (Corvusmonedula) numbering in the hundreds and thousandsof birdsin eachcase (G6mez and De Juana 200

1984, pers. obs.). 100 During 1993, I monitored the number of black kites using the Madrid dump by counting them when they 0 arrived at the only communalroost in the area. The com- FEB MAR APR MAY JUN JUL AGO SEP munal roost was situated in the bank vegetationof the Figure 1. Seasonalabundance of black kites foraging xn Manzanares River, about 4 km east of the dump. The adjacentarea was used for agriculture and cattle grazing the Madrid dump, 1993. (Blancoet al. 1991). Roosttrees were mainly large cot- tonwoods(Populus alba) and elms (Ulmus minor). migration rapidly increasedthe number of kites to About 2 hr beforesunset I beganobserving and counting the kites perchedon the roosttrees from about 50 m with nearly 300. Numbers were lowestduring the breed- binocularsand a 20-60x spottingscope. Later I noted ing season(March-July), and increasedfrom June the numberof kites enteringthe communalroost from the as the firstjuveniles joined the roost.The proportion dump. Observationsof arriving kiteswere conductedfrom of juveniles increasedto the end of July although a fixed vantagepoint locatedbetween the roost and the the highest proportion was reached later (Fig. 2) dump, and lasteduntil all the kites were settledon the trees. when the major influx of migrants occurred(Fig. When conditionsof light and time allowed, I attempted 1). The autumn migration was more lengthy and to age as many kites as possibleon the basisof the numeric increaseof kites was not always con- coloration,shape (Sylven 1977, Cramp and Simmons1980) stant,suggesting the arrival and/or departureof suc- and plumage featuresderived from molt. Black kites were consideredeither adult (definitiveplumage), juvenile (first cessivesurges of migrants.By recordingif blackkites summer), or immature (second summer). in their were molting, I was able to assign 31-86% (N = secondsummer were easily recognizedby remnants of 134-235) of the birds presenteach day to one of the juvenile plumage especiallyin covertsand scapulars,as two age classesdefined by this method.By the more well as by the coloration of old primaries. In addition, kxtesin their secondsummer molting their flight feathers preciseageing technique, I successfullyaged only for the first time, were easily recognizedin flight by their about 13% (N = 2-109). Adults were the highest shape--an irregular appearancebecause of the loss of proportion of the population throughout the period several flight feathers or irregular molt patterns. This in which the more precisemethod was used (Fig. clearly contrastedwith the synchronizedmolt pattern of 2). The percentageof adults increasedto 100% by adults.Because of the impossibilityof exactlyaging a large number of kites by these criteria, I focusedon recording the end of period when black kites were resident in whether they were molting (adults and immatures) or not the area. On the other hand, the sightingsof im- (juveniles),which was easily accomplishedat long dis- mature individualspeaked in early August and then tances.This partial ageing method was used from June, declinedsteadily (Fig. 2). Departure of nearly 850 when the first juveniles appearedin the roost,to the end kites occurredin the first week of August, and black of July, when the developmentstage of molt in adults makesit difficult to assigneach individual to an age class. kites disappearedentirely by the end of that month After the endof July I consideredonly kitesaged by means (Fig. 1). of plumage coloration,shape, and molting pattern. During the breeding seasonkites foraging at the dump showedno seasonaltrends (Fig. 1). Most kites RESULTS observedarriving from the dump usedthe communal Seasonalchanges in abundanceof kites usingthe roost,and breedingindividuals usually roosted close Madrid dumpclearly peaked in correspondencewith to their nestingsites. Hence, it is unlikely that the bothpre-breeding and post-breeding migration (Fig. presenceof active breedersin the roost may have 1). Black kites usually arrived on the study area beeninfluenced by the lack of seasonalvariations in from aboutthe first week of February. Spring the sizeof the populationduring the breedingseason. 244 GUILLERMO BLANCO VOL. 28, NO. 4

I Molting • Adults • Secondsummer • ] Juvendes dant sourcesof food in the surroundingarea. The use of the Madrid dump by black kites has been lOO known for severaldecades (Bernis 1973, pers. obs.) which suggeststhat the high predictabilityand abun- dance of food could have influenced the recruitment of birds from year to year. % 5O No previous studies attempted to examine sea- sonalage-dependent variations in the use of dumps by black kites. Black kitesin their first summerdid not constitutea major part of population foraging o 30-6 5-7 18-7 23-7 25-7 1-8 5-8 16-8 26-8 28-8 in the dump. This studyrevealed that timing of the DATE use of the dump by the different age classessimply reflectedage-related differences in the timing of mi- Figure 2. Percentagedistribution of blackkites by age gration. classesusing the Madrid dumpin summer.Molting kites include birds in their second summer and adults. By foragingin dumps,black kites could benefit from the availability and predictabilityof foodfound there (Pomeroy1975, Donfizar 1992). In contrast, This suggeststhat the communalroost contained severalnegative effects associated with the exploi- mainly nonbreedingbirds. Observationson the age tationof refuseby birdshave been pointed out (Ifiigo and behavior of kites using the communal roost 1987, Donfizar 1992). But, no electric power lines showedthat both breedingand nonbreeding(im- dangerousto black kites were in the vicinity of the maturesand nonbreedingbirds with definitiveplum- Madrid dump, and there was no indicationthat toxic age) foragedin the dump throughoutthe breeding materialswere ingestedby the kites. Five kites were season.However, it was impossibleto obtain a large observedwith piecesof rope, plasticbags, and pieces enoughsample of agedbirds to assessthe proportion of plastic (up to 40 cm long) entangledwith their of each age class. legs,and one immature was seenwith an empty can (approximateweight of 850 g) caughton a leg. The DISCUSSION effectsof theseand other potential dangersto kites The resultsof this studyindicate that black kites foragingin dumpsremains unknown. used the Madrid dump as a feeding station before Further research is needed to determine how much and during their migratorytravels. The increaseof of a trade-off occurs between the costs and benefits black kites foragingat refusedumps during migra- of feedingon refusefor kites. More studiesare also tion has been reportedat severalother Spanishlo- neededto understandthe long-terminfluence of for- calities(Sfinchez et al. 1990, Donfizar 1992, G6mez- aging in dumps on the population size and life- Tejedor and De Lope 1993), suggestingthat such historystrategies of the and other . sitescould act as stopoverand/or congregatingcen- ters for migrant blackkites throughout their migra- ACKNOWLEDGMENTS tory range in Spain. I am grateful to Mari C. Blanco and Mario Herrera The numberof blackkites using the Madrid dump for their companyduring some counts and to Jos6L. Tella, may be muchlarger than the maximumabundance Jos6A. Donfzar, FernandoHiraldo, and Jaime Potti for commentsand improvementson the manuscript. peak indicated.In fact, my results suggestedthat successivesurges of migrantsused the dump at dif- LITERATURE CITED ferent dates.Moreover, the highestcount in the Ma- drid dump in early August was larger than counts Amto¾o, B. 1978. La alimentaci6ndel Milano negro (Milvus migrans)en una localidadde Espafia Central. recordedat other dumpsin Spain. These numbers Ardeola 25:47-58. were only comparablewith roostingand migrating BERNIS,F. 1973. Algunosdatos de alimentaci6ny de- aggregationsof black kites passingsouthward over predacionde Falconiformesy Estrigiformesibgricas. the Strait of Gibraltar (Bernis 1980). Such a cir- Ardeola 19:225-247. cumstancecould be explainedby the superabundant 1980. La migraci6nde las avesen el Estrecho andpermanently available food supply found in Ma- de Gibraltar. Universidad Complutense de Madrid, drid dump, but especiallyby the lack of otherabun- Madrid, Spain. DECEMBER 1994 BLACK KITES IN THE MADRID DUMP 245

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