j. RaptorRes. 31 (1):71-76 ¸ 1997 The Raptor ResearchFoundation, Inc. ROLE OF REFUSE AS FOOD FOR MIGRANT, FLOATER AND BREEDING BLACK KITES ( MILVUS MIGRANS) GUILLERMO BItNet Departamentode Biologfa Animal, Universidadde Alcalti de Henares, Alcaltide Henares,E-28871 Madrid, Spain A•sxv•cx.---The use of refuse by breeding, floating, and migrating Black Kites (Milvus migrans)was studied near a large garbagedump in Madrid, Spain. Refusewas an important food resourcefor non- breeding Black Kites, especiallymigrants that fed almostexclusively at the garbagedump. The dump wasonly of secondaryand variableimportance for kites during the breeding season.Pairs of breeding kites nestingin nearbywooded areasforaged mainly on a wide varietyof wild prey and only sporadically ate refuseat the dump. Floatersroosted in nearbywooded areas but, unlike breedingpairs, they foraged mainly by scavengingat the dump. Exploitation of food resourcesother than refuseby breeding kites may be due to their need for large amountsof prey biomassfor brood provisioning.Perhaps dumps augmentpopulations of Black Kitesby providingrich foraging areaswhere large numbersof nonbreed- ing and migratory kites can scavengefor food. KEYWOP. DS: BlackKite, Milvus migrans;garbage dump; foraging;, refuse. Importancia de la basuraen la diem de Milanos Negros (Milvus migrans)migrantes, flomntes y reprod- uctores. R•SUMF.N.--La impormnciade la basuraen la diem de Milanos Negros (Milvus migrans)reproductores, migradorese integrantesde la poblacionflomnte fue estudiadaen lascercanias del basurerode Madrid, Espafia.La basurafue un recursoimpormnte para los milanos no reproductores,especialmente para Its migrantes,pert su papel fue secundarioaunque variablepara los milanosque se reprodujeron cerca del basurero.Los individuosflomntes forrajearon en el basureropert mmbien consumieronuna amplia variedad de presassalvajes. La abundancia de alimento en el basurero no provoc6 que los milanos nidificaran en suscercanias, ya que la parejasque asi lo hicieron consumieronbasura s61o espofftdica- mente. Por el contrario, Its individuosflomntes percnoctaron a diario en la zona arboladam•ts cercana al basurero. El uso preferencial de presassalvajes frente a la basura por los milanos reproductores puede ser explicado como consecuenciade la necesidadde presasde gran biomasapara su consumo por los pollos en crecimiento. Durante la esmcitn reproductora, el vertedero de Madrid podria tener una impormncia indirecm para el mantenimiento y conservacitnde la poblaci0n reproductoraa travfis del reemplazode las p•rdidas en Its reproductorespor Its individuosflomntes. [Traduccitn del Autor] Breeding and nonbreeding segmentsof raptor 1991), especiallywhere large waste accumulations populations often share the sameforaging habitats occur at garbage dumps (Ceballos and DonS_zar (Newton 1979). Floaters, nonbreeding, nonterri- 1988, Blanco 1994). Rubbish dumps and dung- torial adults and subadults,have larger home rang- heaps are frequently used by scavengingbirds that es and greater mobility than breeders,and usually normally exploit temporary and unpredictable concentrate in areas rich in food (Newton 1979, food sources(Pomeroy 1975, DonS_zar1992). The Ceballos and Don•ar 1990). Interactions between importance of refuse dumps for breeding popula- floaters and breeding conspecificshas been little tions of scavengingbirds has been repeatedly em- studied despite its potential to influence raptor phasized(Pomeroy 1975, Coulson et al. 1987,Don- populationdynamics (Newton 1979). Both breed- 5_zar1992) but, to date, it is not known whether ers and floaters of socialspecies such as the Milvus theseunnatural foraging placesare influencing the kites coexist in breeding areas when food is abun- population dynamicsof scavengingspecies. dant (Espina 1986, Koga et al. 1989, Heredia et al. The Black Kite (Milvus migrans)is a widespread 71 72 BLANCO VOL. 31, No. I speciesthat opportunisticallyexploits a wide array (breeding and floating) and migrant kites. After the of food sources (Delibes 1975, Shiraishi et al. breedingseason, field observationssuggested that breed- ing Black Kites and their fledglingsjoined communal 1990). Floaters form an unknown proportion of roosts together with floaters and northern migrants (J. populations, and usually share habitat with the Vifiuela pers. comm. and pers. obs.). The firstjuveniles breeding segment (Espina 1986, Koga et al. 1989, appeared in the roost soon after the breeding season Blanco 1994). At present, little is known about the (Blanco 1994) when breeders left their territories. There- fore, someremains collected at this time might haveorig- relationship between these two sectors of Black inated from prey of local breederswhich representeda Kite populations. This study deals with the role of high proportion of the birds roosting communallyfrom refuse in the diet of breeding, floating and mi- 23-31 July. Afterwards, the collecting period coincided grating Black Kites living in the vicinity of a large with the major influx of migrants (>1000 kites in early garbage dump near Madrid, Spain. August); at this time local breeders constituted a small proportion of the birds. STUDY AREA AND METHODS The diet of breeding kites and their nestlingswas de- termined by analysisof food remainsfound in and below The studyarea waslocated at an elevationof 500-700 the nestsof 18 pairs. The material was collected after the m in the flood plain of the Jarama River at its confluence w•th River Manzanares (40 ø 19'N, 3ø 31'W) in southeast- breeding seasonto avoid disturbanceat the nests.Pairs studied nested in trees and cliffs about 4-12.5 km from ern Madrid Province, central Spain. It included a com- the dump. plex of riverinc gypsumcliffs, riparian forestsdominated Food remainswere identified by macroscopiccompar- by poplars (Populusalba, P. nigra, P. 34 cultivar),willows (Salixspp.) and elms (Ulmusminor), but most of the area ison with referencecollections and quantified assuming was convertedto agriculture (mainly cereal crops, sun- the smallestpossible number of individuals(Marti 1987). flower, alfalfa and vegetables),cattle grazing and gravel Two main categoriesof food were consideredin the anal- extraction.The Madrid garbagedump waslocated in the ysis:wild prey obtained by hunting and scavenging(e.g., northeastern portion of the study area. From 100 to wild birds, mammals,fish), and refuse (e.g., food items >1000 Black Kites routinely gathered there to feed on intentionally discardedby humans such as domesticre- the refuse (Blanco 1994). fuse, offal from slaughterhousesand butcher shops,and During the breeding seasonof 1994, the Black Kite marine fish). To detect general differencesin food habits population consistedof about 50 pairs nesting on trees, of breeding and nonbreeding Black Kites, prey items cliffs and electric pylons,and about 80-300 nonbreeding were classifiedinto nine major groups (Table 1). The individualsforming a floating population of both adults data were likely biasedin favor of the most durableprey and subadults(Blanco 1994). Each day,resident floaters remains and did not reflect the importance of arthro- roosted communallyabout 4 km southeastof the dump, pods and other prey (Marti 1987). However,the study m a portion of riverine elm forest where 6-10 pairs nest- was not designedto provide detailed descriptionsof the ed Numbers of Black Kites in the area increased consid- Black Kite diet (Delibes 1975, Veiga and Hiraldo 1989, erably after breeding and especiallyduring migration Shiraishi et al. 1990) but to detect broad differences in (Blanco 1994). the consumption of refuse. I determined the diet of nonbreedingkites by analyz- Biomassof each prey specieswas estimated using mean ing food remains collectedbeneath roostingand diurnal weightsof each prey taxon obtained from the literature. communal perching trees during the period when Black Weights of prey taxon identified at nestscould not ex- Kites are resident in the area (Blanco 1994). All prey ceed 300 g even when the mean weight of the taxon remains (bones, fur and feathers) were collected on each exceededthat weight becauseBlack Kitesusually do not visit to the roost, from the final stage of the nestling pe- transport heavier prey to the nest (Espina 1986, pers. riod (June) to the end of August.Prey collectionswere obs.). In this case,I assumedthat prey heavier than 300 grouped into three time periods (22 July, 31 July and g were not deliveredto the nest whole. Weightsof large August) to assesspossible temporal variationsin the con- prey from the roost were estimated based on the daily sumption of refilse. The first period (Period A: until 22 food requirementsof adult Black Kites (100 g, Espina July) included the spring migration, breeding season, 1986; see also Heredia et al. 1991 for Red Kites [Milvus fledgling period and the start of migrant arrivals;Period milvus]).Therefore, I assumedthat the maximum inges- B (23-31 July) coincided with an increasein the influx tion capacityper feeding bout to be equivalent to the of migrants;and Period C (from I Augustto the end of daily food requirements(Blanco et al. 1990). Although that month) included the peak abundance of migrants Black Kites may tear off small pieces of the prey and and the remainder of the residencyperiod of BlackKites bring them to the nest one by one (J. Vifiuela pers in the area (Blanco 1994). Because all food remains comm.), and nonbreedingkites may eat prey over several found below the roosting trees were collected in each days,I felt my criteria