j. RaptorRes. 33(3):207-217 ¸ 1999 The Raptor Research Foundation, Inc.

NEST DISPERSION, DIET, AND BREEDING SUCCESS OF BLACK KITES (MILVUS MIGRANS) IN THE ITALIAN PRE-ALPS

FABRIZIO SERGIO Edward GreyInstitute of Field Ornithology,Department of Zoology,South Parhs Road, OxfordOX1 3PS, U.K.

ALBERTO BOTO Dipartimentodi BiologiaAnimale, Piazza Botta 9, 27100 Pavia, Italy

ABSTRACT.--Westudied a population of Black Kites (Milvus migrans)from 1992-96 in a 100-km')study area in the Italian pre-Alps around Lake Lugano. Population densityincreased from 24 territorial pairs per 100 km'• in 1992 to 38 in 1996. Nearest neighbor distanceswere variable, averaging1288 m for solitary pairs (N = 24) and 306 m for colonial ones (N = 151). Regular spacing of nest siteswas the rule within colonies, but inter-nest distance for solitary breeders increased as new pairs settled in the area. Nests occurred both in trees (58%, N = 84) and on cliffs (42%); 23% and 5% (N = 84) of the nestswere originally built by Common Buzzards (Buteobuteo) and Ravens (C0rvuscorax), respectively. Mean laying date was 25 April (N = 42), mean clutch size was 2.29 eggs (N = 42) and mean number of fledged young was 0.97, 1.11, and 1.78 young per territorial, reproductive, and successfulpairs, respectively(N = 143, 95, 78). The percentageof successfulterritorial pairswas 55% (N = 143). Diet was dominated by fish and birds, which accounted for 62% and 28% of 307 identified prey items, respectively.Compared with other European populations, this population showedan intermediate den- sityand an averageclutch sizebut the lowestfledging and breeding successever recorded for the species. Reasonsfor the low successwere unclear, but may have been related to low food availabilityor water pollution and consequentpesticide contamination. KEYWO•DS: BlachKite,, Milvus migrans; coloniality;density; diet;, breeding success; Italy.

Dispersionde nidos, dieta, y exito reproductivode Milvus migransen los pre-AlpesItalianos R•SWMEN.--Estudiamosuna poblaci6n de Milvus migransdesde 1992-96 en un frea de 100 kms en los pre-Alpes italianos alrededor del Lago Lugano. La densidad poblacional aument6 de 24 parejas terri- toriales por 100 kms en 1992 a 38 en 1996. La distanciamas pr6xima entre vecinosfu6 variable y promedi6 1288 m entre parejassolitarias (N = 24) y 306 m entre las parejascoloniales (N = 151). Los espaciosregulares de los nidos fueron la regla entre las colonias, pero la distanciaentre nidos para los reproductoressolitarios se increment6 a medida que se asentaronnuevas parejas en el firea. Los nidos se encontraron tanto en firboles(58%, N = 84) como en riscos(42%), 23% y 5% (N = 84) de los nidos fueron originalmente construidospor Buteobuteo y Corvuscorax, respectivamente. La media del dia de la puestafu6 el 25 de abril (N = 42). La media del tamafio de la nidada fu6 2.29 huevos (N = 42) y la productividadmedia fu6 0.97, 1.11, y 1.78juveniles por territorio. Las parejasproductivas territori- almente exitosasrepresentaron el 55% (N = 53). La dieta fue principalmente dominada por pecesy aves (62% y 28% respectivamente)de los 307 items de presasidentificadas. A1 comparar con otras poblacioneseuropeas, estas poblaciones mostraron una densidadintermedia y una nidada promedio, pero al mismo tiempo el mas bajo 6xito reproductivo y reclutamiento registrado para la especie. Las causasde esto, son poco claras y pudieron estar relacionadascon la poca disponiblidad de comida o con la poluci6n del agua y la consecuentecontaminaci6n por pesticidas. [Traducci6n de C6sar Mfrquez]

The Black Kite (Milvus migrans)is a medium- 1990) and as one of the most numerous and suc- sized accipitrid that is distributed throughout the cessfulbirds of prey in the world (Brown and Ama- Old World and Australasia. It has been defined as don 1968). Despite its local abundance, the Black an adaptable and opportunistic feeder (Geroudet Kite was recently classifiedas a declining and vul- 1965, Delibes1975, Arroyo 1978,Jones and Manez nerable speciesin Europe (Vifiuela and Sunyer

207 208 SERGIO AND BOTO VOL. 33, NO. 3

1994). Although populationswere reported to be pice system,Matthews 1989) on a 20-30 yr rotation basis. stable or increasing in western Europe between Single mature trees are often kept into the next rotation as seedbearers (coppicewith standardssystem, Matthews 1970-90 (e.g., Bustamanteand Hiraldo 1993,Dou- 1989). Thus, even though single mature trees were pres- meret 1995), pronounced decreaseshave been ob- ent almost everywhere,most woodlotshad been thinned served in Portugal, eastern Europe, and Russia or clearfelled in recent times (<40 yr), resulting in a during the sameperiod (Vifiuela and Sunyer1994, homogeneouscover of secondgrowth forest. Following the recent decline in coppice management throughout Bijlsma1997). Recently,recorded cases of pesticide Europe (Matthews1989), somewoodlots were being con- contamination (Hernandez et al. 1986, Jenni-Eier- verted to mature forest; nevertheless, mature woodland mann et al. 1996), nest robbing (Garcia Ferr6 and wasconfined to a few steep and often inaccessibleslopes. de Juan 1983) and water pollution (e.g., Ceschiet Cultivated fields, mainly grassland and maize (Zea al. 1996) in the species' core distribution areas mays)fields, were located on the valley floors and ac- counted for 3% of the land. Sixteen small villages,all point to an urgent need for quantitativemonitor- confined to the valley floors, covered 13% of the study ing of breeding populations (Vifiuela and Sunyer area. Thus, except for forestry operations, human activ- 1994). ities were mainly confined to lowland and were virtually In Italy, both local population increasesand de- absent from the mountain slopes.The Lake Lugano wa- ter surfaceaccounted for 6% of the studyarea which also clines have been reported (Petretti 1992, De Gia- included two smallerlakes (17 and 25 ha); overall,aquat- como et al. 1993), but information is fragmentary ic habitat covered7% of the studyarea. and mainly nonquantitative (Vifiuela and Sunyer Climate was temperate continental, with wet springs 1994). During the 20th century, the speciesde- and dry summers(Pinna 1978). Annual rainfall ranged clined dramatically in the Po Plain. For example, from 1600-2100 mm, with two peaks, one in the spring and one in the autumn, and the former more pro- one colony in BoscoFontana, near the city of Man- nounced than the latter (Belloni 1975). tova, declined from over 100 pairs in the 1930s (Ar- rigoni degli Oddi and Moltoni 1931) to <10 pairs METHODS in the 1970s (Micheli 1990). The largestItalian We censused Black Kite nests from 1992-96. The ear- populationsare currently concentratedin the pre- liest territorial individualswere observed in the study Alps (Micheli 1990). No quantitative estimatesof area on 18 March. As soon as kites settled on their breed- density,diet or productivityhave been published ing territories, they startedto perch in highly prominent positions, refurbish nests and carry out conspicuousae- for these populations. rial displaysto signal territory ownershipor to attract a In this paper,we report the resultsof a 5-yr study mate (Vifiuela 1993). Nestswere censusedby progressive, on a Black Kite population in a pre-Alpine area of complete searchesas new pairs arrived from migration northern Italy. The aim of the researchwas to pro- and settled on territories. No pairs settled on territories after the middle of April, when somepairs were already vide quantitative data on Black Kite density,nest laying eggs.All nestswere found during the pre-incuba- dispersion,colony size, diet and productivity,and tion period, by watchingthe residentpair's territorial dis- to compare them with estimatesfrom other pop- playsand nest material transfers.This also allowedus to ulations. censusthe nonbreeding sector of the territorial popula- tion (i.e., pairs that failed to lay eggs). STUDY AREA Nest dispersionwas analyzed by means of the G-statistic (Brown 1975), calculatedas the ratio of the geometricto The studyarea was a 100-km'• plot located along the the arithmetic mean of the squarednearest neighbor dis- Italian coastof Lake Lugano. It included two pre-Alpine tances (NNDs) Values ranged from 0-1 with valuesrang- valleys:the CeresioValley along the lake and the Ganna ing from 0.65-1.00 indicatinga regular dispersionof nest Valleyfurther inland from the lake. Altitude rangedfrom sites (Brown 1975). Pairswere defined as solitarywhen 275-1125 m. The landscapewas characterized by pre-Al- they nested>700 m from their nearestneighbor. How- p•ne mountains, mainly of sedimentaryorigin, ranging ever, five nestsbelonging to two territories were defined from 526-1125 m in peak elevation. Mountain slopes as solitarydespite having a NND <700 m. These were were coveredby continuousdeciduous woodland which located on opposite sidesof a mountain and the birds accountedfor 77% of the studyarea and waslocally in- could not see their nearestneighbor while in their nest terrupted by a few open areas.These open areasresulted area; also,they were not seento interact with the neigh- from human activities (regular burning or sheep graz- boring pairs during each of three 1-hr observation ses- ing) and were coveredby dry grassland,ferns, or various sionsduring the pre-layingperiod (seeBrown and Brown bushes,mainly common hazel (Corylusaveliana). Domi- 1996). Apart from thesefive cases,all pairs<700 m from nant tree speciesincluded sweetchestnut (Castaneasati- their nearest neighbor were defined as colonial, and all va), downy oak (Quercuspubescens), sessile oak (Quercus interactedregularly with their neighbors,mostly through petraea), European hop-hornbeam ( Ostrya carpinifolia), contestscaused by territorial intrusions. and locust (Robiniapseudoacada). Forests were managed Whenever possible,nest contentswere checkedthree for timber production by stool shoot regeneration (cop- times:first during incubation to assessclutch size,second SEPTEMBER. 1999 BLACK KITE BREEDING ECOLOGY 209

Table 1. Densityand extent of colonialityin a Black Kite population in the Italian pre-Alpsbetween 1992-96.

VA•ABLE 1992 1993 1994 1995 1996

No. of territorialpairs 27 33 35 39 41 Territorial pairs/100 km2 24 30 32 36 38 No. of colonialpairs (%) 26 (96) 29 (88) 28 (80) 35 (90) 33 (80) No. of colonies 6 6 6 7 5

just after hatching to estimate hatching success,brood 41 in 1996. Density correspondingly increased size, and date of hatching, and third, when the nestlings from 24 to 38 territorial pairs per 100 km2 (Table were 40-45 d old to record the number,of young raised (nestlingsfledge at about 48 d old, Bustamanteand Hir- 1). The loose colony outside the study area con- aldo 1993). We checked nest contents by climbing the tained 3 pair in each of the five study years. The nest tree, going down the nesting cliff with a rope or number of nest areas censused was 48. Of these, simplywatching the nest cup from a distantvantage point six were alwaysoccupied by solitarypairs, 37 by with binoculars or a 20-60X telescope.To minimize dis- turbance, risk of desertion or nest predation by Ravens colonial pairs and five by pairs that switchedfrom ( Corvuscorax), only neststhat could be checkedvery rap- colonial to solitary nesting (N = 3), or viceversa (N idly were visited during incubation or early hatching. = 2), during the studyperiod. Territorial behavior Thus, clutch size, laying date and number of layingpairs wasintense within 50-200 m of nestsbut wasrarely were assessedfrom a subsampleof nests.Hatching date was calculatedby backdating from the feather develop- observed at foraging areas. ment of nestlingsfirst observedwhen <15 d old and by The number of colonies censused was stable and comparison to reference information in Richard (1934), ranged from five to seven.Colonies contained on Cramp and Simmons (1980) and Hiraldo et al. (1990). average5.0 nests (+0.6, N = 30). The percentage Laying date was estimatedby subtracting29 d, the aver- age incubation period (Vifiuela 1993), from hatching of colonial pairs ranged from 80-96% (Table 1). date. Additional data were collectedfrom a loosecolony In particular, the number of pairs in large colonies of three pairs 3 km from the border of the study area. (>5 pair) did not increase significantlythrough We collected prey remains found under nests during the years (X2 = 2.12, df = 4, P = 0.710). Instead, each nest visit and identified them to the genus or spe- the number of pairs in small colonies (--<4pair) cies level assumingthe smallestpossible number of in- dividuals. Fish scaleswere identified following Steinmetz progressivelydecreased (X 2 = 9.76, df = 4, P = and Mfiller (1939) and Baligliniare and Le Louarn 0.040), except in 1995 (Fig. 1) when a new two- (1987); other vertebrate remainswere identified by com- pair colonywas created by settlementby a new pair parisonto a referencecollection. near a previouslysolitary pair. The percentageof Terminologyfollowed Steenh•of(1987) with a repro- ductivepair being one which laid eggs,a successfulpair solitary pairs did not increase significantly over being one which raised at leastone young until it was40 time (X2 = 4.92, df = 4, P = 0.301, Fig. 1). d old and breeding successbeing the percentage of suc- Mean NND was306 m for colonialpairs (+13.7, cessfulterritorial pairs. Raptors generally nest in tradi- N = 151) and 1288 m for solitary pairs (+166, N tional nest areasyear after year (Newton 1979); a nest area was defined as an area where more than one nest = 24). NNDs ranged from 60-690 m for colonial was found in the same or in different years, but where pairs and from 450-4250 m for solitarypairs. Mean only one pair nested each year. NND of solitarypairs did not increasethrough the Stepwiselogistic regression analysis (Norusis 1993) was years(Kruskal-Wallis X •= 1.95, df = 3, P = 0.583, employedto analyzeamong year differencesin the prob- Table 2). For three large colonies (A, B, and C, ability of territorial pairs laying eggsand of eggshatch- ing. Logisticregression uses a linear combination of in- Table 2), we tested the effect of year and colony dependent variables to explain the variance of a on nest spacingby meansof a two-wayANOVA: the dependent dichotomousvariable. To meet the assump- interaction of year and colony was almost signifi- tions of normality,NNDs were alwayslog transformedpri- cant (partialFs,9s = 1.86,P = 0.076) and both year or to parametric tests.Means are given with _+1 SE. All testswere two-tailed and statisticalsignificance was set at and colony significantlyaffected mean NND inde- P < 0.05. pendently (respectively:partial F4,98: 11.01, P = 0.000; partial F2,9s= 20.78, P = 0.000). Variation RESULTS in NND within colonies among years was mainly Density and Nest Dispersion. The number of ter- caused by the progressiveincrease in territorial ritorial pairsincreased steadily from 27 in 1992 to pairs in two colonies around the lake. New pairs 210 SERGIOAND BOTO VOL. 33, NO. 3

.', ! Colonysize

• small(2-4 pr) =• [• solitarypairs •1996 / i large(5-13pr) 1992 1993 1994 1995

Year Figure1. Numberof BlackKite pairs breeding solitarily or in small(•4 pairs)and large (-->5 pairs) colonies between 1992-96 in the Italian pre-Alps.

Table 2. Mean (_SE) BlackKite nearestneighbor distances for 24 solitarypairs and at sevencolonies in the Italian pre-Alpsbetween 1992-96.

ME•'• N•U•ST NEIGHBOR D•ST•CW (m)

NEST TYPE 1992 1993 1994 1995 1996 1992--96 Solitary 934 _ 161 1203- 199 1304_+ 332 1508_ 437 1288+- 166 (4) a 0.82b (7) a 0.72b (4) a 0.68• (8) • 0.38• (24)• 0.54• ColonyA 394 _ 40 373 - 64 288 - 49 252 - 28 241 - 33 295 - 20 (7)• 0.88• (8)a 0.67b (9)• 0.64• (12)• 0.79• (13)• 0.68• (49)• 0.67• ColonyB 292 - 12 169 +__24 (7)• 0.89b 167 - 15 150 - 25 183 - 10 (7) • 0.96• (8)a 0.67• (9)• 0.87• (8) • 0.67• (39)• 0.78b ColonyC 679 - 24 355 - 51 290 - 23 237 - 44 174 _ 23 307 - 34 (3) • 0.10• (5)• 0.85b (5) a 0.75• (7) • 0.83• (25)• 0.59• (5) • 0.95b ColonyD 597 _ 27 597 - 27 567 - 37 567 - 37 433 _ 123 552 - 29 (3) • 0.99• (3) • 0.98• (3) • 0.98• (3) • 0.74• (15) • 0.92b (3) a 0.99 • ColonyE 401 _ 3 317 _ 45 640 +__0 640 _ 0 640 +--0 500 - 44 (3) • 0.10• (3) • 0.93• (2) • (2) • (12)• 0.83• (2) • ColonyF 213 - 10 206 - 0 206 --- 0 206 - 0 208 - 3 (3) • 0.99b (9) a 0.99b (2) • (2) • (2) • ColonyG 403 _+0 (2) 403 _+0 (2) All nests 430_+ 58 (27)• 394 ñ 48 (33)• 492_+ 75 (35)a 388-+ 63 (39)• 493_ 115 (41)• 441 ñ 36 (175)• Sample size. G statistic. SEPTEMBER 1999 BLACI• KITE BREEDING ECOLOGY 211 settlingin thesecolonies did not place their nests place in 84 different nest structures,each one oc- in the outer parts of coloniesor far awayfrom tra- cupied for one to five consecutiveyears (• = 2.1 ditional pairs, but selected areas near already- _+0.1, N = 84). occupiednests. Thus, meanwithin-colony NND de- Of these nests,58% were placed in trees while creased as the number of pairs in the colonies the remainder were on cliffs (N = 84). Of 49 tree increasedthough the years(colony A: F4,44= 2.64, nests, 39% were in sweet chestnut, 33% in sessile P = 0.046; colonyC: F4,20= 11.29, P = 0.000, Table or downyoak, 18% in locust,6% in Europeanhop- 2). Mean NND did not vary significantlywith time hornbeam, 2% in common lime (Tilia europaea), in colony C, where the number of pairs remained and 2% in Scotchpine (Pinussilvestris) trees. The stablethrough the years(F4,34 = 2.07, P = 0.106). 35 cliff nests were placed on bare rock ledges Due to this, NNDs are shown separatelyfor each (29%) and at the base (54%) or in the canopy colony in Table 2. (17%) of trees,generally downy oaks, growing on The G-statisticdeclined progressivelyfor solitary cliff faces. Many pairs used nest structuresorigi- pairs through the years,indicating a regular dis- nally built by other species,to which they added persion of nest sitesbetween 1993-95, but not in new material:23% of 84 occupiednest structures 1996 (Table 2). Pooling years, the overall disper- were originally built by Common Buzzards (Buteo sion of solitarynests was not regular (G = 0.540), buteo)and 5% were old Raven nests. Some nests probablybecause of the progressiveincrease in the were being used by the original occupantswhen mean NND of solitary nesters over time. Instead, they were taken over by BlackKites. Buzzard nests the G-statisticindicated a regular dispersionof nest were taken over during the original owners' pre- siteswithin 22 of 23 colonieswith ->3 pairs. laying period and one Raven nest still contained Nest Sites. All territorial pairs built nests (N = nestlings when it was occupied by kites, which 175 pairs in 48 nest areas).After settlingon terri- preyed on the chicks before starting to add new tories, most pairs brought nest material to one material to the nest. nest,but 13 pairs in six nestareas brought material Breeding Season.There were no year-to-yeardif- to two nestsduring the beginningof the pre-laying ferences in mean laying date (F4,40= 0.68, P = period, before selecting the nest in which they 0.762, Table 3). Laying dates ranged from 12 eventually laid eggs. Altitude of the nest sites April-8 May, averaging25 April (Table 3). No cases ranged from 240-870 m, averaging515 m (+14, N of replacement clutchesin the same nest were ob- = 175). The 175 certsusedterritorial nestingstook served after breeding failures, even when these

Table 3. Mean (+SE) laying date and productivityestimates of an Italian pre-Alpine Black Kite population between 1992-96.

VARIABLE 1992 1993 1994 1995 1996 1992--96

Laying date in April 24.6 - 1 (7) • 23.3 -+ 2 (7) a 25.5 + 2 (8) • 28.7 - 3 (9) • 25.3 + 2 (14) a 25.6 + I (45) a Territorialpairs b % reproductive 17 31 31 33 31 143 Pairs (N) 100 (17) • 76 (21) • 86 (22) • 86 (21) • 74 (19) • 84 (100) • Clutch size (N) 2.5 + 0.2 2.5 _+ 0.2 1.8 + 0.3 1.8 + 0.4 2.6 +_ 0.2 2.3 + 0.1 (42) • (10) • (11) • (8) a (5) • (8) a % hatchingsuccess 68 (25) c 89 (27) c 79 (14) c 89 (9) c 100 (21) c 84 (96) c % successfulpairs 71 58 48 42 61 55 Young fledged per territorial pair 1.06 + 0.2 1.16 _ 0.2 0.71 + 0.2 0.67 + 0.2 1.32 _+ 0.2 0.97 _ 0.1 Young fledged per 1.00 + 0.2 1.24 _ 0.2 0.86 + 0.2 0.76 +_ 0.2 1.72 _ 0.3 (18) • 1.11 _+ 0.1 (97) • reproductive pair (16) • (21) • (21) a (21) a Young fledged per 1.50 ___0.2 2.00 + 0.2 1.47 _ 0.2 1.57 +_ 0.2 2.16 + 0.2 (19) • 1.78 _ 0.1 (78) • successfulpair (12) a (18) a (15) a (14) a Sample size (when different from that in column "territorial pairs"). Those checkedfor nestingsuccess. Number of eggs.Sample size of nestssame as for clutch size. 212 SERGIOAND BOTO VOL. 33, No. 3

Table 4. Diet of breeding Black Kites in the Italian pre- ble 3). On the whole, 84% of 96 eggs checked Alps (1992-97), as estimatedby food remains collected hatched. Stepwise logistic regression analysis under nests. showeda significanteffect of year on probability of egg hatching (B = 0.65, Wald = 5.88, df = 1, P NUMBER OF = 0.015, reclassificationrate -- 84%). PREYCATEGORY ITEMS(%) The number of young•fledged per territorial Fishes 190 (61.9) pair averaged0.97 (N = 143) and differed signifi- Rudd (Scardiniuserythrophthalmus) 59 (19) cantly among years(F4,158 = 2.49, P = 0.045). Pro- Chub (Leuciscuscephalus) 56 (18) ductivitywas significantlyhigher in 1996 than in Unidentified Ciprinidae 37 (12) 1994 and 1995 (Duncan's Multiple Range Test, P Othersa 38 (12) • 0.05, Table 3). Significantamong-year differenc- B•rds 85 (27.7) es alsowere detectedfor mean number of fledged Blackbird (Turdus merula) 20 (7) young per reproductive pair (F4,92= 2.68, P = Othersb 65 (21) Unidentified Passeriformes 20 (7) 0.036). Again, average values were significantly Mammals 24 (7.8) higher in 1996 than in 1994 and 1995 (Duncan's Unidentified Microtidae 15 (5) Multiple Range Test, P • 0.05, Table 3). Mean Others c 9 (3) number of fledged young per successfulpair was Reptiles'• 5 (1.5) 1.78 (N -- 78) and alsovaried among years (F4,73 Amphibianse 1 (0.3) = 3.69, P = 0.009), with more young fledged on Invertebratesr 2 (0.7) averagein 1996 than in 1992, 1994, and 1995 and Total 307 in 1993 than in 1994 (Duncan's Multiple Range a Includes:perch (PercafluviatilisN = 13), bleak (Alburnusalbidus Test, P • 0.05, Table 3). Finally,the percentageof alborella,N = 11), roach (Rutilus rutilus, N = 8), pumpkinseed successful territorial pairs in the population (Lepomisgibbosus, N = 4), large-mouth black bass(Micropterus sal- tooides,N = 2). ranged from 42% in 1995 to 71% in 1992, with no b Includes:Rock Dove (Columbalivia, N = 11), Starling(Sturnus significantamong-year differences (X 2 = 4.98, df = vulgaris,N = 6),Jay ( Garrulusglandarius, N = 5), Italian Sparrow 4, P = 0.301, Table 3). (Passerdomesticus italiae, N = 4), Black Kite nestling (N = 2), Sample size allowed us to test the effect of year Honey Buzzardnestling (Pernis apivorus, N = 2), TawnyOwl (Strix and colony on mean number of fledged young in aluco,N = 2), Hooded Crow (Corvuscorone cornix, N = 2), Song Thrush (Turdusphilomelos, N = 2), Moorhen (Gallinulachloropus, three colonies (A, B, and C) by means of a two-way N = 1), Collared Dove (Streptopeliadecaocto, N-- 1), Barn Swallow ANOVA. The effect of the interaction betweenyear (Hirundo rustica, N = 1), House Martin (Delichonurbica, N = 1), and colony on mean number of young fledged per Chaffinch (Fringillacoelebs, N = 1), Great Tit (Parusmajor, N = territorial pair wassignificant (partial Fs,• = 3.01, 1), Reed Bunting (Emberizaschoeniclus, N = 1). P = 0.005), but only approachedsignificance when c Includes:dormouse (Myoxus glis, N = 3), commonmole (Talpa europaea,N = 3), rabbit (Oryctolaguscuniculus, N = 2), unidenti- using mean number of fledged young per repro- fied mammal from slaughterhouse(N = 1). ductive pair or per successfulpair as the depen- a Includes:Aesculapian snake (Elaphe longissima, N = 2), uniden- dent variable (respectively:partial Fs,oo= 1.80, P = titled Colubridae (N = 3). 0.092; partial F6,42= 1.96, P = 0.093). In thesetwo e Includes: Rana spp. (N = 1). f Includes:Cetonia spp. (N = 2). cases,colony was the only significant main effect (respectively:partial F2,oo= 3.32, P = 0.042;partial F2,42: 3.54, P = 0.038). These patternswere main- happenedearly in incubation.All nestlingsfledged ly caused by the productivity of colony C, where by 15 July and all Black Kites departed the study fledging successremained similar to that in other area by 10 August each year. coloniesuntil 1995, when no young were raisedin Productivity.Each year, 74-100% of territorial any of the eight nests checked. In 1996, only one pairs laid eggs (Table 3), with no clear trend over young was raised in sevennests checked in colony time. Year did not enter a stepwiselogistic regres- C. The reason for such a dramatic decline in pro- sion model with laying or nonlaying of eggsas a ductivity was not clear. dependent variable. The overall mean number of Overall, causesof breeding failure were seldom laid eggs (i = 2.3, N = 42) varied significantly identified: 89% of failures occurred during incu- amongyears (F4,37; 2.75, P = 0.043) and wassig- bation (N = 19) and the rest occurred during the nificantly lower in 1994 than in 1992, 1993, and early part of the fledging period. Probable causes 1995 (Duncan's Multiple Range Test, P • 0.05, Ta- of failure included: failed hatching of eggs and SEPTEMBER 1999 BLACK KITE BREEDING ECOLOGY 213

Table 5. Nesting density of Black Kite populations in Europe, 1966-96.

NEAREST NEIGHBOR DENSITY DISTANCE AREA(PERIOD) HABITAT (N) (pr/100km9) (N) REFERENCE Matas Gordas, Spain Grasslandand (1987-89) marshland 21-45 700-1500 Vifiuela et al. 1994 Rh6ne Plain, France (1970) River plain 140 609 Sermet 1980 Dofiana, Spain (1981-84) Marshland 80 266.6 206 (47) a Hiraldo et al. 1990 Lac Leman, Switzerland Henrioux and Henrioux (1975-90) Farmland and lake 319 100.6 1995 Neuchfitel, Switzerland (1968) Farmland and lake 337 69.6 Sermet 1980 Lorraine, France (1966) Woodland and pas- ture 66 44.6 Thiollay 1967 Lake Lugano, Italy (1992- 96) Woodland and lake 27-41 24-38 441 (175) This study Castelporziano,Italy (1991- Woodland and 92) farmland 16 3.3.3 103 (16) De Giacomo et al. 1993 Lake Constance,Germany (1968-69) Farmland and lake 25-30 18.5-22.2 Heckenroth 1970 Limousin, France (1976- 78) Bocageb 21 9.5 Nore 1979 Dr6mling, Germany (1993- 94) Farmland 8 7.02 2330 (8) c Seelig et al. 1996 Monti della Tolfa, Italy Woodland and pas- (1973-80) ture 42 4.9 Petretti and Petretti 1981 Brandenburg, Germany (1979) Farmland 215 0.74 Fiuczynski 1981 Estimate from Bustamante and Hiraldo (1990), for the period 1985-88. Pastureand woodland enclosedby stonewalls or hedges. Calculated from the published map. consequentdesertion (three cases),predation (two Resultsof the analyseson NND and fledging suc- cases) and disturbance by rock climbers (one cessin the three large colonies showed that each case). colony could be considereda separateunit within Diet. Fish and birds dominated the diet and ac- the population. Each colony was characterized by counted for 61.9% and 27.7% of 307 identified its own nest spacing distance, which tended to be prey items, respectively(Table 4). Mean length of regular, but which differed from that of other col- 15 fish found uneaten in nests was 18.6 cm (-+1.8, onies and sometimesvaried from one year to the range = 7-30 cm). Of 54 avian prey items, 26% next. Similarly,productivity varied among colonies were juveniles. within and among yearswithin each colony.These results emphasizedthe importance of monitoring DISCUSSION populations composedof more than just one col- The number of territorial pairs of Black Kites ony for as many years as possibleto obtain an un- increasedsteadily during our study.It is difficult to biasedestimate of density,nest dispersion,and pro- say whether this was causedby a real population ductivity.To date, many studieson Black Kites have increaseor a simple population fluctuation. Spatial been conducted in just one colony or in areas of and temporal variations in Black Kite population homogeneoushigh density that are functionally density and productivity have been reported by similar to one large colony (e.g., Desai and Mat- many authors (Fiuczynski and Wendland 1968, hotra 1979, Kogaet at. 1989,Vifiueta et at. 1994). Vifiueta et at. 1994, Doumeret 1995, Bijtsma1997). Overall, the population showedremarkable op- 214 SERGIO AND BOTO VOL. 33, NO. 3

Table 6. Productivityof Black Kite populationsin Europa and Asia, 1966-96.

HATCHING

CLUTCH SUCCESS a BREEDING AREA (PERIOD) HABITAT N SIZE (EGGS) SUCCESS Dofiana, Spain (1987-89) Marshland 166 92% Germany (1992-95) 599 79% Nagasaki,Japan (1983-86) Fishing port 32 2.18 (28) b 79% (61) b 75% L•mousin, France (1976-78) Bocaged 22 68% Berlin, Germany (1940-79) Farmland 215 62% Lake Lugano, Italy (1992-96) Woodland and 143 2.29 (42) b 84% (96) b 55% lake Slovakia (1975-89) 162 2.98 (44) b Lac Leman, Switzerland Farmland and (1975-90) lake 165 2.25 Lorraine, France (1966) Woodland and pasture 66 2.26 (45) b New Delhi, India (1973-76) Urban 45 2.3 (60) b 55% (102) b Data on hatchingsuccess not shownin table:75% (N = 36 eggsfrom 14 nests,Hakel, Germany,1957; Stubbe1961) and 64% (N = 28 eggsfrom 10 nests,Mazio, Italy, date unknown;Petretti 1992). Sample size (when different from that in column "N"). Data also from Mammen and Stubbe (1995, 1996). Pastureand woodlandenclosed by stonewalls or hedges. portunism and elasticity.First, variabilityin the ex- which is very common all over the Italian pre-Alps, tent of coloniality between and within years was is very rare elsewhere in the Black Kite's range paralleled by high flexibility in NNDs, the highest (Cramp and Simmons 1980) and only locally re- observedNND being 70 times greater than the ported in Sicily (Massa1985), in an area of central lowest one. Interestingly, as the population in- Spain (Blanco 1997) and near someSwiss lakes creasedover the years,new pairs settlingin large (Sermet 1980). coloniespositioned their nestsnear existingpairs, Finally, diet wasdominated by fish and birds on possibly as a result of conspecific attraction a numeric basis. However, all vertebrate classesand (Stamps 1988), as was also reported by Vifiuela et some occasionalinvertebrate prey were also rep- al. (1994). Advantagesof nesting near traditional resented. This finding agreed with the common older pairs (Vifiuela 1993) could include local en- definition of Black Kites as opportunistic feeders hancement (Hagan and Walters 1990), informa- with local specializationin the most availableprey tion parasitism(e.g., Green 1987) and greaterop- (Delibes 1975, Arroyo 1978, Vifiuela and Veiga portunities for kleptoparasitism (Vifiuela et al. 1992, Blanco 1997). 1994). The opposite trend was recorded for soli- Despite the overall opportunism and adaptabili- tary pairs whose NNDs increasedas new pairs col- ty, productivityestimates for our studypopulation onized the area, suggestingpronounced overall were remarkably low when comparedto other pub- variationsin individual pairs nest dispersion,prob- lished estimatesin Europe,Japan and India (Table ablyin relation to food distribution(Newton 1979, 5). Nesting densities acrossEurope were highest Vifiuela et al. 1994). near large wetlandsand intermediate or very low Second, Black Kite nestswere found in a variety in farmed or extensive livestock rearing land- of situations:in mature and young trees, within scapes.Preference for proximity to large wetlands large forests,in small clumpsof two or three trees, and higher densitiesnear water bodieshave been in isolated trees and on large (>100 m high) and reported by many authors(Heckenroth 1970, De- small (<20 m high) cliffs.Nests originally built by libes 1975, Hiraldo et al. 1990, Henrioux and Hen- other species,especially Common Buzzards,also rioux 1995). Density in the Italian pre-Alpine area were occupiedreadily and taken over by persistent was much lower than that of populations near harassmentof the original occupants.Cliff nesting, large wetlands (Sermet 1980, Hiraldo et al. 1990, SEPTEMBER 1999 BLACtC KITE BREEDING ECOLOGY 215

Table 6. Extended.

MEAN NUMBER OF FLEDGED YOUNG

TERRITORIAL BREEDING SUCCESSFUL PAIR PAIR PAIR REFERENCE

1.77 1.92 (153) b Jones and Manez 1990 1.63 2.07 (471) b Gedeon 1994 c 1 00 1.14 (28) b 1.33 (24) b Koga et al. 1989 1.32 1.93 (15) b Nore 1979 1 2O 1.90 (133) b Fiuczynski1981 O.97 1.1 (95) b 1.78 (78) •' This study

2.31 Danko 1989 2.O2 Henrioux and Henrioux 1.32 1995 1.58 (55) • Thiollay 1967 0 98 Desai and Malhotra 1979

Vifiuela et al. 1994, Henrioux and Henrioux 1995). es of low natality could be low food availabilityor It was similar to that along Lake Constancein the water pollution and consequent prey contamina- 1960s and in low intensityfarmland with scattered tion. DDT contamination of water and fish con- wetlands(Heckenroth 1970, Thiollay 1967, Seelig tamination was reported recently in nearby Lake et al. 1996), but higher than that in farmed or pas- Maggiore (Ceschi et al. 1996). Even though lake ture habitats with few wetlands (Nore 1979, Fiuc- euthrophication and pollution could benefit this zynski1981, Petretti and Petretti 1981). Thus, the speciesby providing dead or moribund fish (Ger- observed density was intermediate, but low, com- oudet 1965, Bijeleveld1974, Doumeret 1995), pes- pared with other populationsnear large lakes or ticide contamination clearly can be harmful for wetlands. Black Kites (e.g., Jenni-Eiermann 1996, Bijlsma Clutch size was similar to that of other popula- 1997) causinglow natalityand eventualpopulation tions (Table 6) and hatching successalso was com- decline in the long term. The aim of current and parable to that of other published estimates,ex- future research on Black Kites in northern Italy is cept the one of New Delhi, although all of these to monitor pesticide concentrations in eggs, pop- data were from studiesconducted outside Europe ulation density and productivity levels in different or during the pesticideera (1960-70s; Newton lakes and correlate them with levelsof pollution in 1979). In contrast, the percentage of successful water and fish prey. There is need for studiesas- pairs and mean number of fledged young per ter- sessingthe level of water contamination above ritorial, reproductive, or successfulpair were the which toxic chemicals concentrations start to be lowestever recorded in Europe and similar only to thosein India and Japan. Reasonsfor such low na- harmful, both for Black Kites and their fish prey. tality were unclear. During the 1960s,trapping and More published estimatesof density and produc- persecutionwere common practice in the county tivity, especiallyhatching success,are also needed of the study area. For example, Bianchi et al. from other European countries.

(1969) reported gamekeepers killing 22 Black ACKNOWLEDGMENTS Kites in one month in a 1-kin 2 area. However, this practice now is rare and we observedno casesof We thank G. Bogliani for continuous encouragement nest robbing, trapping, or persecution.Clutch and during each phaseof the researchand G. Bogliani,C.M Perrins, M. Henning, P.G.W. Salaman,K. Steenhof, and nestling predation was minimal, as important po- D.E. Varland for constructive comments on a first draft tential predatorssuch as Goshawks(Accipiter gentil- of the paper. We thank L. Marchesi for identification of is) and Eagle Owls (Bubobubo) were very scarcein avian prey remains. Part of this studywas funded by Re- the studyarea. Thus, the remainingpotential caus- gione Lombardia, ServizioFaunistico. 216 SERGIO AND BOTO VOL. 33, NO. 3

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