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

NEST DISPERSION,

DIET, AND BREEDING

SUCCESS OF BLACKKITES (MILVUS MIGRANS) IN THE ITALIANFABRIZIO

PRE-ALPSSERGIO

Edward GreyInstituteofField Ornithology,
Departmentof Zoology,
SouthParhsRoad, OxfordOX1 3PS, U.K.
ALBERTO

BOTO

Dipartimento
di BiologiaAnimale,PiazzaBotta9, 27100 Pavia, Italy
ABSTRACT.--We
studieda populationof Black Kites (Milvus migrans)from 1992-96 in a 100-km')study
area in the Italian pre-Alpsaround Lake Lugano. Population densityincreasedfrom 24 territorial pairs
per 100 km'• in 1992 to 38 in 1996. Nearest neighbor distanceswere variable, averaging1288 m forsolitary pairs (N = 24) and 306 m for colonial ones (N = 151). Regular spacing of nest siteswas therule within colonies, but inter-nest distance for solitary breeders increased as new pairs settled in thearea. Nests occurred both in trees (58%, N = 84) and on cliffs (42%); 23% and 5% (N = 84) of thenestswere 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 numberof fledged young was 0.97, 1.11, and 1.78 young per territorial, reproductive, and successfulpairs,

respectively(N = 143, 95, 78). The percentageof successful
territorial pairswas55% (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 densityand 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 waterpollution and consequentpesticidecontamination.
KEYWO•DS: BlachKite,,Milvus migrans; coloniality;density;diet;,breeding
success;
Italy.

Dispersionde nidos, dieta, y exito reproductivode Milvus migransen los pre-AlpesItalianos
R•SWMEN.--Estudiamos
una 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 territoriales por 100 kms en 1992 a 38 en 1996. La distanciamas pr6xima entre vecinosfu6 variable ypromedi6 1288 m entre parejassolitarias(N = 24) y 306 m entre las parejascoloniales(N = 151). Los
espaciosregularesde los nidos fueron la regla entre las colonias,pero la distanciaentre nidos para losreproductoressolitariosse increment6 a medida que se asentaronnuevasparejasen el firea. Los nidos
se encontrarontanto en firboles(58%, N = 84) como en riscos(42%), 23% y 5% (N = 84) de los nidos
fueron originalmente construidospor Buteobuteoy Corvuscorax,respectivamente.La media del dia dela puestafu6 el 25 de abril (N = 42). La media del tamafio de la nidada fu6 2.29 huevos (N = 42) yla productividadmedia fu6 0.97, 1.11, y 1.78juvenilespor territorio. Las parejasproductivasterritorialmente exitosasrepresentaronel 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,estaspoblacionesmostraronuna 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 clarasy pudieron estar relacionadascon la poca disponiblidad de comida ocon la poluci6n del agua y la consecuentecontaminaci6n por pesticidas.
[Traducci6n de C6sar Mfrquez]

The Black Kite (Milvus migrans)is a mediumsized accipitrid that is distributed throughout the
Old

World

and Australasia.

It has been

definedasan adaptable and opportunistic feeder (Geroudet

1965,Delibes1975,Arroyo 1978,Jonesand Manez

207

1990) and as one of the most numerous and suc-cessfulbirds of prey in the world (Brown and Amadon 1968). Despite its local abundance, the Black
Kite was recently classifiedas a declining and vulnerable speciesin Europe (Vifiuela and Sunyer

208

SERGIO AND BOTO

VOL. 33, NO. 3

1994). Although populationswere reported to be pice system,Matthews1989) on a 20-30 yr rotation basis.
stable or increasing in western Europe between Single mature trees are often kept into the next rotationasseedbearers (coppicewith standardssystem,Matthews
1970-90 (e.g., Bustamanteand Hiraldo 1993,Dou- 1989). Thus, even though single mature treeswere presmeret 1995), pronounceddecreaseshavebeen ob- ent almost everywhere,most woodlotshad been thinned
served in Portugal, eastern Europe, and Russia or clearfelled in recent times (<40 yr), resulting in aduring the sameperiod (Vifiuela and Sunyer1994, homogeneouscover of secondgrowth forest. Following
the recent decline in coppice management throughout
Bijlsma1997). Recently,recordedcasesof pesticide Europe (Matthews1989), somewoodlotswere being concontamination (Hernandez et al. 1986,Jenni-Eier- verted to mature forest; nevertheless, mature woodlandmann et al. 1996), nest robbing (Garcia Ferr6 and wasconfined to a few steepand often inaccessibleslopes.
Cultivated fields, mainly grassland and maize (Zeade Juan 1983) and water pollution (e.g., Ceschietmays)fields, were located on the valley floors and acal. 1996) in the species' core distribution areas counted for 3% of the land. Sixteen small villages,allpoint to an urgent need for quantitativemonitor- confined to the valleyfloors, covered 13% of the study
ing of breeding populations (Vifiuela and Sunyer area. Thus, except for forestry operations, human activ1994).

In Italy, both local population increasesand declines have been reported (Petretti 1992, De Giacomo et al. 1993), but information is fragmentaryand mainly nonquantitative (Vifiuela and Sunyer1994). During the 20th century, the speciesdeclined dramatically in the Po Plain. For example,
one colonyin BoscoFontana, near the city of Mantova, declined from over 100 pairs in the 1930s (Arrigoni degli Oddi and Moltoni 1931) to <10 pairs
in the 1970s (Micheli 1990). The largestItalian
populationsare currently concentratedin the preAlps (Micheli 1990). No quantitative estimatesof
density,diet or productivityhave been published
for thesepopulations.
In thispaper,we report the resultsof a 5-yrstudy
on a Black Kite populationin a pre-Alpinearea ofnorthern Italy. The aim of the researchwasto provide quantitativedata on Black Kite density,nest
dispersion,colony size, diet and productivity,andto compare them with estimatesfrom other populations.
STUDY AREA

ities were mainly confined to lowland and were virtuallyabsentfrom the mountain slopes.The Lake Lugano water surfaceaccountedfor 6% of the studyareawhichalso
includedtwo smallerlakes(17 and 25 ha); overall,aquatic habitat covered7% of the studyarea.
Climate was temperate continental, with wet springsand dry summers(Pinna 1978). Annual rainfall ranged
from 1600-2100 mm, with two peaks, one in the springand one in the autumn, and the former more pronounced than the latter (Belloni 1975).
METHODSWe censused Black Kite nests from

1992-96.

The ear-liest territorial individualswere observed in the study
area on 18 March.

As soon as kites settled

on their breed-

ing territories, they startedto perch in highly prominent
positions,refurbish nestsand carry out conspicuousaerial displaysto signalterritory ownershipor to attract amate (Vifiuela 1993). Nestswere censusedby progressive,
complete searchesas new pairs arrived from migrationand settled on territories. No pairs settled on territories
after the middle of April, when somepairswere alreadylaying eggs.All nestswere found during the pre-incubation period, by watchingthe residentpair's territorial displaysand nest material transfers.This also allowedus to
censusthe nonbreeding sectorof the territorial population (i.e., pairs that failed to lay eggs).
Nest dispersionwasanalyzedby means of the G-statistic(Brown 1975), calculatedasthe ratio of the geometricto
the arithmeticmean of the squarednearestneighbordistances (NNDs) Valuesranged from 0-1 with valuesranging from 0.65-1.00 indicatinga regular dispersionof nest
sites(Brown 1975). Pairswere defined as solitarywhen

The studyarea was a 100-km'• plot located along the
Italian coastof Lake Lugano. It included two pre-Alpine
valleys:the CeresioValley along the lake and the Ganna
Valleyfurther inland from the lake.Altitude rangedfrom
275-1125 m. The landscapewascharacterizedby pre-Al- they nested>700 m from their nearestneighbor.Howp•ne mountains, mainly of sedimentaryorigin, ranging ever,five nestsbelonging to two territorieswere definedfrom 526-1125 m in peak elevation. Mountain slopes as solitarydespitehavinga NND <700 m. These were
were coveredby continuousdeciduouswoodlandwhich located on opposite sidesof a mountain and the birds
accountedfor 77% of the studyarea and waslocallyin- could not see their nearestneighbor while in their nest
terrupted by a few open areas.Theseopen areasresulted area; also,they were not seento interactwith the neighfrom human activities (regular burning or sheep graz- boring pairs during each of three 1-hr observationsesing) 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 fromnant tree speciesincluded sweetchestnut (Castaneasati- their nearest neighbor were defined as colonial, and allva), downy oak (Quercuspubescens),sessileoak (Quercus interactedregularlywith their neighbors,mostlythroughpetraea),European hop-hornbeam ( Ostryacarpinifolia), contestscausedby territorial intrusions.
Whenever possible,nest contentswere checkedthreeand locust (Robiniapseudoacada).
Forestswere managed
clutch size,second
for timber production by stool shoot regeneration (cop- times:first during incubation to assess

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

No. of territorialpairs
Territorial pairs/100 km2
No. of colonialpairs (%)
No. of colonies

1992

1993

1994

1995

1996

27
24
26 (96)33
30
29 (88)35
32
28 (80)3936
35 (90)4138
33 (80)

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 pairsper 100 km2 (Table
were 40-45 d old to record the number,of young raised1). The loose colony outside the study area con(nestlingsfledge at about 48 d old, Bustamanteand Hiraldo 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,
simplywatchingthe nest cup from a distantvantagepoint six were alwaysoccupiedby solitarypairs, 37 bywith binoculars or a 20-60X telescope.To minimize disturbance, 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(Nidly were visited during incubation or early hatching. = 2), during the studyperiod. Territorial behavior
Thus, clutch size,laying date and number of layingpairs wasintensewithin 50-200 m of nestsbut wasrarely
were assessedfrom a subsampleof nests.Hatching date
was calculatedby backdatingfrom the feather develop- observed at foraging areas.
The number of colonies censused was stable andment of nestlingsfirst observedwhen <15 d old and bycomparison 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 wasestimatedby subtracting29 d, the averof colonial pairs ranged from 80-96% (Table 1).
age incubation period (Vifiuela 1993), from hatchingdate. Additional data were collectedfrom a loosecolony In particular, the number of pairs in large coloniesof three pairs 3 km from the border of the study area.
(>5 pair) did not increase significantlythroughWe collected prey remains found under nestsduring the years (X2 = 2.12, df = 4, P = 0.710). Instead,each nest visit and identified them to the genus or spethe number of pairs in small colonies (--<4pair)cies level assumingthe smallestpossiblenumber of indecreased(X2 = 9.76, df = 4, P =dividuals.Fish scaleswere identified following Steinmetz progressively
0.040), except in 1995 (Fig. 1) when a new twoand Mfiller (1939) and Baligliniare and Le Louarn(1987); other vertebrateremainswere identified by com- pair colonywascreatedby settlementby a new pairparisonto a referencecollection.
near a previouslysolitarypair. The percentageof
TerminologyfollowedSteenh•of(1987) with a reproductivepair being one whichlaid eggs,a successful
pair solitary pairs did not increase significantly overbeing one which raisedat leastone young until it was40 time (X2 = 4.92, df = 4, P = 0.301, Fig. 1).
d old and breeding successbeing the percentageof sucMean 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 wheremorethan one nest

was found in the same or in different years, but where
only one pair nestedeach year.
Stepwiselogisticregressionanalysis(Norusis1993) was
employedto analyzeamongyear differencesin the probability of territorial pairs laying eggsand of eggshatching. Logisticregressionusesa linear combination of independent variables to explain the variance of adependent dichotomousvariable. To meet the assumptionsof normality,NNDs were alwayslog transformedprior to parametric tests.Means are given with _+1 SE. All
testswere two-tailedand statisticalsignificancewasset at
P <

0.05.

RESULTS

Density and Nest Dispersion.The number of ter-ritorial pairsincreasedsteadilyfrom 27 in 1992to= 24). NNDs ranged from 60-690 m for colonial
pairs and from 450-4250 m for solitarypairs. Mean
NND of solitarypairs did not increasethrough theyears(Kruskal-WallisX•= 1.95, df = 3, P = 0.583,
Table 2). For three large colonies (A, B, and C,
Table 2), we tested the effect of year and colony
on nest spacingby meansof a two-wayANOVA: theinteraction of year and colony was almost signifi-

cant (partialFs,9s= 1.86,P = 0.076) and bothyear
and colony significantlyaffectedmean NND independently (respectively:partial F4,98: 11.01, P =0.000; partial F2,9s= 20.78, P = 0.000). Variationin NND within colonies among yearswas mainly
caused by the progressiveincrease in territorial
pairs in two colonies around the lake. New pairs

210

SERGIOAND BOTO

VOL. 33, NO. 3.', !

Colonysize•/ ilarge
(5-13
pr)
•

=•
1993

1992

1994

small
(2-4pr)[• solitary
pairs

1996

1995

Year

Figure1. Numberof BlackKitepairsbreedingsolitarily
or in small(•4 pairs)andlarge(-->5pairs)colonies
between
1992-96 in the Italian pre-Alps.

Table 2. Mean (_SE) BlackKite nearestneighbordistancesfor 24 solitarypairsand at sevencoloniesin the Italian
pre-Alpsbetween1992-96.
ME•'• N•U•ST
NEST TYPE

1992

Solitary

NEIGHBOR D•ST•CW (m)

1993

1994

1995

1996

1992--96

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(7)• 0.88•

373 - 64(8)a 0.67b

288 - 49(9)• 0.64•

252 - 28(12)• 0.79•

241 - 33
(13)• 0.68•

295 - 20(49)• 0.67•

ColonyB292 - 12(7)• 0.96•

169 +__
24(8)a 0.67•(7)• 0.89b

167 - 15(9)• 0.87•

150 - 25(8)• 0.67•

183 - 10(39)• 0.78b

ColonyC

679 - 24355 - 51

290 - 23237 - 44174 _ 23

307 - 34(3) • 0.10•(5)• 0.85b(5)a 0.75•(7) • 0.83•(25)• 0.59•(5) • 0.95b

ColonyD

597 _ 27597 - 27567 - 37

567 - 37433 _ 123552 - 29(3) • 0.99•(3) • 0.98•(3) • 0.98•(3) • 0.74•(15)• 0.92b

317 _ 45(3) • 0.93•

640 +__0(2) •640 _ 0(2)•

640 +--0

500 - 44(12)• 0.83•(3) a 0.99•

ColonyE

401 _ 3(3) • 0.10•(2) •ColonyF

213 - 10(3)• 0.99b

ColonyG
All nests

206 ---0

206 - 0

(2) •

(2) •

(2) •

403 _+0 (2)208 - 3(9)a 0.99b

403 _+0 (2)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.

206 - 0

SEPTEMBER 1999

BLACI• KITE BREEDING ECOLOGY

settlingin thesecoloniesdid not place their nests
in the outer parts of coloniesor far awayfrom traditional pairs, but selected areas near alreadyoccupiednests.Thus, meanwithin-colonyNND decreased as the number of pairs in the colonies
increasedthough the years(colonyA: F4,44= 2.64,
P = 0.046;colonyC: F4,20= 11.29,P = 0.000, Table2). Mean NND did not vary significantlywith timein colony C, where the number of pairs remained

211

place in 84 different nest structures,each one occupied for one to five consecutiveyears (• = 2.1_+0.1, N = 84).

Of these nests,58% were placed in treeswhile
the remainder were on cliffs (N = 84). Of 49 tree
nests, 39% were in sweet chestnut, 33% in sessile

or downyoak,18% in locust,6% in Europeanhophornbeam, 2% in common lime (Tilia europaea),
and 2% in Scotchpine (Pinussilvestris)
trees.The

stablethroughthe years(F4,34= 2.07, P = 0.106). 35 cliff nests were placed on bare rock ledges(29%) and at the base (54%) or in the canopy(17%) of trees,generallydownyoaks,growingon

Due to this, NNDs are shown separatelyfor eachcolony in Table 2.
The G-statisticdeclinedprogressively
for solitary
pairs through the years,indicating a regular dispersion of nest sitesbetween 1993-95, but not in1996 (Table 2). Pooling years, the overall dispersion of solitarynestswasnot regular (G = 0.540),
probablybecauseof the progressive
increasein themean NND of solitary nesters over time. Instead,the G-statisticindicated a regular dispersionof nest
siteswithin 22 of 23 colonieswith ->3 pairs.
Nest Sites. All territorial pairs built nests (N =175 pairs in 48 nest areas).After settlingon territories, most pairs brought nest material to one
nest,but 13 pairsin six nestareasbrought material
to two nestsduring the beginningof the pre-laying
period, before selecting the nest in which they
eventually laid eggs. Altitude of the nest sites
rangedfrom 240-870 m, averaging515 m (+14, N= 175). The 175 certsused
territorial nestingstook

cliff faces. Many pairs used nest structuresoriginally built by other species,to which they added
new material:23% of 84 occupiednest structures
were originally built by Common Buzzards (Buteo
buteo)and 5% were old Raven nests.Some nests
were being used by the original occupantswhen
they were taken over by BlackKites.Buzzardnests
were taken over during the original owners' prelaying period and one Raven nest still contained
nestlingswhen it was occupied by kites, which
preyed on the chicks before starting to add newmaterial

to the nest.

Breeding Season.There were no year-to-yeardifferences in mean laying date (F4,40= 0.68, P =0.762, Table 3). Laying dates ranged from 12
April-8 May, averaging25 April (Table 3). No casesof replacement clutchesin the same nest were observed after breeding failures, even when these

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

VARIABLE

Laying date in April

Territorialpairsb% reproductive

1992

1993

1994

1995

1996

24.6 - 1 (7) • 23.3 -+ 2 (7) a 25.5 + 2 (8) • 28.7 - 3 (9) • 25.3 + 2 (14) a

1992--96

25.6 + I (45) a

17

31

31

33

31

143

Pairs (N)Clutch size (N)

100 (17) •76 (21) •86 (22) •86 (21) •74 (19) •84 (100) •2.5 + 0.2
(10) •

2.5 _+ 0.2(11) •

1.8 + 0.3(8) a

1.8 + 0.4(5) •2.6 +_ 0.2(8) a2.3 + 0.1 (42) •% hatchingsuccess% successfulpairsYoung fledged per

68 (25) c

89 (27) c

79 (14) c

89 (9) c

100 (21) c

84 (96) c

71

58

48

42

61

55

territorial pair
Young fledged per
reproductive pair
Young fledged per
successfulpair

1.06 + 0.2
1.00 + 0.2

1.16 _ 0.2
1.24 _ 0.2

0.71 + 0.2
0.86 + 0.2

0.67 + 0.2
0.76 +_ 0.2(16) •(21) •(21) a(21) a1.50 ___0.2

2.00 + 0.2

1.47 _ 0.2

1.57 +_ 0.2(12) a(18) a(15) a(14) aSamplesize (when different from that in column "territorial pairs").
Those checkedfor nestingsuccess.
Number of eggs.Samplesizeof nestssameasfor clutch size.

0.97 _ 0.1
1.32 _+ 0.2
1.72 _ 0.3 (18) • 1.11 _+0.1 (97) •2.16 + 0.2 (19) • 1.78 _ 0.1 (78) •212

SERGIOAND BOTO

VOL. 33, No. 3

Table 4. Diet of breeding Black Kitesin the Italian pre- ble 3). On the whole, 84% of 96 eggs checked
Alps (1992-97), as estimatedby food remainscollected hatched. Stepwise logistic regression analysis
undernests.

showeda significanteffect of year on probabilityof egg hatching (B = 0.65, Wald = 5.88, df = 1, P
NUMBERPREYCATEGORYOF

ITEMS(%)

Fishes190 (61.9)

Rudd (Scardiniuserythrophthalmus)
Chub (Leuciscus
cephalus)
Unidentified Ciprinidae
OthersaB•rds
Blackbird (Turdus merula)
Othersb
Unidentified Passeriformes
Mammals
Unidentified Microtidae

59 (19)56 (18)37 (12)38 (12)85 (27.7)
20 (7)
65 (21)20 (7)
24 (7.8)
15 (5)

Othersc

9 (3)Reptiles'•Amphibianse

5 (1.5)1 (0.3)Invertebratesr

2 (0.7)= 0.015, reclassificationrate -- 84%).

The number of young•fledged per territorial
pair averaged0.97 (N = 143) and differed significantly among years(F4,158= 2.49, P = 0.045). Productivitywas significantlyhigher in 1996 than in
1994 and 1995 (Duncan's Multiple Range Test, P• 0.05, Table 3). Significantamong-yeardifferences alsowere detectedfor mean number of fledged
young per reproductive pair (F4,92= 2.68, P =0.036). Again, average values were significantly
higher in 1996 than in 1994 and 1995 (Duncan'sMultiple Range Test, P • 0.05, Table 3). Mean
number of fledged young per successfulpair was

1.78 (N -- 78) and alsovariedamongyears(F4,73= 3.69, P = 0.009), with more young fledged on
averagein 1996 than in 1992, 1994, and 1995 and
Total
307
in 1993 than in 1994 (Duncan's Multiple Rangea 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(Lepomis
gibbosus,
N = 4), large-mouth black bass(Micropterus
salranged from 42% in 1995 to 71% in 1992,with notooides,N = 2).
differences(X2 = 4.98, df =bIncludes:RockDove (Columba
livia,N = 11), Starling(Sturnus significantamong-year
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(Pernisapivorus,
N = 2), TawnyOwl (Strix
and colony on mean number of fledged young in
aluco,N = 2), Hooded Crow (Corvuscorone
cornix,N = 2), SongThrush (Turdusphilomelos,
N = 2), Moorhen (Gallinulachloropus, three colonies(A, B, and C) by meansof a two-way
N = 1), Collared Dove (Streptopelia
decaocto,
N-- 1), Barn Swallow ANOVA. The effect of the interaction betweenyear

and colony on mean number of young fledged per
territorial pair wassignificant(partial Fs,• = 3.01,
P = 0.005), but only approachedsignificancewhen
using mean number of fledged young per reproductive pair or per successfulpair as the dependent 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).
cases,colony was the only significant main effect
f Includes:Cetonia
spp. (N = 2).
(respectively:
partialF2,oo= 3.32, P = 0.042;partial
F2,42: 3.54, P = 0.038). These patternswere mainhappenedearlyin incubation.All nestlingsfledged ly caused by the productivity of colony C, whereby 15 July and all Black Kites departed the study fledging successremained similar to that in otherarea by 10 August each year.
coloniesuntil 1995, when no young were raisedin
Productivity.Each year, 74-100% of territorial any of the eight nestschecked. In 1996, only one
pairs laid eggs (Table 3), with no clear trend over young was raised in sevennestschecked in colony
time. Year did not enter a stepwiselogisticregres- C. The reasonfor such a dramatic decline in prosion model with laying or nonlaying of eggsas a ductivitywas not clear.
dependent variable. The overall mean number ofOverall, causesof breeding failure were seldom
laid eggs (i = 2.3, N = 42) varied significantly identified: 89% of failures occurred during incuamongyears(F4,37; 2.75, P = 0.043) and wassig- bation (N = 19) and the rest occurred during thenificantly 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(Hirundo rustica, N = 1), House Martin (Delichonurbica,N = 1),

Chaffinch (Fringillacoelebs,
N = 1), Great Tit (Parusmajor,N =1), Reed Bunting (Emberizaschoeniclus,
N = 1).
cIncludes:dormouse(Myoxusglis,N = 3), commonmole (Talpa
europaea,
N = 3), rabbit (Oryctolagus
cuniculus,N = 2), unidentified mammal from slaughterhouse(N = 1).
a Includes:Aesculapian
snake(Elaphelongissima,
N = 2), uniden-

SEPTEMBER 1999

BLACK KITE BREEDING ECOLOGY213

Table 5. Nesting densityof Black Kite populationsin Europe, 1966-96.
NEAREST
NEIGHBOR
DENSITYDISTANCE

HABITAT(N)(pr/100km9)(N)

Grasslandand
marshland

21-45

700-1500

AREA(PERIOD)
Matas Gordas,Spain(1987-89)

REFERENCE

Vifiuela

et al. 1994

Sermet1980

Hiraldoet al. 1990

Rh6ne Plain, France(1970)
Dofiana, Spain (1981-84)

River plain
Marshland

140

609

80

266.6

206 (47) a

Lac Leman, Switzerland

Henrioux(1975-90)

andHenrioux

Farmland and lake

319

100.6

1995

Farmland and lake

337

69.6

Sermet66

44.6

Thiollay 1967

27-41

24-38

441 (175)

This study

16

3.3.3

103 (16)

De Giacomo

et al. 1993

25-30

18.5-22.2

Heckenroth

1970

Bocageb

21

9.5

Farmland

8

7.02

ture

42

4.9

Petretti

Farmland

215

0.74

Fiuczynski 1981

Neuchfitel, Switzerland(1968)

Lorraine, France (1966)

1980

Woodlandand pasture

Lake Lugano, Italy (199296)

Woodland and lake

Castelporziano,Italy (199192)

Woodland andfarmlandLake Constance,Germany
Farmland(1968-69)
Limousin, France (197678)

and lake

Nore

1979

Dr6mling, Germany (199394)

2330 (8) cSeelig et al. 1996

Woodland and pas-

Monti della Tolfa, Italy(1973-80)
Brandenburg, Germany(1979)and Petretti

1981

Estimate from Bustamante and Hiraldo (1990), for the period 1985-88.

Pastureand woodland enclosedby stonewallsor hedges.
Calculated from the published map.

consequentdesertion (three cases),predation (twocases) and disturbance by rock climbers (onecase).
Diet.

Fish and birds

counted for 61.9%

dominatedthe diet

and ac-and 27.7% of 307 identified

prey items, respectively(Table 4). Mean length of
15 fish found uneaten in nests was 18.6 cm (-+1.8,

range = 7-30 cm). Of 54 avianprey items, 26%
were juveniles.
DISCUSSION

The number of territorial pairs of Black Kites
increasedsteadilyduring our study.It is difficult tosaywhether this was causedby a real population
increaseor a simple population fluctuation. Spatial
and temporal variations in Black Kite population
density and productivity have been reported bymany authors (Fiuczynski and Wendland 1968,

Vifiuetaet at. 1994,Doumeret 1995,Bijtsma1997).

Resultsof the analyseson NND and fledging successin the three large colonies showed that eachcolony could be considereda separateunit within
the population. Each colony was characterized byits own nest spacing distance,which tended to be
regular, but which differed from that of other colonies and sometimesvaried from one year to thenext. Similarly,productivityvaried among colonies
within and among yearswithin each colony.These
resultsemphasizedthe importance of monitoring
populations composedof more than just one colony for as many yearsas possibleto obtain an unbiasedestimateof density,nestdispersion,and productivity.To date, many studieson Black Kiteshave
been conducted in just one colony or in areas ofhomogeneoushigh density that are functionallysimilar to one large colony (e.g., Desai and Mat-hotra 1979, Kogaet at. 1989,Vifiueta et at. 1994).
Overall, the population showedremarkable op-

214
Table

SERGIO AND BOTO
6.

VOL. 33, NO. 3

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

AREA (PERIOD)

Dofiana,Spain (1987-89)
Germany (1992-95)
Nagasaki,Japan (1983-86)
L•mousin, France (1976-78)
Berlin, Germany (1940-79)
Lake Lugano, Italy (1992-96)

HABITATMarshland

N

CLUTCH

SUCCESS a

BREEDING

SIZE(EGGS)SUCCESS

166
599

2.18 (28) b

79% (61)b

143

2.29 (42) b

84% (96) b

162

2.98 (44) b

Fishing port
Bocaged

32
22

Farmland215

Woodland

and

92%
79%
75%
68%
62%
55%

lake

Slovakia (1975-89)
Lac Leman, Switzerland

Farmland and(1975-90)
Lorraine, France (1966)

lake
Woodland and

165

2.25

New Delhi, India (1973-76)

pasture
Urban

66
45

2.26 (45) b
2.3 (60) b

55% (102) b

Data on hatchingsuccessnot 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).
Samplesize (when different from that in column "N").
Data also from Mammen and Stubbe (1995, 1996).

Pastureand woodlandenclosedby stonewallsor hedges.

portunism and elasticity.First,variabilityin the extent of coloniality between and within years wasparalleled by high flexibility in NNDs, the highest
observedNND being 70 times greater than thelowest one. Interestingly, as the population increasedover the years,new pairs settlingin largecoloniespositionedtheir nestsnear existingpairs,
possibly as a result of conspecific attraction(Stamps 1988), as wasalso reported by Vifiuela etal. (1994). Advantagesof nesting near traditional
older pairs (Vifiuela 1993) could include local enhancement (Hagan and Walters 1990), information parasitism(e.g., Green 1987) and greateropportunities for kleptoparasitism (Vifiuela et al.
1994). The opposite trend was recorded for solitary pairs whoseNNDs increasedas new pairs colonized the area, suggestingpronounced overallvariationsin individual pairs nest dispersion,probablyin relation to food distribution(Newton 1979,

which is very common all over the Italian pre-Alps,is very rare elsewhere in the Black Kite's range(Cramp and Simmons 1980) and only locally reported in Sicily (Massa1985), in an area of central

Spain (Blanco 1997) and near someSwisslakes(Sermet 1980).

Finally, diet wasdominated by fish and birds ona numeric basis. However, all vertebrate classesandsome occasionalinvertebrateprey were also represented. This finding agreed with the common
definition of Black Kites as opportunistic feeders
with local specializationin the most availableprey(Delibes 1975, Arroyo 1978, Vifiuela and Veiga
1992, Blanco 1997).
Despite the overall opportunismand adaptability, productivityestimatesfor our studypopulation
were remarkablylow when comparedto other published estimatesin Europe,Japan and India (Table
5). Nesting densitiesacrossEurope 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 landof 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 beenin isolated trees and on large (>100 m high) and reported by many authors(Heckenroth1970, Desmall (<20 m high) cliffs.Nestsoriginallybuilt by libes 1975, Hiraldo et al. 1990, Henrioux and Henother species,especiallyCommon Buzzards,also rioux 1995). Densityin the Italian pre-Alpine area
were occupiedreadily and taken over by persistent was much lower than that of populations nearharassmentof the original occupants.Cliff nesting, large wetlands (Sermet 1980, Hiraldo et al. 1990,

SEPTEMBER 1999

Table

6.

BLACtC KITE BREEDING ECOLOGY

Extended.

MEAN NUMBER

OF FLEDGED YOUNGTERRITORIAL

BREEDING

SUCCESSFUL

PAIR

PAIR

PAIR

1.77
1.63
1 00

1.14 (28) b
1.32

1 2O

O.97

215

1.1 (95) b

REFERENCE

1.92 (153) b
2.07 (471)b
1.33 (24) b
1.93 (15) b
1.90 (133)b
1.78 (78) •'Jones and Manez 1990
Gedeon1994 cKoga et al. 1989
Nore

1979

Fiuczynski1981
This study

2.31

Danko

2.O2

Henrioux

1989

and Henrioux

1995

1.32

1.58 (55) •Thiollay 1967

0 98

Desai

and Malhotra

1979

es of low natality could be low food availabilityor
It was similar to that along Lake Constancein the water pollution and consequent prey contaminaof water and fish con1960sand in low intensityfarmland with scattered tion. DDT contamination
wetlands(Heckenroth 1970, Thiollay 1967, Seelig tamination was reported recently in nearby Lakeet al. 1996), but higher than that in farmed or pas- Maggiore (Ceschi et al. 1996). Even though laketure habitats with few wetlands (Nore 1979, Fiuc- euthrophication and pollution could benefit thiszynski1981, Petretti and Petretti 1981). Thus, the speciesby providing dead or moribund fish (Gerobserved densitywas intermediate, but low, com- oudet 1965, Bijeleveld1974, Doumeret 1995), pespared 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 successalsowascom- decline in the long term. The aim of current andparable 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, popdata 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 in1979). In contrast, the percentage of successful
water and fish prey. There is need for studiesaspairs and mean number of fledged young per tersessingthe level of water contamination above
ritorial, reproductive, or successfulpair were the which toxic chemicals concentrations
start to belowestever recorded in Europe and similar only tothosein India and Japan. Reasonsfor suchlow na- harmful, both for Black Kites and their fish prey.
talitywere unclear.During the 1960s,trapping and More published estimatesof density and producpersecutionwere common practice in the county tivity, especiallyhatching success,are also neededof the study area. For example, Bianchi et al. from other European countries.
(1969) reported gamekeepers killing 22 Black ACKNOWLEDGMENTS

Vifiuela et al. 1994, Henrioux and Henrioux 1995).

Kites in one month in a 1-kin 2 area. However, thispractice now is rare and we observedno casesof
nest robbing, trapping, or persecution.Clutch andnestlingpredation was minimal, as important potential predatorssuchas Goshawks(Accipitergentilis) and Eagle Owls (Bubobubo)were very scarcein
the studyarea.Thus, the remainingpotentialcaus-

We thank G. Bogliani for continuous encouragement
during eachphaseof the researchand G. Bogliani,C.M
Perrins, M. Henning, P.G.W. Salaman,K. Steenhof, andD.E.

Varlandforconstructive

comments

on a first draftof the paper. We thank L. Marchesi for identification ofavian prey remains. Part of this studywas funded by Regione Lombardia, ServizioFaunistico.

216

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Received29 August 1998; accepted3 April 1999