The Auk 115(4):955-963, 1998

FOREST SIZE AND ISOLATION HAVE NO EFFECT ON REPRODUCTIVE SUCCESS OF EURASIAN NUTHATCHES (SITTA EUROPAEA)

ERIK MATTHYSEN 1 AND FRANK ADRIAENSEN Departmentof Biology,University of Antwerp,B-2610 Antwerpen, Belgium

ABSTRACT.--Wecollected data on breedingsuccess, nestling mass, fledging date, and re- cruitmentof EurasianNuthatch (Sitta europaea) nesting attempts in a numberof small (<30 ha) forestfragments and parks,as well asin two larger(>200 ha) forests.Most study areas were dominatedby well-developedoak stands.We foundno differencesin reproductivepa- rametersbetween fragments and the two largeforests, nor any relationshipwith the sizeor degreeof isolationof individual fragments.Failed nests more often were takenover by nest competitors(particularly starlings) in fragments,but this did not seemto affectoverall suc- cessrates. Pairs nesting in parkshad a lowerchance to producerecruits than pairs in similar- sized oak fragments,and early broodsrecruited more offspringthan late broods.We con- cludethat fragmentation does not affect the suitability of matureoak stands for reproduction of nuthatcheswithin the sizerange of oakstands frequented by thisspecies. Received 14 July 1997, accepted12 March 1998.

FRAGMENTATION OF NATURAL HABITATS is a effectsof habitatfragmentation on fitnesspa- potential threat to the persistenceof animal rameters.The prominentexception is the re- and plant populationsin many different kinds productive success of open-nesting birds, of landscapesand habitat types (Saunders et al. whichgenerally decreases in smallerfragments 1991,Andr•n 1992,Robinson et al. 1995).Many and/or nearhabitat edges owing to increased studieshave demonstrateda lower diversity predationor brood parasitism(Gates and Gy- and/or abundanceof various organismsin sel 1978,Brittingham and Temple 1983, Moller habitatfragments compared with largerhabi- 1988, Kurki and Linden 1995, Robinson et al. tat tracts (e.g. Soul• 1986, Opdam 1991, Mat- 1995). This pattern has been verified experi- thysenet al. 1995b).Two majorgroups of hy- mentallyby the useof artificialnests in a wide pothesesto explain these patterns involve variety of habitats(Andr•n 1992, Santosand changesin populationstructure and habitat Telleria 1992, Burkey 1993, Nour et al. 1993). quality, respectively.Changes in population However,nest predation and broodparasitism structure include a reduction in population are unlikely to provide a generalexplanation size,diminishing dispersal between patches or for the effectsof habitatfragmentation on bird betweenlocal populations within a metapopu- speciesthat nest in moreprotected sites such as lation,and loss of geneticvariation (Gilpin and tree cavities.Reproductive success of cavity- Hanski1991). Changes in habitatquality may nestingbirds may be influencedby otherfac- resultfrom the increasinginfluence of abiotic tors suchas changesin competitionfor nest andbiotic elements from the surrounding land- sitesor in food availability,but very few studies scape(the matrix) on the habitatremnant, or haveaddressed reproductive success in forest from changesin communitystructure within fragments(Kuitunen and Helle 1988,Tjernberg the habitat(e.g. abundance of prey,parasites, et al. 1993, Nour et al. 1998). predators,competitors; Soul• 1986,Saunders et The EurasianNuthatch (Sitta europaea)ap- al. 1991).Consequently, fitness parameters and pearsideally suited to testwhether forest frag- ultimatelypopulation growth may be negative- mentationaffects reproductive success of a cav- ly (or positively)related to the natureof adja- ity-nestingbird species.The effects of fragmen- centhabitats as well as the size,shape, and de- tation are well documentedat the population greeof isolationof the remainingpatches. level but the underlyingcauses are not re- Relativelyfew studieshave documented the solved. Nuthatches have been shown to be less abundant in small and isolated forest frag- E-mail:[email protected] mentsand in that respectare amongthe most

955 956 MATTHYSENAND ADRIAENSEN [Auk, Vol. 115 fragmentation-sensitiveforest bird species (Opdamand Schotman 1987, van Dorp andOp- dam 1987,Enoksson et al. 1995,Matthysen un- publ.data). These negative effects have gener- ally been attributedto lack of dispersal,or to highermortality during dispersal among frag- ments(Verboom et al. 1991,Enoksson et al. 1995,Matthysen et al. 1995a,Matthysen and Currie1996), but effects of resource availability (nestsites and/or food) have not been studied. Inthe breeding season, nuthatches aregener- alistinsectivores that mainly glean and probe for arthropodson bark andleaves. They are ob- ligatesecondary cavity nesters, and competi- tion for suitable nest sites is well documented (e.g. Nilsson 1984). Here, we report on repro- o ductive success of nuthatches in a set of forest o 5 lO 15 28 and parkland fragmentsof varying sizes, as Size (ha) well as in two larger forests. FIG.1. Sizedistribution of forestfragments with at leastone observationof EurasianNuthatches (n = METHODS 38), excludingone large (30 ha) fragmentwith 16ob- servations (see text). Study speciesand areas.--In western Europe, Eur- asian Nuthatchestypically occur at relativelylow densities(one to five pairsper 10 ha) in maturede- ciduouswoodland. They normally raise a single large, degree surrounded by agricultural land, broodper yearin a naturalcavity or old woodpecker whereasthe otherhalf of the parkswere largely sur- nest. Pairs defend the same territory year-round roundedby residentialareas. Only a few fragments (Matthysen1990). Most of the data were collected were connectedby hedgerows,and thesewere usu- from 1990 to 1994 in a 200-km2 study area closeto ally only onetree-width wide. Nestboxes of various Antwerp, northern Belgium. The study area con- types and sizeswere presentin only a few areas. tained scatteredforest patchesor parkland areas Fragment sizes as measuredfrom aerial photo- with extensivewooded cover (henceforth"frag- graphsvaried from 0.8to 30 ha (Fig. 1). Adjacentgar- ments")that togethercovered only 1.5%of the area dens with scatteredtrees were not incorporatedin (seeMatthysen et al. 1995a).Additional data were this measure, and lanes connected to a woodlot were collected in two study plots within large forests only incorporatedif at leastthree or four treeswide. (>200 ha; seebelow). For this study,fragments were Becausemost fragments were small (Fig. 1), sizes definedas discretepatches of a singlehabitat type were logarithmicallytransformed to obtaina normal ("oak forest"or "park") separatedfrom similarhab- distribution for statistical analysis (Kolmogorov- itat patchesby at least100 m (typically•-500 m). Us- Smirnov test, untransformed P = 0.02, transformed ing this definition,we neverobserved color-banded P > 0.9). On average,parks were somewhatsmaller individuals to include more than one fragment in (;• = 4.6 ha, range1.5 to 14 ha) than oak stands(;• = their homerange. A few patcheswith nuthatchesbut 6.9 ha, range0.8 to 28 ha), but the differencewas not belongingto other foresttypes were not considered significant(Mann-Whitney U-test, z = -0.97, P > in this study.All oak forestfragments were well-de- 0.3). The degreeof isolation(I) from othernuthatch velopedstands (>20m canopyheight) of common habitatwas calculated by a modifiedversion of Whit- oak (Quercusrobur), with at most 10% other trees comb et al.'s (1981) gravitationindex, calculatedfor suchas Europeanbeech (Fagus sylvatica), birch (Bet- eachfragment as: ula spp.), and red oak (Quercusrubra). Park areas I = -log •, (T,/d?), (1) were mostly old private estates,often turned into public parks, with a mixture of woodland, ponds, whered = distancefrom the focalfragment to the ith lawns and paved areas.The wooded vegetationin fragment(including all fragmentswithin the 200- theseparks consistedof large old trees,often •-30 m km2 study area and the 2-km zonearound it), and T tall, mainly oak and beechbut alsoornamental trees = the maximum number of nuthatch territories in suchas lime (Tiliaplatyphyllos), horse chestnut (Aes- the ith fragmentduring the study.We used number culushippocastanum), and someconifers. All of the of territories rather than size because this allowed us oak fragmentsand abouthalf of the parkswere to a to integratedifferent types of habitatsupporting dif- October1998] BreedingSuccess ofNuthatches 957 ferent population densities.In particular, densities per fragment,with the exceptionof the largestfrag- in parks were 50% higher than those in oak frag- mentZR. In this30-ha fragment, of which16 ha were ments(Matthysen unpubl. data), which may be ex- studied, 28 nesting attempts were monitored; ZR plained by differencesin canopyvolume, tree-spe- was treated separatelyin all analyses. ciesdiversity, or other factors.The number of terri- Most nests were observed only from a distance. toriesin both habitattypes was a simplelinear func- Breeding stage was inferred at least every 10 days tion of size and was not related to isolation basedon parentalbehavior (i.e. nestbuilding, incu- (Matthysenunpubl. data). Logarithmictransforma- bation, feedingnestlings and fledglings).Nests were tion was used to obtain a normal distribution (Kol- scoredas successfulif they producedlarge nestlings mogorov-Smirnov test, untransformed P • 0.001, or fledglings, or unsuccessfulif they failed before transformed P > 0.5). producing large nestlings.For each successfulnest, In additionto the fragments,we studiedtwo plots the range of possiblefledging dateswas estimated of about28 ha within largerforests: Peerdsbos (PB; using the following assumptions(Matthysen 1998): >200 ha, data collected1991 through 1994), and incubationor brooding (on eggsor small young) oc- Meerdaalbos(MB; ca. 1,500 ha, data collected1992 curred until the young were at most 10 days old; through1994). These forests are located15 km north nestlingswere fed for a maximum of 25 days, and and 40 km southeastfrom the fragments,respective- were not seenin the nest entranceuntil 3 daysbefore ly. Both study plotswere situatednear the edgeof fledging;and fledged young stayedwith their par- the forestbordering on agriculturalland. The plots ents for no more than 20 days. If this range couldbe were dominated by oaks (>80%), and only nests narrowed down to 10 days or less,the midpoint was within oak stands were studied. taken as the best estimateof fledging date (n = 57). In all oak forests we measured trunk circumfer- A subsampleof nests(n = 88) wasinspected when ence at breast height of 20 common oaks and con- the youngwere suspectedto be between10 and 18 vertedthis to diameteras an indexof habitatquality. days of age.Nestlings were counted(i.e. brood size), This measurement was chosen both for ease of mea- aged (Winkel 1970), individually color banded, and surementand becauseoak standswere very similar weighed(in 1993 and 1994only). In 11 broodsan ex- in other respectssuch as canopycover, tree-species act count was not possible.Young were assumedto composition,and understory.Trees were chosenar- fledge at 23 days (Matthysen1998) unless fledging bitrarily by tossinga pencilin the air at roughlyequi- was observeddirectly. Each spring and summer distant intervals along an arbitrary transect, and most fragmentswere searchedfor color-banded measuringthe nearesttree at about 15 m in the in- nestlings(Matthysen et al. 1995a).Recruitment was dicated direction that had a circumference of at least defined as the probabilitythat at leastone nestling 50 cm. Mean trunk diameterswere larger in large from a successfulnest was observedagain at least foreststhan in fragments(œ = 55 vs. 46 cm;F = 4.7, one month after fledging, i.e. after the dispersalpe- df = 1 and 14, P = 0.047),but theranges overlapped riod had begun. completely(large forests,53 to 58 cm; fragments,37 Data analysis.--Variationin reproductiveparame- to 60 cm). Habitatquality wasnot estimatedin parks ters was studied on two levels. On the first level, we becauseof the large variation in tree species,tree analyzed variation among five categoriesof study size, and other characteristics that were difficult to area:oak fragments,park fragments,the 30-hafrag- assess(i.e. disturbance,degree of canopy cover, ment (ZR), and the two large forests.Because of the paved areas,lawns, etc.). smallsamples for brood size and fledglingmass, the Nestobservations.--Each spring we locatedas many threelargest forests were pooled for analysisof these nestsas possible.Nests were found by following fe- variables.On the secondlevel, we related reproduc- males during nestbuilding and maleswho were pro- tive parametersto characteristicsof individual frag- visioningtheir incubatingmates. A few nestswere ments,excluding the 30-ha fragment.These charac- found during the nestlingstage. Including renesting teristicswere habitat type (oak or park), fragment attempts, we found 8 nests in 1990, 61 to 68 from size (log transformed), and isolation as defined 1991 to 1993, and 43 in 1994 (253 neststotal). There above. For oak and park fragments where sample were no secondbroods. Only 16% of the nestswere sizes were sufficiently large, additional analyses in nest boxes. When we detected a nest failure, based comparedthe variablestrunk diameter (measuredin on nest inspectionor lack of parent attendanceon oak fragmentsonly) or matrix type (agricultural or successiveobservations, we searchedthe territory residentialin park fragmentsonly, becauseall oak againfor renestingattempts. We couldnot searchall fragmentsbelonged to the formertype). For the anal- fragmentsfor renestingattempts, but we conducted ysisof recruitment,we alsoincluded the distancebe- suchsearches without bias asto fragmentcharacter- tween the fragmentcenter and the centerof the en- istics. Causes of failure were not established, but we tire studyarea, because birds born in the periphery noted whether the nest was taken over by another are lesslikely to be foundwithin thestudy area (Mat- speciesat the first observationfollowing failure. We thysen et al. 1995a). We assumedthat nestswithin studied from 1 to 12 (median = 3) nesting attempts fragmentsprovide independentinformation relative 958 MATTHYSENAND ADRIAENSEN [Auk, Vol. 115

TABLE1. Reproductiveparameters of EurasianNuthatches in oakfragments, parks, and oak stands within largerforests (i.e. ZR, PB,MB). Valuesbetween brackets are samplesizes (in the two top rows,number of pairs or broods/numberof fragments).

Proportionof Proportionof Standardized Brood Fledgling broodswith Area pairs successful fledgingdate a size mass(g) recruits Oak fragments 0.69 (52/12) 0.8 (34/11) 7.2 (20/8) 24.6 (10/5) 0.35 (17/8) Parks 0.84 (25/5) -1.6 (51/16) 6.0 (35/15) 24.4 (9/4) 0.14 (37/16) ZR (30 ha) 0.79 (24) 0.03 (19) 6.2 (4) __b __ PB (300 ha) 0.86 (35) 0.9 (29) 6.8 (13) 23.4 (10) -- MB (1,500ha) 0.61 (23) 3.3 (16) 6.8 (5) __b __ Total 0.75 (159) -- 6.5 (77) 24.1 (29) 0.20 (54) Relativeto overallyearly mean. Pooled with PB. to the investigatedfragment variables.Also, we not vary between years or areas (logisticre- could not checkfor independenceof nestsby the gression,year effect:X 2 = 2.4, df = 3, P > 0.3; sameindividuals because only a sampleof themwas area effect: X2 = 8.0, df = 4, P = 0.09). Pairs in banded. However, because the total number of nests oakfragments had an intermediatesuccess rate per fragmentwas ratherlow (median= 3, different years combined), violation of these assumptions betweenthose in the two largestforests (Table would have only a limited effect. 1). Breedingsuccess, nest-site takeover, and recruit- We analyzedbreeding success in relationto ment were analyzedas 0/1 responsevariables in lo- fragment characteristicsfor oak fragments gisticregression models using GLIM (Crawley1993). only,because for parkareas both the number of For analysesinvolving only one independentvari- failures (n = 4) and the number of different able, we calculatedFisher exact probabilitieswith fragments(n = 5) wassmall. The initial model STATXACT.Breeding success was analyzedas suc- for oak fragmentsincluded year, three habitat cess(0/1) of a pair, either from a first or a repeat nest.For this analysiswe omitted a numberof areas parameters(size, isolation, trunk diameter), where we did not systematicallysearch for repeat and two-wayinteractions among the three hab- nests.Fledging date, brood size,and meanfledgling itat parametersonly. None of the variablesex- masswere analyzedwith PROCGLM usingtype III plained a significantamount of variation in sumof squares(SAS 1994), or with ANOVAif all in- breedingsuccess (Ps > 0.1).Because the effect dependentvariables were factorial.Fledging dates of isolationwas rejectedat P = 0.06 (X2 = 3.6 in werestandardized per year (seeResults). In bothlo- a model also containing size, diameter, and gisticregression and GLM, model selectionwas per- their interaction),we reenteredthis variablein formedby backwardelimination of leastsignificant terms starting from a model including all interac- eachsuccessive step of themodel selection, but tions, unlessspecified otherwise. In particular,in- it nevercame close to significance(all Ps > 0.2). teractionsbetween habitat parameters and year were The final modelwas slightlyoverdispersed (X 2 not tested in analysesof brood size, fledging mass, = 64.2, df = 51) but this doesnot affectthe con- or recruitmentwhere annual sample sizes were clusions. small. Lower-order terms were not eliminated as Nest-site takeover.--Failednesting cavities long asthey were incorporated in higher-orderterms weretaken over by EuropeanStarlings (Sturnus still in the model.All testparameters refer to the last vulgaris)in 11 casesand Great SpottedWood- modelthat includedthe parameteror interactionbe- peckers(Dendrocopos major) in 9 cases(Table 2). fore it was removed.All final logisticmodels were Because of the small number of failed nests in checkedfor overdispersion(Crawley 1993). Chi- squaretests have one degreeof freedomunless in- parks(see above), the analysis was restricted to dicated otherwise. oak fragmentsand large forests.Takeover by starlingswas more common in fragmentsthan RESULTS in the three large forestscombined (29% vs. 6%,respectively; Fisher exact test, P = 0.01;Ta- Breedingsuccess.--Overall, 120 of 159 of nut- ble 2). Takeoverby woodpeckersfollowed the hatch pairs (75%) successfullyraised large sametrend, but the differencewas not signifi- nestlingsor fledglings.Breeding successdid cant (19% vs. 9%; P = 0.3; Table 2). Within the October1998] BreedingSuccess ofNuthatches 959

TABLE2. Proportionof failed EurasianNuthatch not shown) revealedthat in oak fragments, nests(%) that were takenover by EuropeanStar- lingsor GreatSpotted Woodpeckers; n = number nestingwas earlier in small, nonisolatedfrag- of failures,with numberof fragmentsin parenthe- mentsand in largeisolated fragments, whereas sesfor oak fragmentsand parks. in park fragmentsthe reversewas true. In ad- dition, we found an interactionbetween frag- Takeoverspecies ment size and trunk diameterin oak fragments Star- Wood- (F = 5.2, df = 1 and 28, P = 0.03). Broods Area ling pecker None n fledgedearliest in smallfragments with small Oak fragments 29 19 52 31 (12) treesand in largefragments with largetrees. It Parks 0 0 100 4 (4) should be noted that when the interaction ZR (30 ha) 13 13 75 8 termswere deletedfrom the previousmodels, PB (300 ha) 0 10 90 10 MB (1,500 ha) 7 7 87 15 noneof the main effectsof habitattype, isola- Total 16 13 71 68 tion, fragment size,or trunk diameterwere sig- nificant(all Ps • 0.1). In the park fragments, however, there was an additional effect of ma- trix type on fledgingdate (F = 7.0, df = 2 and oak fragments,takeover by starlingswas not 46, P < 0.01).Broods in parkswithin residen- related to fragment size, isolation,or trunk di- tial areasfledged an averageof 5.3 daysearlier ameter(logistic regression, all Ps • 0.1;year ef- thanthose in parkssurrounded by agricultural fectsnot tested).Takeover by woodpeckerswas land.This effect remained significant even in a influencedby a size x diameter interaction model without interactions. term (X2 = 7.0,P < 0.01).Inspection of the data Broodsize.--Brood size varied from 2 to 10 (œ parameterestimates suggested that takeovers occurredmore often in large fragmentsif trees = 6.5, n = 77). The three largestforests were were large.However, this effectwas not very lumped in the analysis.Brood size varied robustbecause it disappearedwhen a single among areas(F = 4.2, df = 2 and 73, P = 0.02) nest in a very small fragment(0.8 ha) was re- andwith fledgingdate (F = 5.0,df = 1 and 73, moved (X2 = 2.1, P > 0.1). P = 0.03) but not amongyears (P > 0.5). Brood Fledgingdates.--Because area MB was not size was largestin oak fragments,followed by studied in 1991, we first verified that no area x larger forestsand parks (Table 1), and it de- yearinteraction was present in the 1992to 1994 creasedwith time by 0.08 + SE of 0.03 fledg- data (two-way ANOVA, F = 0.7, df = 8 and 84, lings per day. Mean brood size differed be- P • 0.5). A secondtest, without an interaction tweenoak and parkfragments, but neitherdif- term, on the four yearsof data showedthat fered from brood size in large forests(Tukey fledgingdates varied betweenyears (F = 13.7, test, c• = 0.05). df = 3 and 130, P < 0.001) but not between ar- An analysison broodsize in relationto frag- eas (F = 1.8, df = 4 and 130, P > 0.1). For fur- ment characteristics(not includingthe three theranalysis, we recalculatedfledging dates as large forests)confirmed the effectsof habitat differencesfrom the overall yearly means. type (F = 7.7, df = 1 and 52, P < 0.001)and date Thesevaried from 25 May (1993and 1994) to 1 (F = 6.2, df = 1 and 52, P = 0.02) but revealed June(1992). Mean standardizedfledging dates no effectsof year, fragment size, or isolation per area are shownin Table1. (all Ps • 0.1). Accordingto this analysis,suc- Within the fragments(n = 85 broods),fledg- cessfulbroods fledged 1.5 more young in oak ing datewas related in a complexway to hab- fragmentsthan in parks when we controlled itat, isolation,and fragmentsize (three-way in- for theeffects of fledgingdate. The seasonal de- teraction, F = 11.3, df = 1 and 77, P = 0.001). cline in brood size was similar to that in the Whenthe analysiswas repeatedfor eachhab- completedata set (0.09per day).In the follow- itat type (includingextra variables,see Meth- ing analysesbased on habitattype, year effects ods), we found significantsize x isolationin- were no longer tested. teractionsboth in oak and park fragments,but In the sampleof oakfragments used to study with oppositesigns for the parameteresti- brood size, the three habitatparameters (size, mates (oak: F = 15.8, df = 1 and 28, P < 0.001; isolation, trunk diameter) were significantly park: F = 4.7, df = 1 and 46, P = 0.03). Inspec- correlated (r 2 between 0.22 and 0.7, all Ps • tion of the predictedresponse surfaces (details 0.05)and were thereforeentered one by onein 960 MATTHYSENAND ADRIAENSEN [Auk, Vol. 115 separatemodels. None of thesethree parame- the later stagesof model selection.Fragment tershad a significanteffect on brood size (all size and isolationalso had no effect(both X2 < Ps > 0.1), but the effect of fledging date re- 0.3, P > 0.5). The final model containedonly mained(P < 0.05).In the park fragmentsthere fledgingdate (X2 = 9.9, P < 0.01)and habitat was also a date effect (F = 6.1, df = 1 and 32, type (X2 = 6.3, P = 0.01). Broodsin oak frag- P = 0.02)and a significanteffect of matrixtype mentswere more than twice as likely to pro- (F = 6.2, df = 1 and 32, P = 0.02);nests in parks duce recruitsthan were thosein parks (Table surroundedby agriculturalland containedan 1), andbroods with recruitsfledged an average averageof 1.8 more nestlings(corrected for of 5.5 daysearlier than those without recruits. date)than thosein residentialareas. Again, we foundno effectsof fragmentsize or isolation(P DISCUSSION > 0.2).Note that in this analysis,no higher-or- der interactions were included in the initial Our mainobjective was to testthe hypothesis model. that habitatfragmentation reduces the repro- Mass at fiedging.--Meannestling mass was ductive successof a cavity-nestingbird. We recorded for 29 broods in 1993 and 1994 at es- found no evidence for reduced reproductive timated agesof 14 to 17 days. Body massdid output in fragmentsversus large forests,nor not vary with agebetween days 14 and 17 (F = for relationshipswith fragmentsize or isola- 6.2, df = 3 and 25, P = 0.09) and thus is con- tion. Nuthatchesin oak fragmentshad an in- sideredto representmass at fledging(f = 24.1 termediatesuccess rate compared with thosein g). the two largerforests and had relativelyhigh Becauseof the small samplesizes, the three brood sizes and fledgling masses,but no sig- largestforests were againlumped for analysis nificant differences were found. The mean (total n = 10 broods),and no higher-orderin- numberof youngper pair was very similar in teractionswere tested.Fledging mass did not oak fragmentsand parks (5.0 young/pair) and differ amongareas (F = 2.6, df = 2 and 25, P = largeforests (5.1 young/pair; three largest for- 0.1) or years (F = 2.0, df = 1 and 21, P > 0.1) estspooled). Fragmentation had no effecton and was not related to fledging date (F = 0.3, fledgingdate, which is importantgiven the ef- df = 1 and 24, P > 0.3) or brood size (F = 2.3, fect of date on the chance of recruitment within df = 1 and 27, P > 0.1).Within the fragments the study area. A premium on early fledging (19 broodsin 9 fragments),there was no rela- had been predictedbased on the severecom- tionshipwith year,habitat type, fragmentsize petition for summer territoriesvery soonafter or isolation,fledging date, or brood size (all Ps fledging (Matthysen1987, 1990) but is con- > 0.1). No further testswere doneon parksand firmedhere for the first time.The complexsta- oak fragmentsseparately. tisticalinteractions among the effectsof frag- Recruitment.--Withinthe fragmentareas (ex- mentsize, isolation, and habitattype on fledg- cludingthe 30-hafragment), 15 recruitswere ing date are difficultto interpret,but they do found from 11 differentbroods (Matthysen et not suggesta consistenteffect of fragmenta- al. 1995a). We performed a logistic regression tion. on the probabilitythat a brood producedat One couldargue that breedingperformance least one recruit, with the independentvari- is not a good indicatorof habitatquality be- ableshabitat type, log fragmentsize, isolation, causebirds may follow an "ideal free" distri- distancefrom the centerof the study area, bution among territories of different quality fledging date, and number of young banded within and between habitats (Fretwell 1972, per brood. Becauseof the small sample,we Matthysen1990) such that expectedfitness only includedfour two-wayinteractions, cho- doesnot vary with the averagequality of a hab- sena prioriin the initial model (habitatx size, itat. Accordingto this reasoning,the combi- habitatx isolation,habitat x fledgingdate, size nationof high breedingsuccess and low den- x isolation). sity in oak fragmentsmight imply that on av- Surprisingly,brood size and distancefrom erage,territories are poorer,but that only the the centerwere droppedin the early stagesof best territories are occupied.However, obser- model selection(both X2 < 0.2, P > 0.5) and re- vationson dispersaland territory establish- mainednonsignificant even when reentered in mentsuggest that the best territories are not al- October1998] BreedingSuccess ofNuthatches 961 ways selected(Matthysen and Currie 1996). areas.Dhondt et al. (1984)found a similarpat- Furthermore, the fact that nuthatch densities tern in Great Tits but not Blue Tits, and sug- are not correlated with the size or isolation of gested an effect of anthropogenicfood (i.e. individualfragments (Matthysen unpubl. data) seeds,bread, fat) that is morereadily takenby rules out any "ideal free" explanationfor the GreatTits, and alsoby nuthatches. lackof a relationshipbetween breeding success Our conclusionthat forest fragmentation and fragmentcharacteristics. does not affect the quality of oak forest as The only parameterthat differed between breedinghabitat agrees with thefew published fragmentsand largeforests was the probability studiesof other cavity nesters.Kuitunen and of nest takeoverby other species.Aggressive Helle (1988)found no relationshipbetween dis- takeoverby EuropeanStarlings is a common tanceto forestedge and nesting success, clutch cause of nest failure in Eurasian Nuthatches size, or laying date of EurasianTreecreepers (L6hrl 1956,1958; Nilsson 1984) and othercav- (Certhiafamiliaris). Although they did not con- ity nesters(e.g. Troetschler1976, Kerpez and siderfragment size, any changein thisparam- Smith 1990). Our observationsare not suffi- eter necessarilyleads to closer proximity to ciently detailedto distinguishbetween active edges.Nour et al. (1998)found no consistentef- evictions that cause brood failure, and take- fectsof fragmentsize on reproductiveparam- overs of previously abandonednests. Even eters of Great Tits and Blue Tits in forests sim- thoughit did not seemto affectthe overall suc- ilar to thosein ourstudy. Tjernberg et al. (1993) cessrate in fragments,the frequent occurrence found no differencein breedingsuccess or lay- of takeoverssuggests that competitionfor nest ing date of BlackWoodpeckers (Dryocopus mar- sites was intense.Indeed, starlingswere very tius)in a large forestand thosein an agricul- abundantin somefragments but ratherscarce tural area where each territory encompassed in thelarge forest plots. In a verydifferent hab- manysmall fragments. However, none of these itat, Kerpezand Smith(1990) found that prox- studiesinvestigated the effectsof size or iso- imity to agriculturalland increasedthe com- lation of a large numberof fragments. petition for nest sites between starlings and The contrastbetween the resultsfor cavity woodpeckers. nestersand open-nestingbirds is not entirely The differencesin reproductiveparameters unexpected,because cavities are better pro- betweenoak fragmentsand parks, although tected from predatorsand brood parasites.A unrelatedto habitat fragmentation,revealed potential cost of cavity nesting is increased that our studyis sufficientlypowerful to detect competitionfor nest sites,and our study sug- effectsof habitatquality. Broods in parkswere gests that such competitionincreases with significantlysmaller by 1.5 nestlings,which is fragmentation,even though the effect on breed- probablynot causedby variationin clutchsize. ing successappears to be limited. The above- In similar habitats,Dhondt et al. (1990) found cited studies on cavity nesters could not ad- considerable between-area differences in brood dress this issue, either because customized sizes (but not clutch sizes)of Great Tits (Parus boxeswere provided (treecreeper,tits) or be- major) and Blue Tits (Parus caeruleus).Food causethe focal specieswas a dominantcom- availabilitymight be lowerin parks,where in- petitor (woodpecker).Food availability does sectpopulations on exotictrees can be expected not appear to be lower in smaller or more iso- to be less diverse (Kennedyand Southwood latedfragments, as shownby largebrood size 1984), but this was not reflectedin lower nest- and high nestling body mass. This result ling masses.Broods in parks also were less agreeswith studieson food provisioningand likely to producerecruits. Again, food couldbe breedingsuccess in tits in similar areas(Nour an explanationthrough effects on predispersal et al. 1998). Previousstudies have reacheddi- mortality or on predispersalcondition affect- vergent conclusionson the effects of forest ing futuresurvival. Another possibility is pre- fragmentationon arthropod populations. Some dationon youngnuthatches by corvids,which found reductionsin abundanceor speciesdi- were moreabundant in parksthan in oak frag- versity (Faeth and Kane 1978, Nilsson and ments(Matthysen unpubl. data). A final result Ebenmann1981), whereasothers found no ef- is that parks surroundedby residentialareas fects (Morse 1977, Middleton and Merriam had earlierfledging dates than thosein rural 1983, Nilsson et al. 1985). 962 MATTHYSENAND ADRIAENSEN [Auk, Vol. 115

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