j. Field Ornithol., 66(2):231-235

CLUTCH SIZE AND FLEDGING RATE IN THE EURASIAN BREEDING IN NATURAL CAVITIES IS UNRELATED TO NEST CAVITY SIZE

VLADIMIR V. PRAVOSUDOV • Instituteof theBiological Problems of theNorth Academyof Scienceof Russia Karl Marx p•: 24, Magadan, Russia Abstract.--Clutchsize and fledgingrate of the EurasianNuthatch (Sitta europaea)breeding in natural cavitiesand their relation to nest cavityparameters were studiedin northeastern .The sizeof 25 natural cavitiesmeasured bore no relationshipwith either clutch size or fledging rate.

SIN RELACION EL TAMAI•O DE LA CAVIDAD Y EL TAMAgIO DE LA CAMADA Y LA TASA DE PICHONES QUE DEJAN EL NIDO DE SITTA EUROPEA Sinopsis.--Seestudiaron en la parte norestede Siberia,la relaci6n entre el tamafio de la cavidadutilizada para anidar pot individuosde Sitta europea,y el tamafio de la camada de •stosy el n6mero de pichonesque dejabanel nido. No se encontr6relaci6n entre el tamafio de 25 cavidadesnaturales y el tamafio de la camada o nfmero de pichones que dejaron dichos nidos.

For some hole breeding it has been suggestedthat clutch size could be affected by nest cavitysize (Karlssonand Nilsson 1977, L6hrl 1973). To date, however,the evidencehas been quite controversial.The relation between clutch size and the area of the cavityfloor was positive and significantin Marsh Tits (Paruspalustris, Ludescher 1973). For Pied Flycatchers(Ficedula hypoleuca)breeding in nest boxes, a reduction in the sizecorrelated with a decreasein clutch size,but conspecifics breeding in natural holes did not show any relationship between cavity sizeand clutch size (Alatalo et al. 1988, Gustafsonand Nilsson 1985, Slags- vold 1987). No significantrelationship between clutch size and cavitysize existed in European Starlings(Sturnus vuilgaris)breeding in nest boxes (Karlsson1983), but L6hrl (1987a,b) found that both larger floor areas and deeper cavitiescaused larger clutchesin box-nestingEurasian Nut- hatches (Sitta europaea).In open-nesters,birds with larger nestcupshad larger clutchesand greater breeding success(Slagsvoid 1989a,b, Slagsvoid and Amundsen 1992). It is still unclear why cavitysize should be a determinant of clutch size in hole nestingbirds. L6hrl (1973) suggestedthat better insulationof the clutch within larger cavitiesdue to a thicker layer of nest material allows a female to decreaseher energy expenditure for thermoregulation.She somehowanticipates such savings and, therefore,increases her clutch size (L6hrl 1973). Another possibleexplanation is that nestlingsin larger cav- ities have more spaceto avoid energetically-wastefuloverheating during

• Current address:Behavioral Ecology Research Group, Departmentof Zoology,The Ohio State University,1735 Nell Avenue, Columbus,Ohio, 43210-1293 USA.

231 232] v.v. Pravosudov j. FieldOrnithol. Spring 1995

TABLE 1. Characteristicsof natural cavitiesoccupied by the EurasianNuthatch.

Characteristic Mean SD Min Max

Floor area (cm '•) 153.6 45.1 86.6 280.4 Depth (cm) 16.5 3.2 11.0 22.0 Entrance (cm) 3.7 1.0 2.5 7.0 Volume (cm-•) 3006 711 1759 4775

hot days (Alatalo et al. 1988, Karlsson1983) or that sibling competition for spaceand favorable feeding positionsis reduced (Slagsvoid1989a). Here, I report findingsfrom the EurasianNuthatch (Sitta europaeaasia- tica) breeding in natural cavitiesthat bear on the issueof whether clutch size dependson chamber size.

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2 I I I I I I I 1500 2000 2500 5000 5500 4000 4500 5000

CAVITY VOLUME, CM

FIGURE1. Scatterplotof clutchsize and volumeof naturalcavities in EurasianNuthatches. Vol.66, No. 2 Reproductionand Cavity Size in EurasianNuthatch [233

T^BLE 2. Correlation coefficients between clutch size, number of fledglings and nesting cavitydimensions. P-value is in brackets.

Clutch size 4/fledglings Floor area -0.283 (0.17) -0.025 (0.90) Depth -0.066 (0.75) 0.239 (0.25) Entrance 0.325 (0.12) -0.021 (0.92) Volume -0.239 (0.25) 0.121 (0.57)

METHODS A detailed description of the study area in the Magadan region of northeastern Siberia has been published previously(Pravosudov 1993). Every cavitystudied during 1986-1990 breeding seasonswas used only once for analysis.If the samecavity was used more than once, the average

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2 I I I I I 50 100 150 200 250 500

?LOOR AREA, CM FIGURE2. Scatterplotof clutchsize and floor areaof naturalcavities in EurasianNuthatches. 234] V.V. Pravosudov I. FieldO•nithol. Spring 1995 values of clutch size and number of fledglings were used. Entrance size and cavitydepth (from the bottom of the entrance to the bottom of the nest) was measured with a ruler. After gaining accessto the nest chamber by cutting a replaceabledoor in a tree trunk, I measured the distance from the front to the rear wall of the cavity (X) and the width of the cavity (Y) at the bottom level. The area of the cavityfloor was estimated as an ellipse (3.142 X X/2 X Y/2), and the volume of the cavitywas calculated by multiplying the floor area times the cavitydepth including the entrance height (Alatalo et al. 1988, van Balen 1984). I measured 25 nest cavities, and employed simple regression and correlation to check possiblerelations between breeding parametersand cavitysize.

RESULTS AND DISCUSSION Most of the nuthatch cavitieswere old holes (Pravosudov 1993). No significant relationship was found between either clutch size or number of fledglingsand any cavitydimension I measured (Tables 1- 2; Figs. 1-2, r ( 0.32, P ) 0.12 for all comparisons). Thus, these as well as the Pied Flycatchersbreeding in nat- ural cavities(Alatalo et al. 1988) failed to show any relationship between cavity size and either clutch size or number of fledglings. The ranges of cavitysize in this studywere quite similar to those of nest-boxesused for experiments (bottom area from approx. 103 cm:• in small boxes to 314 cm3 in large ones, L&hrl 1987a,b). It is puzzling that most nestboxma- nipulation studieshave shown such a trend whereassome studiesof nat- ural cavitieshave not. Perhaps the inconsistencyarises because natural holes differ in some way other than cavity size. Different tree species, different extent of heart rot or some other factor could well affect the thermal climate inside a cavityand, therefore, be a confounding factor that preventsdetection of relationshipsbetween breeding parametersand nest hole size in natural cavities.

ACKN()1Aq •EDGMENTS

I thank T. C. Grubb, Jr., T. Slagsvoidand an anonymousrexSewer for proxSdingvaluable commentson this paper.

LITERATURE CITED

AL&IALO,R. V., A. CARLSSOX,AND A. LUNDBERG.1988. Nest cavitysize and clutch size of Pied FlycatchersP¾cedu[a hypo[eucct breeding in natural tree-holes.Ornis Scand.19:317- 319. BAL•X,J. H. 1984. The relationshipbetween nest-boxsize, occupationand breeding parametersof the Great Pctrusmajor and someother hole-nestingspecies. Ardea 72: 163-175. GUSTAFSSON,L., AND S.G. NILSSON.1985. Chttch size and breeding successof Pied and CollaredFlycatchers Ficedula spp. in nest-boxesof difibrent sizes.Ibis 127:380-385. KARLSSON,J. 1983. Breeding of the starling (Sturnus vulgaris). Ph.D. thesis,Univ. Lund, Lund, Sweden. --, AND S.G. NILSSON. 1977. The influence of nest-box area on the chttch size in some hole-nestingpassetines. Ibis 119:207-211. Vol.66, .No. 2 Reproductionand CavitySize in EurasianNuthatch [235

LOHRL,H. 1973. Einfiussder Brutraumfiacheauf die Gelegegrosseder Kohlmeise(Parus major).J. Ornithol. 114:339-347. 1987a. Influenceof cavitydepth on clutchsize of the Nuthatch (Sitta europaea). Okol. Vogel 9:69-70 (in German with English summary). 1987b. The breedingsuccess of the Nuthatch (Sitta europaea)with regardto nesting spaceand habitat. Okol. Vogel. 9:53-63. LUDESCHER,E-B. 1973. Sumpfmeise(Parus p. palustrisL.) und Weidenmeise(P montanus salicariusBr.) als sympatrischeZwillingsarten. J. Orn. 114:3-56. PRAVOSUDOV,V. V. 1993. Breedingbiology of the EurasianNuthatch in northeasternSiberia. Wilson Bull. 105:475-482. SLAGSVOLD,T. 1987. Nest site preferenceand clutch size in the Pied FlycatcherFicedula hypoleuca.Ornis Scand.18:189-197. 1989a. On the evolution of clutch size and nest size in passefinebirds. Oecologia 791300-305. ß 1989b. Experimentson clutch size and nest size in passerinebirds. Oecologia 80: 297-302. •, A•NDT. AMUNDSEN.1992. Do Great tits adjusthatching spread, egg size and offspring sex ratio to changesin clutch size?J. Anim. Ecol. 61:249-258. Received14 Jan. 1994; accepted29 Jul. 1994.