TORE SLAGSVOLD, 1 JOSTEIN SANDVIK, 2 GUNNAR ROFSTAD, 2 )YSTEIN LORENTSEN, 2 AND MAGNE HUSBY 2

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1 ON THE ADAPTIVE VALUE OF INTRACLUTCH EGG-SIZE VARIATION IN BIRDS TORE SLAGSVOLD, 1 JOSTEIN SANDVIK, 2 GUNNAR ROFSTAD, 2 )YSTEIN LORENTSEN, 2 AND MAGNE HUSBY 2 Zlgical Museum, University f Osl, Sarsgt. 1, Osl 5, Nrway; and 2Department f Zlgy, University f Trndheim, N-7055 Dragvll, Nrway ABSTRACT.--Using data frm the field and the literature n 67 species f birds, we analyzed intraclutch variatin in egg size, especially the deviatin f the last egg frm the clutch mean (D). Values f D are clser t zer in preccial than in altricial species; D is negatively crrelated with bdy size in interspecific cmparisns, i.e. large birds, including preccial species, lay small final eggs; and D is higher in pen-nesting passerines (n average D = %, 17 species) than in hle-nesting species (n average D = -0.05%, 13 species). Within ppulatins f birds, a negative relatinship exists between D and clutch size, particularly in species that have a generally lw value f D. The results supprthe view that intraclutch variatin in egg size has an ultimate, adaptive value. We suggesthat birds adpting the "brd-reductin strategy" have a small final egg, particularly thse birds with large clutches, whereas birds adpting the "brd-survival strategy" have a relatively large final egg, particularly thse birds with large clutches. Received 5 December 1983, accepted 19 April BIRDS pssesseveral mechanisms by which they can adjust the magnitude and pattern f their breeding effrt in relatin t envirnmental cnditins and t their wn breeding cnditin. The mst imprtant f these factrs is clutch size (Lack 1954, O'Cnnr 1978, Lundberg and V iis inen 1979). Other prpsed ables them t adjust the number f ffspring they rear in relatin t the envirnmental cnditins that prevail during the nestling stage. This is brught abut by hatching asynchrny, a phenmenn that results in a size hierarchy within the brd (Lack 1954, Ricklefs 1965, Hahn 1981, Slagsvid 1982a). Clark and Wilsn mechanisms are the sex rati f the brd (1981), hwever, cited data n the hatching patterns f several species f altricial birds and claimed that, rather than supprting the brdreductin hypthesis, these data supprted predictins adduced frm a mdel based n the (Trivers and Willard 1973, Hwe 1976, Fiala 1981), egg quality (Schifferli 1973, Hwe 1978, Ricklefs et al. 1978, O'Cnnr 1979, H6gstedt 1981, Birkhead and Nettleship 1982), hatching pattern (Lack 1954, Ricklefs 1965), and intraclutch egg-size variatin (Parsns 1970, 1976; Hwe 1976; Ryd n 1978; Ojanen et al. 1981). When clutch-size adjustments and the influence f nest predatin are excluded, a prnunced variatin still is fund amng birds in the prprtin f eggs that are successful. The hatchability f the eggs (Kenig 1982) and nestling mrtality vary significantly. In many passerine birds, fr example, nestling mrtality frm starvatin is rather lw (Nice 1957, Ricklefs 1969, Slagsvid 1982a), cmpared with that f sme birds f prey in which ne f the tw yung in the brd always dies (Stinsn 1979, Edwards and Cllpy 1983). It has been assumed that birds that lay relatively many eggs in relatin t the number f yung that they are nrmally able t feed (e.g. raptrs) have adpted a brd-reductin strategy that en~ 685 prbabilities f nest failures ccurring. In many species f passerine birds egg size increases with laying rder, and this phenmenn is indeed difficult t explain by means f the brdreductin hypthesis (Clark and Wilsn 1981). In the present paper, we reprt the results f an inter- and intraspecific cmparisn made with regard t such within-clutch egg-size variatin. A large bdy f data nw exists fr a wide variety f bird species, and the time is ripe fr a cmparative analysis. The patterns f such egg-size variatin seem fundamental t an understanding f verall reprductive strategies in birds. THEORY It has been assumed that the intraclutch variatin in the size f birds' eggs has an ultimate, The Auk 101: Octber 1984

2 686 SL^CSVOLD ET AL. [Auk, Vl EE ' Turdus piiarls r =0.926 n=56 p< 0.01 Cr us c½e r = n= ' 1 6 ' 1'8 20 d2 2'4 Mean vlume f all preceding eggs(cm 3) Fig. 1. The vlume f the final egg laid in a clutch pltted against the mean value f the egg vlume f all the preceding eggs laid in the same clutch. Data frm Fieldfare and Hded Crw clutches f 4 r 5 eggs. The y = x lines have been entered. adaptive value (Paludan 1952, Parsns 1970, Nisbet and Chen 1975, Hwe 1976, Ryd n 1978, Lundberg and V isgnen 1979, O'Cnnr 1979, Edwards and Cllpy 1983). This cannt yet be cnsidered as prven, as the variatin may simply reflect the cnditin f the egglaying female (cf. Pinwska 1979, Drent and Daan 1980, Ojanen et al. 1981, Hustn et al. 1983). The assumptin seems reasnable, hwever, as distinctly different patterns are fund between species, differences that are difficult t explain frm energetic cnstraints alne [see als the experiments dne by Paludan (1952) and Parsns (1976), and the review by Klmp (1970)]. Fr instance, the Fieldfare (Turdus pilaris) prduces, relative t parental bdy size, a ß larger egg mass per clutch than des the Hded Crw (Crvus crne crnix). Yet the final egg laid by the Fieldfare is, in general, larger than the preceding eggs laid in the same clutch, whereas the final egg f the Hded Crw is nt larger than its preceding eggs (Fig. 1). The results f several studies have indicated that the chances f survival f any particular nestling are related t the size f the egg frm which it hatched (Parsns 1970; Schifferli 1973; Murtn et al. 1974; O'Cnnr 1975, 1979; Ryder 1975; Hwe 1976; Nisbet 1978; Lundberg and Vgis nen 1979; Williams 1980a; Mss et al. 1981; Birkhead and Nettleship 1982), and we fund that the final eggs f bth the Black-billed Magpie (Pica pica) (t = 3.56, df = 15, P < 0.01) and the Hded Crw (t = 2.74, df = 8, P < 0.05) that did nt hatch were significantly smaller than thse that did hatch (Table 1). If the egg-size variatin within the clutch has an adaptive value, this variatin may be related t the hatching pattern f the bird. Richter (1982) argued that the nest-failure mdel f Clark and Wilsn (1981) is nt a bilgically plausible, general explanatin fr hatching asynchrny, but rather is an alternative t the brd-reductin hypthesis, applying t species whse nestlings seldm starve but wh frequently suffer nesting failures. Thus, sme species f birds may hatch asynchrnusly t facilitate brd reductin, thers t escape heavy nest predatin (Tyrv inen 1969, Hussell 1972). If such different breeding strategies d exist, viz. a "brd-reductin" and a "brd-surviv- al" strategy, they may be accmpanied by differences in intraclutch egg-size variatin (cf. O'Cnnr 1978, 1979). In birds that hatch their eggs asynchrnusly, the nestling that hatches frm the last-laid egg is at a disadvantage cmpared with its siblings. Laying a small final egg may therefre be anther mechanism that facilitates brd reductin. On the ther hand, birds adpting the brd-survival strategy are expected t have an egg-size increase with the laying rder (Clark and Wilsn 1981). MATERIAL AND METHODS Bird nests were lcated near Trndheim (63øN, 10øE), Nrway, during the perid We fund that there was a crrelatin between the rel- ative size f the final egg laid within a clutch and the general egg-size variatin within the same clutch,

3 Octber 1984] Egg-size Variatin 687 TABLE 1. Mean vlume (cm ) f Black-billed Magpie and Hded Crw eggs. a Pica pica C 'vtt$ ½ 'tle Mean SD n Mean SD n All eggs, cmplete clutches Last eggs laid (hatched) Last eggs laid (did nt hatch) a Frm nests in the vicinity f Trndheim (present study). i.e. if the final egg was relatively small, there was als a decrease in the size f the eggs frm the first t the penultimate egg. When we expressed the relatinship between egg size and laying rder f the per clutch size was at least five nests. Bdy weight, as used in this paper, refers t the weights f adult females recrded in spring and was taken frm the same papers frm which the egg-measurement data x I first eggs fr each clutch by means f a regres- were btained, wherever pssible, r frm ther sin slpe and cmpared the value f this slpe with the relative size f the final egg laid fr the same surces, mainly frm Hartman (1961), Haftrn (1971), and Clench and Leberman (1978). clutch, we fund r = 0.55 (16 clutches, P < 0.05) fr the Hded Crw. Thus, we use nly the relative size RESULTS f the final egg laid, calculated as the percentage deviatin frm the mean size f all the eggs in the clutch (D-value), in ur cmparisns. The final egg is mre frequently identified than the ther eggs in the clutch, and it is als bilgically relevant t use this measure, as the hyptheses mentined abve pay Penguins.--Measurements f penguin's eggs are included in the Appendix t allw a cmparisn t be made with thse f ther birds, but these data were nt used in ur analyses because f the peculiar bilgy and unique special attentin t the cnditin f the last yung phylgenetic psitin f penguins (e.g. Wilhatched. liams 1981). Mst f the species wuld have fitted nicely int the patterns that emerged because, in all penguins except thse belnging Using field data and data frm the literature, we estimated egg vlume frm egg length (L) and maximal breadth (B) by means f the frmula 0.51 LB 2 (Hyt 1979), except where therwise stated (Appendix). Thus, all eggs within a clutch were assumed t have the same shape. Althugh this is nt necessarily true, at present we d nt have enught data available n egg-shape variatin within clutches t adjust the vlumes calculated. We cmpared the D-value calculated frm egg-vlume data with that calculated frm fresh-egg-weight data fr the same species and fund a high crrelatin. Using nly measurements f the same clutches fr each species (at least fur clutches measured), we fund that r = 0.98 (9 species, y = 0.973x ); when adding 5 species with measurements mstly f the same clutches we fund als that r = 0.98 (14 species, y = 0.986x ); excluding penguins, we fund that r = 0.87 (9 species, y = 0.941x ). Thse mean values f egg size that were based n egg-weight data (see Appendix) were first reduced by 8% (Manning 1978) befre being used in the analyses in rder t make them cmparable with egg- vlume data, because the specific weight f fresh eggs is abve unity. In the Appendix, the relative size f the final egg laid is calculated fr varius ppulatins. In the interspecific cmparisns, the weighed mean D-value fr each species is used. The data have been presented separately fr particular clutch sizes in the Appendix in all cases where the sample size t the genus Eudyptes, the final egg laid in a clutch was smaller than the thers. The secnd (final) egg f Eudyptespp. is unique, hwever, in being much larger than the first ne laid, and, generally, the nestling that hatches frm this secnd egg is the nly ne t survive (Gwynn 1953, Warham 1975). The large final egg f Eudypte spp. is very difficult t explain, as it seems t cntradict what thery wuld predict. PrecciaI and aitriciai birds.--if the relative size f the eggs in a particular clutch represents an adaptive mechanism that facilitates brd reductin, r prevents it frm ccurring, then we shuld expect t find such a mechanism primarily amng altricial species f birds, as mst f these species hatch asynchrnusly (Clark and Wilsn 1981) in cntrast t preccial birds. Thus, we predict that the deviatin f the size f the final egg laid frm that f the mean size f all the eggs in the clutch will be clser t zer in preccial than in altricial species f birds. The results f ur analyses supprt this predictin (Table 2), althugh the data fr the pre-

4 688 SLAGSVOLD ET AL. [Auk, VL 101 TABLE 2. Cmparisn f the relative size f the final egg laid." Num- Mean bet f devi- Mannø spe- atin Whitney Species grup cies (%) SD U-test Nn-passerines Preccial U = 35 Altricial P < 0.02 Passetines Hle-nesters U, = 37 Open-nesters P < "Data frm Appendix; thse fr the penguins excluded. ccial birds were rather few and mstly referred nly t waders, and whether r nt waders shuld be regarded as preccial may be questined. The relative size f the final egg was rather similar in altricial and preccial species as a whle (-0.29, n = 49, fr altricial, and -0.68, n = 9, fr preccial), but the standard deviatin was much higher in altricial birds, viz versus 1.77 (P < 0.001, test between the variances). The mean egg size f ducks (Kskimies 1957, Bezzel 1968) and f Willw Ptarmigan (Lagpus lagpus; Mss et al. 1981) seems t decrease as the laying sequence prgresses, but this is nt true f Black Swans (Cygnus atratus; Cutten 1966). Bdy size.--large birds are less vulnerable t nest predatin than are small birds, and, thus, they wuld be expected t fllw the brdreductin strategy. Mrever, they lay relatively smaller eggs than thse f smaller birds (Heinrth 1922, Lack 1968, Rahn et al. 1975), and the energy cst f incubating an egg is als relatively less fr such birds (Ricklefs 1974). Large birds, therefre, wuld nt seem t be s rigrusly cnstrained by egg prductin and incubatin, althugh this des nt prve that N O ø '. [] ß [] ß ii -12 [] [] lg 50g 100g 500g 1 g 5kg 1Ok Female bdy weight Fig. 2. The relative size f the final egg laid pltted in relatin t the bdy weight f the adult female: D = In(bdy weight). Mean values fr each species have been used (frm Appendix, penguins excluded). Symbls: pen circles = passerine species; pen squares = altricial, nnpasserine species; slid squares = preccial, nnpasserine species.

5 Octber 1984] Egg-size Variatin 689 T^BI E 3. Multiple crrelatin analysis f the relative size f the final egg. a Species Multiple Simple crrelatin cefficients Partial regressin b crrelatin cefficient grup n Bdy weight Nest site a Clutch size Bdy weight Nest site a R Nnpasserines *** *** *** Passetines ** 0.49** *** 0.54*** 0.76*** Ttal *** ** -0.85*** 0.28** 0.79*** a Same data as used in Table 2; ** P < 0.01, *** P < b Standardized regressin cefficients after inclusin f the tw variables; clutch size yielded n significant cntributin and was mitted. c Lg values fr bdy weight f adult females. a Hle-nesters vs. pen-nesters. these prcesses are less demanding in such birds (cf. Hustn et al. 1983). The csts shuld preferably nt be measured in energy units but in the cst f btaining the amunt f energy needed. Ricklefs (1974) cncluded that the energetic cst f egg prductin relative t BMR in passerine birds des nt change with bdy weight but that such a change is fund in ther grups f species. If large birds have a tendency t lay a relatively greater excess f eggs per clutch than d small birds (i.e. nt in abslute terms but in relatin t the number f yung that they are able t feed; e.g. raptrs, Edwards and Cllpy 1983), then mst mrtality will be due t starvatin, a situatin prmting the brd-reductin strategy. Thus, we wuld expect that large birds wuld be less likely t attempt t raise the last-hatched yung and thus wuld nt tend t give it a size advantage. We wuld therefre expect that a negative relatinship shuld exist between the mean bdy size f a bird species and the relative size f the final egg laid in the clutch. The results supprt this predictin (Fig. 2, Table 3). The tendency fr the final eggs laid by small birds t be relatively large was fund t be as true fr altricial birds (r = -0.77, n = 49, P < 0.001) as fr preccial birds (r = -0.74, n = 9, P < 0.05). The lg values f the bdy weights f adult females were used in these calculatins, althugh the values f the crrelatin cefficients btained fr the relative size f the final egg pltted againsthe lg value f the mean egg vlume were f similar magni- tude. Open-nesting and hle-nesting passerine species.- Open-nesting species f passerines lay fewer eggs per clutch than d hle-nesters, nt nly in abslute terms (Nice 1957, Lack 1968) but perhaps als in relatin t their ability t rear the brd (Slagsvid 1982a). The hatchability f their eggs seems t be higher (Kenig 1982), but they als suffer a higher rate f nest predatin (Lack 1968). Thus, pen-nesters may be mre likely t shw hatching asynchrny as a respnse t nest failure and wuld be expected t try t ffset the disadvantage t the lasthatched yung (the brd-survival strategy). We therefre predict that pen-nesters lay relatively larger final eggs than d hle-nesters. The results supprt the predictin (Tables 2 and 3). In hle-nesters, the last egg laid was similar in size t the preceding eggs laid, whereas in pen-nesters the final egg was abut 4% abve mean size. Fr passefine birds, the crrelatin cefficient fr the relatinship between the relative size f the final egg laid and the lg values f bdy weight was r = (n = 30, P < 0.01). When the type f nesting site was included, a further 28.6% f the variatin culd be accunted fr. The multiple cr- relatin cefficient then btained was R = 0.76 (Table 3), accunting fr a ttal f 58% f the variatin amng the passerine species in the relative size f the final egg laid. Clutch size.--within species, but nt between species (Table 3), a negative crrelatin was fund between the relative size f the final egg laid and clutch size. This is shwn by cunting the number f cases when a decrease r an in- crease in the mean D-value was fund fr an increase in clutch size frm x t x + 1 eggs (Fig. 3). The Dx+ values were less than the respective Dx values in as many as 18 f the 24 investi-

6 690 SLACSVOLD ET AL. [Auk, Vl " 4 ' [] % deviatin f final egg, clutch size = x Fig. 3. The relative size f the final egg laid pltted against successively increasing clutch size (data frm Appendix, penguins excluded). The Dx, = Dx line has been entered. Symbls as in Fig. 2. gated cases (x 2 = 6.00, P < 0.02). Ntice als that mre f the data pltted in Fig. 3 lay belw the Dx = Dx, line fr thse species that yielded lw Dx values than fr thse that yielded high Dx values. In ther wrds, this decrease f the relative size f the final egg seems t be mre prnunced in species adpting, by ur interpretatin, the brd-reductin strategy than in thse adpting the brd-survival strategy. Fr instance, the average plt fr each f the 6 large species in Fig. 3 fell belw the line as cmpared with the average plt fr nly 5 f the 9 small species [large species: magpie (210 g) r larger; small species: Fieldfare (103 g) r smaller]. Mean egg'vlume and time f egg laying.--we cllected field data t determine whether r nt there is any relatinship between the relative size f the final egg, the mean egg vlume f the same clutch, and the time f egg laying (Table 4) n the Fieldfare and the Hded Crw (pen-nesters, the latter with a bdy weight f 489 g) and the magpie (hle-nester). In nne f these species was any tendency fund in thse individual birds that lay large eggs t lay a particularly deviant (either small r large) final egg in the clutch (Table 4), nr was there any cnsistentendency fr the final egg in clutches laid late in the breeding seasn t be particularly small r large (Table 4). The nly stalls- TABLE 4. Crrelatin cefficients between the relative size f the final egg laid and (A) the mean egg vlume fr the same clutch, and (B) the date f egg laying (first egg). Against Against Clutch mean egg laying Species size n vlume date a Turdus pilaris Pica pica * Crvus crne ' *P < tically significant crrelatin btained was that fr the magpie, when the data fr all clutch sizes were pled. The partial crrelatin cefficients btained were als nnsignificant fr all three bird species investigated when clutch size and egg laying date were kept cnstant. DISCUSSION The results f the present study supprt the view that the intraclutch variatin in the size f birds' eggs is a mechanism that has an ultimate, adaptive value that is used in cmbinatin with asynchrnus hatching. The eggs f mst altricial bird species hatch asynchrnusly (Clark and Wilsn 1981). A large final egg will increase the prbability that the nestling hatched frm such an egg will fledge successfully. Thus, the parents that practice the brdsurvival strategy will benefit frm certain advantages ffered by asynchrnus hatching, ther than facilitatin f a reductin in brd size, such as a shrtening f the time between the nset f egg laying and the time the first nestling fledges (Tyrvainen 1969) and a staggering f the perids f peak fd demand by the yung (Bryant 1978). We cnsider that the cntrast between the relative size f the final egg f pen-nesting and that f hle-nesting species f passerines is f particular imprtance in prviding a clue fr an understanding f the breeding strategies adpted by birds in general. The pen-nesting A B

7 Octber 1984] Egg-size Variatin 691 A. Brd reductin B. C. Brd survival -1 x x+l x-1 x x+l x 1 x x+l Clutch sl=e Fig. 4. Prpsed relatinship between the relative size f the final egg laid and change f clutch size, fr three patterns f breeding strategies. A third dimensin has als been included, viz. the cnditins fr breeding, illustrated by the dashed lines (arrws indicating expected change as cnditins are imprved). passerines lay fewer eggs than the hle-nesters, but the final egg they lay in a clutch is large. The smaller clutch size f pen-nesters cannt therefre simply be explained by energetic cnstraints n the egg-laying females. There are at least three explanatins fr the reductin in clutch size: the higher csts f incubating many eggs, in particular in pen nests (Biebach 1981, Slagsvid 1982a); the increased mrtality f nestlings because f vercrwding and falling ut f the nest (Slagsvid 1982a); and the higher rate f nesting failures. Opennesting passerines run a serius risk f nesting failure because f a sudden nset f bad weath- er and a high degree f nest predatin (Nice 1957, Ricklefs 1969). These factrs may select fr a reductin in the number f eggs laid in each single attempt t nest and fr an increase in the ability f pen-nesters t renest quickly (Skutch 1949, Snw 1978, Ewald and Rhwer 1982, Slagsvid 1982b). As mentined abve, it is pssible that the birds use within-clutch egg-size variatin t adjust the clutch and the brd t the prevailing cnditins fr breeding, althugh n direct evidence exists as far as we knw. Figure 4 illustrates three pssible patterns f these adjustments n the individual-bird level. Fr a certain clutch size, it is assumed in Fig. 4A that a psitive relatinship exists between the relative size f the final egg laid and the prevailing breeding cnditins. The switch frm ne level f clutch size t a higher level at im- prved cnditins (indicated by dashed lines and arrws) is ne frm a clutch f x eggs, with a relatively large final egg, t a clutch f x +! eggs, with a relatively small final egg. Figure 4C represents just the ppsite pattern, whereas a third pssibility, that n such relatinship exists at all between clutch size and the relative size f the final egg laid, is illustrated in Fig. 4B. We wuld expect the pattern in Fig. 4A t apply in species f birds that have adpted the brd-reductin strategy; if the clutch is in- creased by ne egg, it is imprtant that this egg is a small ne s that the brd-reductin mechanism wuld perate effectively. On the ther hand, birds adpting the brd-survival strategy shuld lay a particularly large final egg n the changever frm x t x + 1 eggs r at least a final egg f n reduced size, because the last yung t hatch wuld be mre at a disadvantage in a large brd (i.e. Fig. 4C r B). The patterns in Fig. 4 are qualitatively supprted by the results given abve (Fig. 3). We encurage further studies f these relatinships, hwever, as pssible cncmitant changes in general egg size and in hatching pattern have nt been included in the mdel because they are nt knwn. Fr instance, if the general egg size is small, it may be f greater imprtance that the final egg is large than when all eggs are large. Mrever, if the cnditins fr breeding are imprved, it is pssible that the mechanism f respnse adpted by

8 692 SLAGSVOLV ET AL. [Auk, Vl. 101 birds is t hatch mre synchrnusly. This wuld imprve the prbability f survival f the yung hatched frm the final egg laid, and then n cncmitant change in the relative size f the final egg may be needed (i.e. the pattern shwn in Fig. 4B may then be expected). If the mean egg size f a clutch reflects the cnditins fr breeding (e.g. H/Sgstedt 1981), we wuld expect a psitive crrelatin t be fund between mean egg size and the relative size f the final egg laid fr species adpting the brd-reductin strategy but nt fr thse adpting the brd-survival strategy (cf. Fig. 4). Thus, fr selected clutch-size levels, we wuld expect psitive crrelatin cefficients t be fund fr the magpie and the Hded Crw, and negative values fr the Fieldfare. Table 4 indicates that 6 f 7 cefficients had the predicted sign. The randm prbability that all, r all-but-ne, f the cefficients shuld have the predicted sign is P = 8-(0.5) 7 = (netailed test). Statistically, hwever, n single cefficient significantly deviated frm zer (Table 4). Karlssn (1983) prvided Eurpean Starlings (Sturnus vulgaris) with extra fd. The birds advanced the time f egg laying and laid larger eggs, but they changed neither clutch size nr the relative size f the final egg laid (which was 1.9% and 1.0% smaller than the mean egg size, respectively, fr the experimental and cntrl clutches). Thus, this species seems t represent the pattern in Fig. 4B. It shuld be added, hwever, that, althugh the availability f fd at the time f egg laying influences the cnditin f the female at this stage f the breeding perid, it may nt necessarily indicate t the female that the cnditins fr rear- ing yung wuld be favrable later n. When prvided experimentally with additinal fd, passerines lay larger eggs but d nt seem t respnd by laying larger clutches (Ewald and Rhwer 1982, Karlssn 1983; see Slagsvid 1982a fr further references). We wuld like t stress that the tw breeding "strategies" prpsed, brd reductin and brd survival, are indeed nt well-dcumented entities and that a greater amunt f lss by brd reductin fllwing hatching asynchrny has nt been demnstrated cnvincingly (Clark and Wilsn 1981; but see Hahn 1981, Slagsvid 1982a). Thus, the existence f tw such breeding strategies is pssible but has nt yet been prved. The present data supprt the in- terpretatin f the relative size f the final egg as an indicatin f a brd-reductin r a brd- survival strategy. The mdel shwn in Fig. 4 is cmprised f three dimensins: clutch size, relative size f final egg, and cnditin fr breeding (dashed lines). Preferably, as mentined abve, we shuld als have included tw mre dimensins: general egg size f the clutch and the hatching pattern. Unfrtunately, we have nt yet infrmatin available fr ding this, but we hpe that ur study wuld encurage further research t find ut if, and hw, birds use and cmbine all these factrs t maximize their reprductive fitness. ACKNOWLEDGMENTS We thank A. B. Clark, G. H6gstedt, and W. Kenig fr helpful cmments n the manuscript, J. T. Lifjeld and T. Albu fr assistance in the field, P. A. Tallantire fr imprving the English, and I. Harder fr help in preparing the manuscript. The study was supprted by grants frm the Nrwegian Research Cuncil fr Science and the Humanities. LITERATURE CITED ANTIKAINEN, E The breeding adaptatin f the Jackdaw Crvus mnedula L. in Finland. Savnia 2: ARN-WILLI, H Bilgische Studien am Alpensegler. Slthurn, Vgt-Schild. BARTH, E.K Egg dimensins and laying dates f Larus marinus, L. argentatus, L. fuscus, and L. canus. Nytt Mag. Zl. 15: BEHLE, W. H., &: W. g. GOATES Breeding bilgy f the Califrnia Gull. Cndr 59: BEZZEL, E Ergebnisse der Messungen yn Eil nge und -breite in einer Entenppulatin. Anz. Ornithl. Ges. Bayern 8: BIEBACH, H Energetic csts f incubatin n different clutch sizes in Starlings (Sturnus vulgaris). Ardea 69: BIRKHEAD, T. R., &: D. IN. ]NETTLESHIP The adaptive significance f egg size and laying date in Thick-billed Murres Uria lrnvia. Eclgy 63: BLOESCH, M Sechsergelege beim Weissenstrch Cicnia cicnia. Ornithl. Beb. 79: BRYANT, D. g Establishment f weight hierarchies in the brds f Huse Martins Delichn urbica. Ibis 120: CH^PFELE, B The dissimilar egg and ther prblems, part 3. Ol. Rec. 22: 1-6. CEARIC, A. B., & D. S. WILSON Avian breeding adaptatins: hatching asynchrny, brd reductin, and nest failure. Quart. Rev. Bil. 56: CLENCH, g. H., &: R. C. LEBERMAN Weights f

9 Octber 1984] Egg-size Variatin species f Pennsylvanian birds analysed by mnth, age and sex. Bull. Carnegie Mus. Nat. Hist. 5: COULSON, J. C Egg size and shape in the kittiwake (Rissa tridactyla) and their use in estimating age cmpsitin f ppulatins. Prc. Zl. Sc. Lndn 140: , G. R. POTTS, & J. HOROBIN Variatin in the eggs f the Shag (Phalacrcrax aristtelis). Auk 86: CUTTEN, F. E.A Clutch-size and egg dimensins f the Black Swan Cygnus atratus at Lake Pliesmere, Canterbury, New Zealand. Emu 65: DAVIS, J. W. F Age, egg-size and breeding success in the Herring Gull Larus argentatus. Ibis 117: DRENT, R. H., a: S. DAAN The prudent parent: energetic adjustments in avian breeding. Ardea 68: EDWARDS, T. C., JR., & M. W. COLLOPY Obligate and facultative brd reductin in eagles: an examinatin f factrs that influence fratri- cide. Auk 100: EWALD, P. W., & S. ROHWER Effects f supplementary feeding n timing f breeding, clutchsize and plygyny in Red-winged Blackbirds Agelaius pheniceus. J. Anim. Ecl. 51: FIALA, K. L Sex rati cnstancy in the Redwinged Blackbird. Evlutin 35: GARGETT, V A 13-year ppulatin study f the Black Eagles in the Matps, Rhdesia, Ostrich 48: GEMPERLE, M.E., a: F. W. PRESTON Variatin f shape in the eggs f the Cmmn Tern in the clutch-sequence. Auk 72: GI 3 3, J The breeding bilgy f the Great and Blue titmice. Ibis 92: GOCHFELD, M Intraclutch egg variatin: the uniqueness f the Cmmn Tern's third egg. Bird-Banding 48: GROEBBELS, F., F. M SBERT, a: G. TIMMERMANN Ueber die Beziehungen swischen Eigewichten und Legeflge. Beitr. Frtpfl. Bil. V6gel. 6: GWYNN, g. M The egg-laying and incubatin perids f Rckhpper, Macarni and Gent penguins. A.N.A.R.E. Rept. Series B. Zl. 1: HAFTORN, S Nrges fugler. Osl, Universitetsfrlaget. HAHN, D.C Asynchrnus hatching in the Laughing Gull: cutting lsses and reducing rivalry. Anim. Behav. 29: HARRIS, M.P Aspects f the breeding bilgy f the Gulls Larus argentatus, L. fuscus and L. ma- rinus. Ibis 106: HARTMAN, F Lcmtr mechanisms f birds. Smithsni an Misc. Cil. 143: HEINROTH, O Die Beziehungen swischen Vgelgewicht, Eigewicht, Gelegegewicht und Brutdauer. J. Ornithl. 70: H SGSTEDT, G Effect f additinal fd n reprductive success in the Magpie (Pica pica). J. Anim. Ecl. 50: HOL½OMB, L. C Breeding bilgy f the American Gldfinch in Ohi. Bird-Banding 40: HOUSTON, D.C., P. J. JONES, & R. M. SIBLEY The effect f female bdy cnditin n egg laying in Lesser Black-backed Gull Larus fuscus. J. Zl., Lndn 200: HOWE, H. F Egg size, hatching asynchrny, sex, and brd reductin in the Cmmn Grackle. Eclgy 57: Initial investment, clutch size, and brd reductin in the Cmmn Grackle (Quiscalus quiscula L.). Eclgy 59: HOYT, D. F Practical methds f estimating vlume and fresh weight f bird eggs. Auk 96: HUSSELL, D. J. T Factrs affecting clutch size in arctic passerines. Ecl. Mngr. 42: KARLSSON, J Breeding f the Starling (Sturnus vulgaris). Unpublished Ph.D. dissertatin. Lund, Sweden, Univ. Lund. KENDEIGH, S.C., T. C. KRAMER, a: F. HAMERSTROM Variatin in egg characteristics f the Huse Wren. Auk 73: KLOMP, H The determinatin f clutch-size in birds. A review. Ardea 58: KOENIG, W. D Eclgical and scial factrs affecting hatchability f eggs. Auk 99: KOSKIMIES, J Variatin in size and shape t eggs f the Velvet Screr, Melanitta fusca (L.). Sumal. Elain-Ja Kasvit. Seur. Van. Tiedn. P6yt ik. 12: LACK, D The natural regulatin f animal numbers. Oxfrd, Clarendn Press Eclgical adaptatins fr breeding in birds. Lndn, Methuen. LUNDBERG, C-A., & R. A. V IS NEN Selective crrelatin f egg size with chick mrtality in the Black-headed Gull (Larus ridibundus). Cndr 81: MANNING, t. H Measurements and weights f eggs f the Canada Gse, Branta canadensis, analyzed and cmpared with thse f ther species. Can. J. Zl. 56: MEYBURG, B-U Sibling aggressin and mrtality amng nestling eagles. Ibis 116: MILLER, E.H Egg size in the Least Sandpiper Calidris rninutilla n Sable Island, Nva Sctia, Canada. Ornis Scandinavica 10: MILt s, J. A Factrs affecting the egg size f Red-billed Gulls Larus nvaehllandiae scpulinus. Ibis 121: Mss, R., g. WATSON, P. ROTHERY, a: W. W. GLENNIP.

10 694 SLACSVOLD ET AL. [Auk, Vl Clutch size, egg size, hatch weight and laying date in relatin t early mrtality in Red Gruse Lagpus lagpu scticus chicks. Ibis 123: MURTON, R. K., N. J. WESTWOOD, & A. J. ISAACSON Factrs affecting egg-weight, bdy-weight and mult f the Wdpigen Clumba palumbus. Ibis 116: NICE, M.M Nesting success in altricial birds. Auk 74: RICKLEFS, R.E. NISBET, I. C. T Dependence f fledging success n egg-size, parental perfrmance and eggcmpsitin amng Cmmn and Rseate terns, Sterna hirund and S. dugallii. Ibis 120: ß, & M. E. CO EN Asynchrnus hatching in Cmmn and Rseate terns, Sterna hirund and S. dugallii. Ibis 117: O'CONNOR, R Initial size and subsequent grwth in passefine nestlings. Bird-Banding 46: ß Grwth strategies in nestling passerines. Living Bird 16: Egg weights and brd reductin in the Eurpean Swift (Apus apus). Cndr 81: OJANEN, M., M. ORELL, & R. A. V IS NEN Egg size variatin within passefine clutches: effects f ambient temperature and laying sequence. Ornis Fennica 58: PALUDAN, K Cntributin t the breeding bilgy f Larus argentatus and Larus fuscus. Vidensk. Meddr. Dansk Naturh. Fren. 114: PARSONS, J Relatinship between egg size and pst-hatching chick mrtality in the Herring Gull (Larus argentatus). Nature 228: Egg size, laying date and incubatin perid in the Herring Gull. Ibis 114: ß Factrs determining the number and size f eggs laid by the Herring Gull. Cndr 78: PIKULA, J Die Variabilit it der Eier der Ppulatin Turdus philmels, Brehm 1831 in der CSSR. Zl. Listy 20: PINOWSKA, B The effect f energy and building resurces f females n the prductin f Huse Sparrw [Passer dmesticus (L.)] ppulatins. Ekl. Plska 27: P NOWS, J., & A. MYRCHA Bimass and prductin rates. Pp in Granivrus birds in ecsystems (J. Pinwski and S.C. Kendeigh, Eds.). Lndn, Cambridge Univ. Press. PRESTON, F. W., & E. J. PRESTON Variatin f the shapes f bird's eggs within the clutch. Ann. Carnegie Mus. 33: PRYL, M Breeding bilgy f the Thrush Nightingale Luscinia luscinia in suthern Finland, I. Ornis Fennica 57: RAHN, H., C. V. PAGANELLI, & A. AR Relatin f avian egg weight t bdy weight. Auk 92: REID, B The Ad lie Penguin (Pygscelis adeliae) egg. New Zealand J. Sci. 8: RICHDALE, L. E A ppulatin study f pen- guins. Oxfrd, Clarendn Press. RICHTER, W Hatching asynchrny: the nest failure hypthesis and brd reductin. Amer. Natur. 120: Brd reductin in the Curvebilled Thrasher. Cndr 67: An analysis f nesting mrtality in birds. Smithsnian Cntrib. Zl. 9: Energetics f reprductin in birds. Nuttall Ornithl. Club 15: ß D.C. HAHN, & W. A. MONTEVECCHI The relatinship between egg size and chick size in the Laughing Gull and Japanese Quail. Auk 95: RUNDE, O. J., & R. T. BARRETTß 1981ß Variatin in egg size and incubatin perid in the Kittiwake Rissa tridactyla in Nrway. Ornis Scandinavica 12: RYDgN, O Egg weight in relatin t laying sequence in a Suth Swedish urban ppulatin f the Blackbird Turdus merula. Ornis $candina- vica 9: RYDER, J.P Egg-laying, egg size, and success in relatin t immature-mature plumage f Ringbilled Gulls. Wilsn Bull. 87: $CHIFFERLI, L The effect f egg weight n the subsequent grwth f nestling Great Tits Parus majr. Ibis 115: SCHRANTZ, F.G Nest life f the Eastern Yellw Warbler. Auk 60: SCHREIBERß R. W., & J. M. LAWRENCEß Organic material and calries in Laughing Gull eggs. Auk 93: SKUTCH, A D trpical birds rear as many yung as they can nurish? Ibis 91: $LAGSVOLD, t. 1982a. Clutch size, nest size, and hatching asynchrny in birds: experiments with the Fieldfare (Turdus pilaris). Eclgy 63: b. Clutch size variatin in passefine birds: the nest predatin hypthesis. Oeclgia 54: SNw, B The breeding bilgy f the Shag Phalacrcrax aristtelis n the Island f Lundy, Bristl Channel. Ibis 102: SNw, D. W The nest as a factr determining clutch-size in trpical birds. J. Ornithl. 119: STUNSON, C. H On the selective advantage f fratricide in raptrs. Evlutin 33: SUMMERS, R. W., & J. COOPER The ppulatin, eclgy and cnservatin f the Black Oystercatcher Haematpus mquini. Ostrich 48:

11 Octber 1984] Egg-size Variatin 695 TAYLOR, R.H The Ad lie Penguin Pygscelis The bilgy f penguins (B. Stnehuse, Ed.). adeliae at Cape Ryds. Ibis 104: Lndn, Macmillan Press. TRIVERS, R. L., & D. E. WILLARD Natural se- WILLIAMS, g.j. 1980a. Variatin in weight f eggs lectin f parental ability t vary the sex rati f ffspring. Science 179: and its effect n the breeding bilgy f the Great Skua. Emu 80: TVCK, L.M The snipes: a study f the genus 1980b. Penguin prprtinate egg weight. Capella. Can. Wildl. Serv. Mngr. Ser. 5: Ntrnis 27: TYRV INEN, H The breeding bilgy f the Why d penguins have lng laying Redwing (Turdus iliacus L.). Ann. Zl. Fennica intervals? Ibis 123: : V'/i. IS NEN, A., O. HILD N, M. SOIKKELI, & S. VOULAN- TO Egg dimensin variatin in five wader species: the rle f heredity. Ornis Fennica 49: VERMEER, K Egg measurements f Califrnia WINKEL, W Experimentelle Untersuchungen zur Brutbilgie vn Khl- und Blaumeise (Parus majr und P. caeruleus). J. Ornithl. 111: YTREBERG, N-J Cntributin t the breeding bilgy f the Black-headed Gull (Larus ridibundus L.) in Nrway. Nytt Mag. Zl. 4: and Ring-billed gull eggs at Miqueln Lake, Al- ZAC, R Hatching asynchrny, egg size, berta, in Wilsn Bull. 81: WARHAM, J The Firdland Crested Penguin Eudyptes pachyrhynchus. Ibis 116: grwth, and fledging in Tree Swallws. Auk 99: ZIMMERMANN, D Zur Brutbilgie der Dhle, The Crestdd Penguins. Pp in Cleus mnedula. Ornithl. Beb. 48: APPENDIX. Data frm the literature and frm ur fieldwrk n the clutch sizes, egg vlumes, and sizes f last eggs laid f a number f species. Mean Relative egg size f Clutch size vlume last egg Species (range) n (cm ) (%)a Surce Megadyptes antipdes Richdale 1957 Pygscelis papua 2.19 (2-3) Gwynn ? b Williams 1980b Pygscelis adeliae Taylr Taylr (2-3) Taylr Reid 1965 Eudyptes chryslphus Gwynn ? b Williams 1980b Eudyptes atratus Warham 1975 Eudyptes crestatus 2.10 (2-3) Gwynn ? b Williams 1980b Eudyptes pachyrhynchus Warham 1974 Eudyptes rbustus Warham 1975 Spheniscus demersus 2? b Williams 1980b Phalacrcrax aristtelis Snw b Snw b Snw (2-4) b Snw Culsn et al Culsn et al Culsn et al (2-4) Ctrisn et al Cicnia cicnia Blesch 1982 Branta canadensis Manning 1978 Melanitta fusca ca ca.-4 Kskimies 1957 Aquila verreauxii 2 I Meyburg Gargett 1977 Hieraaetus fasciatus 2 I Meyburg 1974 Haematpus mquini Summers and Cper 1977 Charadrius hiaticula Viiislinen et al. 1972

12 696 SLAGSVOLD ET AL. [Auk, VL 101 APPENDIX. Cntinued. Mean Relative egg size f Clutch size vlume last egg Species (range) n (cm 3) (%)a Surce Calidris temminckii Calidris minutilla Calidris alpina Gallinag gallinag Phalarpus lbatus Larus nvaehllandiae (2-3) Larus atricilla 2.79 (2-3) Larus ridibundus Larus delawarensis Larus canus Larus fuscus Larus argentatus Larus califrnicus Larus marinus Rissa tridactyla (2-3) (2-3) Sterna dugallii b Sterna hirund b b Clumba palumbus b Apus apus b b (2-3) b 0.02 Apus melba Tachycineta biclr b b (4-7) b 2.09 Hirund rustica 5.00 (4-6) Trgldytes aedn (4-7) V iis inen et al Miller 1979 Vaisanen et al Tuck 1972 V iis inen et al Mills 1979 Mills 1979 Mills 1979 Prestn and Prestn 1953 Schreiber and Lawrence 1976 Ytreberg 1956 Lundberg and V iis nen 1979 Vermeer 1969 Ryder 1975 Barth 1967 Paludan 1952 Harris øN, Barth øN, Barth øN, Barth 1967 Paludan 1952 Harris øN, Barth øN, Barth øN, Barth 1967 Parsns 1972 Davis 1975 Behle and Gates 1957 Vermeer 1969 Harris øN, Barth øN, Barth øN, Barth 1967 Culsn 1963 Culsn 1963 Culsn øN, Runde and Barrett øN, Runde and Barrett øN, Runde and Barrett øN, Runde and Barrett 1981 Nisbet and Chen 1975 Gemperle and Prestn 1955 Nisbet and Chen 1975 Gchfeld 1977 Nisbet 1978 Muttn et al Present study O'Cnnr 1979 O'Cnnr 1979 O'Cnnr 1979 Arn-Willi 1960 Zach 1982 Zach 1982 Zach 1982.lbu pers. cmm. Kendeigh et al Kendeigh et al Kendeigh et al. 1956

13 Octber 1984] Egg-size Variatin 697 APPENDIX. Cntinued. Mean Relative egg size f Clutch size vlume last egg Species (range) n (cm ) (%)' Surce Prunella mdularis 5.50 (5-6) Present study Erithacus rubecula b 0.20 Chappell 1948 Luscinia luscinia Pryl 1980 Phenicurus phenicurus Ojanen et al Turdus merula b 2.40 Ryd n 1978 Turdus pilaris Slagsvid 1982a Slagsvid 1982a 5.54 (4-7) Slagsvid 1982a Turdus philmels 5? Pikula 1971 Turdus iliacus Present study Present study 5.55 (5-6) Present study Sylvia atricapilla Present study Phyllscpus cllybita 5.60 (5-6) Grebbels et al Present study Phyllscpus trchilus Grebbels et al Muscicapa striata Present study Ficedula hypleuca Ojanen et al Ojanen et al (6-7) Ojanen et al Present study Present study Present study 6.19 (5-8) Present study Parus palustris Present study Parus ater 8.00 (7-9) Present study Parus caeruleus ca Winkel (10-11) Present study Parus majr ca Gibb 1950 ca Winkel Ojanen et al Ojanen et al (9-10) Ojanen et al Present study Pica pica Present study Present study Present study 6.47 (3-9) Present study Crvus mnedula Antikainen Antikainen (3-6) Antikainen Zimmermann 1951 Crvus crne Present study Present study 4.64 (3-6) Present study Sturnus vulgaris Ojanen et al Ojanen et al (5-6) Ojanen et al Passer dmesticus Pinwski and Myrcha Pinwski and Myrcha b Pinwski and Myrcha Pinwski and Myrcha (2-5) Pinwski and Myrcha 1977 Fringilla celebs 4.86 (4-5) Present study Carduelis tristis 5.26 (5-6) Hlcmb 1969 Dendrica petechia Schrantz 1943 Emberiza scheniclus Present study Quiscalus quiscula Hwe 1976 Deviatin (in %) f the size f the final egg laid frm that f the mean size f all the eggs in the clutch. Mean egg weight.