TheCondor96:170-177 8 The Cooper Ornithological Society 1994

CLUTCH SIZE IN OF SOUTHERN SOUTH AMERICA’

YORAM YOM-TOV Department of Zoology, Tel Aviv University,Tel Aviv 69978. Israel

MIGUEL I. CHRISTIE AND GUSTAVO J. IGLESIAS Villegas369, 8400 Bariloche,Rio Negro,Argentina

Abstract. Clutch size of the passerinesof southern South America was studied by com- paring data taken from the literature for 33 1 (177 genera), which breed in Chile, Argentina, Uruguay, and southernBrazil. Mean overall clutch size was 2.98 eggs,appreciably smaller than clutch size of passerinesat similar latitudes in the northern hemisphere. There was a highly significantdifference between mean clutch size of the two sub-orders representedin the study area, with the Deutero-Oscineshaving on averagesmaller clutches (2.73) than the Oscines(3.32). This difference between the two sub-orderswas maintained in each of the five zoogeographicalregions of the study area. There were also significant differencesin clutch size between the five main zoogeographicalregions: overall mean clutch size was smallest in the Amazonianregion (2.67) and largestin the Chaco(3.29). Deutero- Oscineseggs were proportionally larger than thoseof Oscines(14% and 11%of bodymass, respectively). Therewas no correlationbetween clutch size and body mass, nor was clutch size related to either nest type or migration. There is no evidence for latitudinal gradient in clutch size in the area studied. Key words: Clutch size; Passeriformes;South America.

INTRODUCTION clutch size increasesby as much as one egg per Clutch size varies both between and within spe- 6-10”latitude. Information about the occurrence cies, and the reasons for these variations have of this phenomenon in the southern hemisphere been the subject of many studies. Clutch size is is scarce,but Moreau (1944) showed that clutch constrained by several factors, which can be sizes of populations in South Africa average less groupedinto two main categories-ecologicaland than one egg more than those of populations in phylogenetic or taxonomic (Klomp 1970; Lack equatorial Africa, about 30” to the north. Simi- 1947, 1968; Perrins and Birkhead 1983). The iarly, Yom-Tov (1987) has shown that a very most commonly discussed factors in the first cat- weak but significant trend exists in eastern Aus- egory are latitude and habitat, while relatively tralia, where mean clutch size of passerinesin- little has been done concerning the connection creasesby about 0.8 eggover 30”. Hence, it seems between phylogeny and clutch size. that although a latitudinal trend in clutch size The trend of increase in clutch size of passer- exists in both northern and southern hemi- ines with latitude is one ofthe most striking vari- spheres,it is much more marked in the northern ations between and within speciesof , and hemisphere,where clutch size increasesat a much is especially marked in multibrooded passerines steeperrate. Clutch size is also related to habitat: (Perrins and Birkhead 1983, Murray 1985). within the tropics, clutches tend to be larger in Clutches of tropical passerinestend to have two savanna and grassland than in the rain forest to three eggs(Moreau 1944, Skutch 1985), those (Lack and Moreau 1965), and in temperate breeding in middle latitudes laying four or five regions clutches of great tits are larger in decid- eggs,and those in the Arctic five to seven (Rick- uouswoodland than in pine forests(Klomp 1970), lefs 1969). This trend was studied extensively in and blackbirds lay larger clutches in rural than the northern hemisphere, where it was shown to in urban sites (Snow 1958). Other ecologicalfac- occurin both the Old (Lack 1968) and New (Cody tors such as population density (Perrins 1965) 197 1) worlds and Klomp (1970) claimed that and type of nest (Klomp 1970) have also been shown to influence clutch size. Several hypotheseshave been suggestedto ex- I Received24 May 1993. Accepted 28 September plain the latitudinal trend and habitat type in- 1993. fluence (herein termed as “ecology”) on clutch

[I701 CLUTCH SIZE OF SOUTH AMERICAN PASSERINES 171 size. Murray (1985) distinguished between two phylogeny and ecology. However, becausethere alternative general hypothesesregarding the evo- are almost no published data on clutch size in lution of clutch size, per se. The view developed relation to latitude in this vast region, we studied by Lack (1947, 1968) and his followers (Ashmole clutch size in relation to habitat type, which 196 1, Cody 1966, Ricklefs 1980) statesthat birds probably has a major effect on the ecologyof the reproduce up to the maximum energy that the birds. Clutch size is known to be affectedby type parents can devote to laying eggsand rearing the of nest (Perrins and Birkhead 1983), and we ex- young. Excessreproduction is eliminated by sub- amined this as well as our hypothesis that mi- sequent density-dependent mortality. A second gratory passerines should have larger clutches group of views comprising the ideas often re- than residents. ferred to as “adjusted reproduction,” according More than 460 species(about 240 genera) of to which the clutch size is adjusted to the mor- passerinebirds belonging to 25 families breed in tality rate of the species,with those having long southern South America. Eight of these (Den- life expectanciesalso having small clutches and drocolaptidae, Furnariidae, , vice versa. There is, however, little agreement Rhinocryptidae, Cotingidae, Pipridae, Tyranni- on how this adjustment evolves (Stresemann dae and Phytotomidae) belong to the infraorder 1927-1934, Rensch 1938, Skutch 1969, Wynne- Tyranni (Sibley and Ahlquist 1985) of the sub- Edwards 1962, Murray 1985). Deutero-Oscines (Voous 1977), and the Although the single most important factor pro- rest belong to the suborder Oscines (Passeres). ducing variation in clutch size between species The Tyranni evolved in South America after it is their ecology (i.e., food availability and pre- was separated from the Old World portions of dation; Perrins and Birkhead 1983) there are Gondwanaland during the Cretaceous period, major differences in clutch size between taxo- about 80 million years ago. The Oscines evolved nomic groups. Lack (1968) noted that some or- in parallel. Some groups (i.e., Emberizidae, Ic- ders have a typical clutch size which is common teridae, Parulidae) in the New World, either in to most of their species.For example, Procellar- South America (i.e., Thraupidae, Icteridae, Vire- iformes lay only one egg,Columbiformes mostly onidae) or North America (i.e., Emberizidae), two. The (Passeriformes)have a vari- while others (i.e., Hirundinidae, Corvidae, Cin- able clutch size, and relatively little researchhas clidae, Turdidae) evolved mainly in the Old been carried out on the effect of phylogeny on World and dispersedto North America and then clutch size in this group. However, even within South America (Sibley and Ahlquist 1990). this order some families have larger clutchesthan Our study area includes Chile except for the others. For example, among the Australian pas- Tarapaca region in the tropical north, Argentina, serines, the “old endemic? families (i.e., Meli- Uruguay, and Rio Grande de Sul, the southeast phagidae, Maluridae and Acanthizidae) have province of Brazil, and excluding all oceanic is- clutches of 2-3 eggs,while the “new invaders” lands. This region (about 3.95 million sq. km) (i.e., Hirundinidae, Ploceidae and Corvidae) lay stretchesabout 4,000 kms between 22”S-56”s. 3-6 eggs(Yom-Tov 1987). Hence, phylogenetic We have divided it into five main zoogeograph- differencesmight also be important in determin- ical regions, namely the Amazonian, Chaco, An- ing clutch size. dean, Patagonian, and Araucanian (Cabrera and Most studies on variation in clutch size have Willink 1980, Narosky and Yzurieta 1989), which been carried out in the Northern Hemisphere (for occupy approximately 7%, 54%, 14%, 14% and review, see Klomp 1970) and comparisons have 11% of the total area, respectively. The Ama- been carried out between these regions and the zonian region occupiesthe northern half of Rio tropics (Skutch 1985). Little has been done on Grande de Sul and two small areasof north east- this subject in the Southern Hemisphere (but see em and north western Argentina and comprises Moreau 1944 and Yom-Tov 1987) and there is subtropical forest; the Chaco region comprises no study of clutch size in mid-latitudes in South subtropical deciduous xerophytic woodland, America. We studied the clutch size of passerines scruband grasslandscovering central Chile, most breeding in the southern cone of South America, of lowland north and central Argentina, all of to determine whether the clutch size of these Uruguay and the southern half of Rio Grande birds was small, like those in other two southern de Sul; the Andean region covers the Andes of continents. We try to relate our results to both Argentina and Chile from latitude 36”s to the 172 Y. YOM-TOV, M. I. CHRISTIE AND G. J. IGLESIAS north, including the Puna and the lowlands of approximation of both eggvolume and eggmass northern Chile, mostly subtropical to temperate (van Noordwijk et al. 198 1). deserts,xeric scrub and montane grasslands.The TYPE OF NEST Patagonian region comprises the cold scrub and grass steppes of southern Argentina and Chile. All nests were divided into four types: cup nests The Araucanian region occupies the temperate made of vegetational matter; nests in a hole in and cold forests of southern Chile and south- the ground or a tree; enclosednests made of veg- western Argentina. The Andean and Patagonian etational matter; or enclosed mud nests, typical regions could be considered as sub-units of a to the genus Furnarius and some Hirundinidae. larger Andino-Patagonian region. ANNUAL MOVEMENTS MATERIALS AND METHODS Although most birds of the study area are year- We studied several factors related to clutch size round residents, some speciesmigrate or move of passerinesby comparing clutch sizes in rela- from one region to another during the year. Three tion to taxonomic relationships, habitat, body types of annual movements are distinguished by size, mode of migration, and type of nest. All Narosky and Yzurieta (1989) and Belton (1984, these factors have been shown to be related to 1985): A) Birds that breed in the Northern Hemi- clutch size in one or more studies. sphere and winter in South America. Birds be- longing to this type were excluded from the anal- BODY MASS ysis as they do not breed in the study area. B) Most data on body mass were gathered from Birds which breed in the study area and migrate published literature (Belton 1985, Fiora 1933, to the neotropics for the austral winter. C) birds 1934; Contreras 1983a, 1983b, 1975, 1979; Ma- which breed in Patagonia or southern Chile and son 1985; Salvador 1988) with some additional migrate to other parts of the study area or further data provided by two of the authors (MC and north for the austral winter. GI). Sample size varied greatly between these DISTRIBUTION sourcesand between species.In caseswhere data for a specieswere given by more than one source All specieswere allocated to one or more of the the largest sample was chosen. five main zoogeographicalregions of the study area (Amazonian, Chaco, Andean, Patagonian CLUTCH AND EGG SIZE and Araucanian) using the distribution maps in There are few sources which give data on the Belton (1984,1985), Narosky and Yzurieta (1989) clutch size, eggsize, and nest type of the passer- and Johnson (1967). Introduced species (the ines of the study area. We used data from several house sparrow and two Carduelis species)were works (Belton 1984, 1985; Cavenari et al. 1991; not included in the analysis. DelaPena 1987,1988,1989;FragaandNarosky 1985; Johnson 1967; Narosky et al. 1983; Mason STATISTICS 1985) which generally give a range of the com- Ordinary least square regressionequations and mon clutch sizes or in which a value is given their statistics were calculated for the relation- with the comment that sometimes a clutch size ships between body mass and clutch size. Mean is smaller or larger by one egg.Whenever we had clutch size of the various families, regions, nest data from more than one source for the same types, and migration types were compared by species, we selected data based on the largest Student t-tests and analysis of variance (ANO- sample. We calculated a mean from the common VA). range and the word “sometimes” received a score The use of specieswithin a taxon for compar- of 0.3 egg; e.g., if a clutch size was said to be 2, ative analyses of variables has been crit- sometimes 1, it received a value of 1.7. Similarly, icized, becausesuch speciesmay share common a clutch of 2, sometimes 3, is scored 2.3. Data genetic information and therefore do not provide on egglength and width are usually given as means independent values (Harvey and Mace 1982). of a sample, and the largest sample was selected Ideally, the most satisfactory comparative anal- for this work. Egg volume was calculated using ysis should be based on comparing phylogenies, the equation Volume = 0.5 x Length x Width2 but because accurate evolutionary histories of (van Noordwijk et al. 198 1). This index is a good speciesare scarce,taxonomic classificationscan CLUTCH SIZE OF SOUTH AMERICAN PASSERINES 173

TABLE 1. Mean clutch size (+ SD) of passerinefam- TABLE 2. Mean clutch size ( f SD) in relation to type ilies in the studyarea, calculatedfrom means ofgenera. of migration. B-breed in the study area and migrate to the Neotropics; C-breed in Patagoniaand migrate

Family n Meall SD within the study area; D-residents.

7 2.547 0.718 Migration type n MeaIl SD Fumariidae- 27 3.099 G.626 Formicariidae 8 2.225 0.256 B 32 2.976 0.956 Rhinocryptidae 7 2.357 0.341 C 29 3.283 0.790 Cotingiciae 7 2.703 0.751 D 145 2.954 0.667 Pipridae 4 2.000 0.000 Tirannidae 41 2.697 0.551 Phytotomidae 1 3.000 Our sample includes 61 genera which have All Deutero-Oscines 102 2.728 0.615 migratory species.Mean clutch size of migratory Hirundinidae 6 4.583 0.516 birds and residents are given in Table 2. Al- Corvidae 1 4.000 Troglodytidae 2 5.000 0.000 though genera that breed in Patagonia and mi- Cinclidae 1 4.000 grate to northern parts of the study area for the Mimidae 1 4.060 winter have on average larger clutches than res- Turdidae 1 3.000 idents, there was no significant difference be- Motacillidae 1 3.500 tween the groups in clutch size in relation to Sylviidae 1 3.000 Vireonidae 2 3.500 0.707 migration type (ANOVA: F2,203= 2.444, P = Parulidae 4 2.725 0.320 0.0894, ns). Of the total number of 472 species Coerebidae 2 2.750 0.354 of passerinesobserved in the study area and for Tersinidae 1 3.500 which we have data, only 2% are Northern Thraupidae 13 2.858 0.560 Embehzidae 26 3.179 0.376 Hemisphere migrants, 11% migrate to the Neo- Frinsillidae 1 3.670 tropics and 10% are Patagonean specieswhich IcteGdae 12 3.297 0.896 migrate within the study area. There were no All Oscines 75 3.322 0.746 significant differences in clutch size in relation Total 177 2.980 0.733 to nest type (Table 3; ANOVA: F3,,67= 1.629; P = 0.1845). To test the hypothesisthat clutch size is influ- form an ad hoc basis of comparative studies enced by both systematicposition (suborder) and (Harvey and Purvis 199 1). Hence, in the present region we performed a two-factor ANOVA on work, we analyze data allometrically at the genus the five main regions. There were highly signif- level by calculating means for each character for icant differencesin mean clutch size between the each genus for which we had data. two suborders (F, = 20.303; P = 0.0001) and between the regions (F, = 4.198; P = 0.003), RESULTS without interaction between the two factors We gathered data on clutch size for 33 1 species (Tables 4 and 5). Mean clutch size of Deutero- of 177 genera which constitute 72% and 73% of Oscines is smaller than that of the Oscines in the total number of speciesand genera breeding every region. Overall mean clutch size is smallest in the study area. in Amazonean genera (2.67) and largest in the Overall mean clutch size is 2.98 (SD = 0.73, Chaco (3.29). IZ = 177). Mean clutch size by family is given in Mean body mass and absolute eggvolume did Table 1. There was a highly significant difference not differ between the two suborders(mean body (t = 5.66, P < 0.001) in clutch size between the mass was 30.94 and 33.34 g, ANOVA: F,,,,, = two suborders of passerines represented in the study area, with the Deutero-Oscines having on TABLE 3. Mean clutch size (*SD) in relation to type average smaller clutches(2.73, n = 102) than the of nest. Oscines (3.32, n = 75). There was no significant relationship between body mass and clutch size Nest type n Meall SD in the sample as a whole (R2= 0.005,P = 0.3837, CUP 96 2.941 0.605 n = 160) or in either of the suborders (Deutero- Hole 35 2.892 0.917 Oscines: R2 = 0.01, P = 0.353, n = 90; Oscines: Enclosednest 38 3.144 0.841 Enclosed mud nests R2 = 0.002, P = 0.739, n = 70). 2 3.750 0.354 174 Y. YOM-TOV, M. 1. CHRISTIE AND G. J. IGLESIAS

TABLE 4. Mean (&SD) clutch size of passerinessuborders in southernSouth America in relation to their main distribution regions.

Deutero-Oscines Oscines All Region n Clutch SD n Clutch SD n Clutch SD

Amazonian (A) 41 2.515 0.719 17 3.029 0.822 58 2.666 0.780 Chaco (B) 32 3.078 0.647 33 3.496 0.689 65 3.290 0.676 North Andes (C) 9 2.432 0.504 4 3.625 0.479 13 2.799 0.745 Patagonian (E) 6 2.750 0.392 3 3.667 0.289 9 3.056 0.572 Araucanian (D) 10 2.675 0.450 3 3.267 0.929 13 2.812 0.167

0.2594, P = 0.6111; egg volume 3.56 and 3.20 latitude. . . Not only do species nesting south cmz, F,,,,, = 0.7644, P = 0.3832; Deutero-Os- of the 50th parallel lay the same number of eggs tines and Oscines, respectively), but Deutero- to the clutch as other members of the same fam- Oscine eggswere proportionally larger than those ilies nesting further north, but no difference in of Oscines (14.13% and 11.26% of body mass, the number is discernible even where the same respectively. Mann-Whitney U test: n, = 104, n, species nests both in the extreme south and in = 44, U = 1,375.5, P = 0.0001). However, body the semi-tropical north.” mass explains only 9% of the variation of this The small clutch size of the southern South character. American passerines (2.98; 177 genera) is similar to that of the other two Southern Hemisphere DISCUSSION continents: mean clutch size of 299 species of Our results indicate that mean clutch size of Australian passerines is 2.69 (Yom-Tov 1987) southern South America passerines is consider- and that of 353 species of South African passer- ably smaller than would have been expected from ines is 2.8 (Rowley and Russell 199 1). We suggest the latitude range of most of the study area and that Ashmole’s (I 96 1) hypothesis as formulated that in all five main biogeographic regions mean by Ricklef s (1980) can account for the difference overall clutch size is about three eggs. Small clutch in clutch size between the Northern and Southern size is expected for the Amazonian region which Hemispheres. According to this hypothesisgeo- is closest to the tropics, but not for the rest of graphical trends in clutch size are caused pri- the regions which extend to much higher lati- marily by factors that limit populations during tudes. Even in the southernmost regions of our the nonreproductive period. Birds which do not study area (Patagonia and Araucanian region) endure long migrations or severewinters survive overall mean clutch sizes are 3.0 and 2.8, re- better, and by the end of the nonreproductive spectively. Because there is no trend of increase period they could be faced with stronger com- in clutch size from tropical (Amazonian) to tem- petition from conspecificsthan birds in which perate (Patagonia and Araucanian) regions, it does mortality during the nonreproductive period is not appear that in this region clutch size of pas- high. Hence, the former birds would be capable serines changes in relation to latitude in the same of feeding fewer young than the latter, and thus way that it does in the northern hemisphere. This lay smaller clutches. The high and mid-latitude conclusion was also reached by Johnson (1965) areas of northern hemisphere continents form for the birds of Chile. He stated that “there is no large land masseswhere much of the area has a evidence that clutch size tends to increase with continental climate and the moderating effect of the surrounding seas is not pronounced. Birds TABLE 5. Two factor analysis of variance of clutch living in theseareas face harsh winters or migrate size of the passerinesof southernSouth America. The test was carried out on the groupsincluded in Table 4. south to avoid them, and consequently may suf- fer from heavy winter lossesand produce large clutches. On the other hand, the winter climate in southern continents is much milder than that Suborder 1 9.173 9.173 20.303 0.0001 of the Northern Hemisphere due to: (1) the tri- Region 4 7.588 1.897 4.198 0.003 angular shape, with the tip pointing south, of Interaction 4 1.717 0.429 0.950 0.4371 southern Africa and South America, which en- Error 148 66.872 0.452 ables the moderating effectof the seato influence CLUTCH SIZE OF SOUTH AMERICAN PASSERINES 175 large areas; and (2) most of the area of the three ronmental conditions there is one solution. How- southern continents lying in mid- rather than ever, this is a simplistic approach to high latitude (i.e., below 40”). Consequently, the (Lewontin 1979), and adaptation and selection majority of passerines are residents and rela- might occur with no selective basis for differ- tively few speciesare migratory, as those that are ences among adaptations (Gould and Lewontin migrant travel shorter distances than Northern 1979). In Lewontin’s words “There are multiple Hemisphere migrants (Rowley and Russell 199 1). selective peaks when more than a single gene is Hence, the resident birds may suffer fewer losses involved in inlluencing a character. The exis- than Northern Hemisphere passerines, and so tence of multiple peaks means that for a fixed face more competition from conspecificsduring regime of natural selection there are alternative the breeding seasonand lay smaller clutches. paths of evolution and the particular one taken That passerines breeding in southern conti- by a population depends upon chance events. nents lay on averagesmaller clutchesthan North- Thus, it is not meaningful to ask for an adaptive ern Hemisphere does not necessarily mean that explanation of the differencebetween two species they lay fewer eggsper breeding season.Breeding that occupyalternative peaks.Hence, related taxa seasonsofbirds are longer in the Southern Hemi- often develop different adaptations as solutions sphere(Baker 1938) partly becauseoftheir mild- to the same problem. When multiple adaptive er climate. Thus, Southern Hemisphere passer- peaks are occupied, we usually have no basis for ines may lay more clutches per seasonthan do assertingthat one solution is better than another” their Northern Hemisphere counterparts. How- (Gould and Lewontin 1979). It seemsthat there ever, very little was published on breeding fre- is no adaptive explanation for the existence of quency of passerinesin our study area. small clutch size in the Deutero-Oscines and a Another result of this study is that clutch size larger one in the Oscines. Rather, they are alter- of passerinesin the study area is related not only native outcomes of the same general selective to ecological,but also to phylogenetic (taxonom- forces, and each of these two groups has reacted ic) factors. In each of the regions, Deutero-Os- within its own genetic framework, and reached tines lay smaller clutchesthan the Oscines. This a different adaptive peak. result is quite similar to the finding that among Australian passerines“ old endemics” lay smaller ACKNOWLEDGMENTS clutches than “new invaders” (Yom-Tov 1987). Deutero-Oscines lay also relatively larger eggs, Yoram Yom-Tov wishes to thank Jorge Rabinovich, and both results indicate that they lie more to- Eddy Rapoport and Jorge Protomastro for their hos- wards the “K-selected” end of the K and r-se- pitality in Argentina, for providing sourcesof infor- lection continuum (Pianka 1974). It is interesting mation and for many helpfulsuggestions; to EuanDunn, to note that on both continents the group which Rosendo Fraga, Bertram G. Murray Jr., Ian Rowley, Eleanor Russell and an anonymousreferee for helpful lays smaller clutches evolved there-the Deu- commentson the manuscript,to Jon Wright for fruitful tero-Oscines (or, according to another division, discussions,and to Arie Landsman who was very help- infraorder Tyranni, Sibley and Ahlquist 1990) in ful in computerizing the data. The data supplied by South America and parvorder Corvida (sensu Miguel Christie and Gustav0 Iglesias were collected while working under grants from the Argentine Na- Sibley and Ahlquist 1990) in Australia. Most tional Parks Administration. other families of South American passerines evolved elsewhere, either in North America or in the old world, and invaded South America LITERATURE CITED later. It seemsthat their laying of larger clutches is one of the characteristics which enabled the ARAYA,B. M., ANDH. MILLIE. 1986. Guia de campo latter birds to expand their range in the past to de las aves de Chile. 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