On the Relationship Between Avian Clutch Size and Life Span

On the relationship between avian clutch size and life span

ERKKI HAUKIOJA & TUOMO HAKALA

HAUKI0JA, E. & T. HAKALA 1979 : On the relationship between avian clutch size and life span . - Ornis Fennica 56 :45-55 . The negative correlation between avian clutch size and life span is usually explained on the basis of trade-off between fecundity and survival . The main difficulty in such explanations, as in evolutionary ecology in general, lies in the obscurity of the criterion of fitness. Our analysis produced the following a posteriori goal for organisms : maximal probability of bridging gaps in time . This makes it possible to see longevity and fecundity as equal components of fitness. Our interpretation does not demand selection above the level of the individual, but we point out that theories based on the benefit of individuals and of genes produce different measures of success for individuals. The evolution of low reproductive rates is not in itself a problem, but the reasons for it have not been adequately studied. Variation in the ratio clutch size/life span obviously largely depends on differences in the intake of resources. The observation of trade-off between avian fecundity and survival is complicated by some special features of birds, such as potentially high life span and parental care . Due to the former it may not be adaptive to endanger adult survival for the sake of reproduction. And, since the young are dependent on parental care, a high reproductive effort leading to the death of an adult, obviously kills the whole brood . Some difficulties encountered in empirical testing of avian life history theories are discussed. Erkki Haukioja, Department of Zoology, University of Turku, SF-20500 Turku 50, Finland Tuomo Hakala, Department of Biomedicine, University of Turku, SF- 20520 Turku 52, Finland

"Most trees, if they bear too much fruit, Introduction wither away after the crop when nutriment is not reserved for themselves, and this seems to In birds, clutch size, i .e. number of be what happens to annuals, as leguminous plants, corn, and the like . For they consume eggs produced in a breeding attempt, all their nutriments to make seed, their kind varies between one and about twenty. being prolific . And some fowl after laying too Evolution of clutch size by natural much, so even to lay two eggs in a day, have selection is a general biological pro- died after this . For both the birds and the plants become exhausted, and this condition blem. Research and speculation on the is an excess of secretion of residual matter" subject go farthest in the field of (Aristotle in De Generatione Animalium) . ornithology (see Klomp 1970 for a re- 46 ORNis FENNICA Vol. 56, 1979 view), and here, too, the greatest theo- the environment, showing that adult retical advances were made in the survival over reproductive failures is 1940s and 1950s (Lack 1947, 1948, a way which may lead directly to the 1954, Skutch 1949, v. Haartman 1954, fixation of iteroparity with accom- Wynne-Edwards 1960). panying lower reproductive rates. When defining K-selection, Mac nega- Clutch size shows an overall Arthur & Wilson (1967) also suggest- life span which tive correlation with ed a hypothetical way in which repro- is especially clear in nidicolous birds. ductive capacity could be compromis- Annual production of eggs has an ed for adult survival. even stronger correlation with life span, because only short-lived species Thus there is no shortage of potent- tend to lay more than one clutch per ial explanations for the evolution of season . The evolution of the com- low reproductive rates. All of them bination low reproductive rate - long assume that when the reproductive life span has been found hard to ex- capacity decreases something else is plain. Lack's answer was the same for won. This other thing is the direct or all cases : birds reproduce as efficient- indirect improvement of adult sur- ly as they can and long-lived birds are vival. But what is the role of survival not able to rear more than small in the theory of evolutionary ecology? broods. But he ignored the core of the Research has traditionally been based problem - how low rearing capacity on population genetics and demo- of adults has evolved. Wynne-Ed- graphy, where the rate of change (in wards' answer was totally different gene frequencies or in population size) and suggested an explanation for the has been emphasized. Consequently, overall negative correlation of fecund- the fitness measure, rm or the innate ity and longevity : reproduction needs capacity of increase, has been adopted to be just high enough to balance in evolutionary ecology, too. But if mortality. Higher clutches are not only rm is the sole measure of success, it unnecessary but also deleterious. leaves little space for trade-off be- The most remarkable theoretical ad- tween fecundity and survival, because vance in the explanation of low repro- an increase in survival is inefficient ductive rates took place when Williams in increasing the value of r, (1966) concluded that reproductive Birds are characterized by low re- rates should not be seen as separate productive rates - lower, presumably, problems but are connected with adult than in their reptile ancestors. Hence survival. If efficient reproduction in- evolution towards lower reproductive creases adult mortality, i.e. there is a capacity and rm has taken place . trade-off between survival and fe- Trade-off between fecundity and sur- cundity, then lower than maximal re- vival gives the only orthodox explana- productive rates may maximize the tion and birds should be excellent total lifetime fitness of parents. Many study objects for testing the prediction variations of and slight improvements experimentally . Some attempts have on this explanation were presented been made in ornithology - mainly as later (e.g. Cody 1966, Haukioja 1970, by-products of population studies. To Hussell 1972, Charnov & Krebs 1973, our knowledge unambiguous results Goodman 1974, Anderson 1978) . are lacking which conclusively prove Murphy (1968) emphasized the role of that efficient reproductive perform- E. Haukioja & T. Hakala : Avian clutch size and life span 47 ance means higher mortality among tion due to selection 1 and the mode adults. Earlier studies have given only of heredity. Let us study these differ- weak indications that this is the case ent levels separately. (e.g. Snow 1958, Perrins 1965, v. Haartman 1971, Hussell 1972) or fail- Natural selection as differential reed to show it (e.g. Kluyver 1963, Hau- production of genotypes is the equi- kioja 1970, Ricklefs 1977). Yet, if we valent of selection 2 . Now the whole cannot demonstrate trade-off, how can process of natural selection is easily the evolution of low reproductive rates seen only as a statistical phenomenon. be understood? What are the conse- This line of reasoning regards the quences of neglecting rm as the only population as an evolutionary unit, measure of success? Is the theoretical because the change in the constitution basis sound? Are there possible biases of the population is studied. Here fit- in the empirical procedures? These nesses are end products of phenotypes types of questions are treated in this and environments, i .e. they inform us paper. of realized success, or probability of realized success. However, fitnesses without environments can be used pro- Two views of the same theoretical ductively in genetic models, because basis stochastic variation and the . mode of heredity also modify the final result, The central difficulty in evolutionary change in gene frequencies. This kind ecology is that the term fitness is not of approach does not need individuals: clear. However, fitness is what natural "The individual is simply a device selection is said to favour, and high constructed by genes to ensure the fitness is often, although anthropo- production of more genes like them- morphically, called the ultimate goal selves" (Maynard-Smith 1977). But, of an organism. But what does fitness strangely enough,the widely accepted mean for the ecologist? Is the concept ultimate goal of an organism is proof natural selection of population gen- duced by such reasoning neglecting etics ("differential reproduction of individuals : maximal genetic repre- genotypes") relevant for ecologists sentation in a population. If this goal trying to explain how certain charact- is taken literally - and it generally eristics have evolved in relation to is - only traits leading to high rm certain environmental factors? and to an asexual type of reproduction First we must analyse the basic are features which natural selection nature of the term natural selection. should favour. In our opinion, how- When studying the use of natural ever, producing "more genes like them- selection in different types of life selves" is simply a description of a history models, Tuomi & Haukioja tactic to produce surviving individuals (1979) proposed that the process of when successive generations of orga- natural selection must include two nisms tend to meet similar environ- levels : selection 1, survival and repro- mental conditions. We see no reason duction of individuals as the function why genes should be "less willing" to of their phenotypes and the environ- form new types of genes, if this solu- mental factors encountered, and se- tion would lead to phenotypes which lection 2, characters (and genes behind were better in contests with environ- them) becoming frequent in a popula- ments, i.e. in selection 1 . 48 ORNIs FENNICA Vol. 56, 1979

An a priori fitness can be used pro- that he did not use phrases like ductively in studying selection 2 but struggle for reproduction, struggle for it is totally unsuitable in studying se- evolution or struggle for rapid evolu- lection 1, where fitness is the end pro- tion. The "goal" presented above also duct and not the starting point. In accords with the possible evolutionary selection 1 we have no a priori fitness goal given by Stearns (1977) -follow- for certain types of organism, but fit- ing Mountford (1973) : to minimize the ness is always the result of contests probability of leaving no young. We between the phenotype and the en- prefer the version of the present paper, vironment. Now the ultimate goal of because Stearns' definition easily over- an organism can be seen from a differ- looks a very important point : the time entangle. The traits of organisms do period within which an adult produces not have their present constellation a replacement for itself may be of any because genes in their ancestors strove length. In theory survival without re- to ensure production of genes similar production is also a possible tactic . to their own. Rather, particular genes A gap in time can be bridged by exist now, and may even be common, longevity of the particular organism enable in- because they happened to or/and by reproduction and survival dividuals to function in agreement of the young. The longer the time with the environment. interval, the greater will be the prob- Taking the above view, let us now ability that reproduction is involved. proceed from the following axiom: The importance of reproduction is each ancestor of present organisms clear, because it also permits rapid has produced at least one replacement evolution. What is not so generally for itself. This is an a posteriori out- understood is that longevity per se is look, and it yields an a posteriori goal of similar inherent value. This is be- for an organism : to maximize the cause longevity is not a tactic for probability of bridging gaps in time. rapid evolution but to keep organisms It is the uninterrupted continuity of existing. But when tactics based on generations up to the moment of study fecundity fail, longevity may also help which determines the individuals that to make a type of organism frequent can be studied, and it is their chain a population - even to replace racters that we are trying to explain. other types of organisms . We wish to empasize that the above The above rationale relaxes the as- "goal" should not be interpreted as sumption that natural selection meaning acceptance of group selection. "maximizes" only rm Natural selec- Continuity of a particular chain of generations is quite different from avoid- tion as a process "maximizes" every- existing. ance of extinction by populations or thing that keeps organisms species . But, of course, solutions en- Adaptations leading to high r, ,, are abling a chain of generations to bridge one path to survival . Another, but gaps in time also lead to survival of equally good, path is longevity. Which populations and species, because these tactic gives a phenotype a higher rate are collections of surviving individ- of actual increase (r) depends on the uals. phenotype, the environment and The above reasoning closely agrees heredity . No explanation neglecting with the phrase used by Darwin one of these can be sufficient. (1859) : struggle for existence. Note Hence we have no difficulty in ex- E. Haukioja & T. Hakala : Avian clutch size and life span 49 plaining why natural selection has in most theoretical predictions con- produced low reproductive rates or cerning the behaviour of RE . long life spans or iteroparity. They Empirical determination of the accord well with the original Darwin- above parameters is tedious. REp and ian theory. Difficulties are created by REa can be measured from detailed models which replace the dynamic time and energy budgets. REs can be effects of environmental factors by the measured indirectly from evidence of basically static concept of a priori fit- the strain of reproduction on breeding ness. adults .

Reproductive effort - definitions Trade-off between fecundity and survival Not only production of the clutch, but also pairing, nest-building, incubation Comparison of fecundity and survival and rearing the young make demands is complicated due to the different on the adult bird. All these activities factors contributing to them. Both of may be characterized by the term re- them are based on a certain design productive effort (RE). It is usually and it is only in the design that evo- defined as the part of resources which lution can occur. As used here the the adult directs towards reproduction term design does not bear any teleo- instead of towards maintenance and logical label. Survival is a product of growth. The following three related the design in relation to resources us- aspects of RE are separated in the ed and in relation to potentially present paper: fatal environmental factors (predation, diseases, competition, accidents, etc.) . (1) The proportion of resources Fecundity, on the other hand, is a pro- which the parent uses for reproduction duct of the design for reproduction (REp) . If REp is high, the proportion and the amount of resources processed of total resources used for adult main- through that design. Within certain tenance and/or growth is low. limits, fecundity can be increased more resources absolute amount of resources simply by processing (2) The the existing design, but sur- used for reproduction (REa) . REa through which the vival is no longer enhanced if more gives the limits within than are de- reproductive processes are realized, resources are processed but it does not inform us how many manded for performing the vital activ- offspring (small and many or .large ities. Hence, fecundity may depend are produced. more directly on resources than sur- but few) vival. (3) The stress of reproduction on Theories of trade-off between fe- the adult (REs). REs should not be cundity and survival often include the confused with high mortality. . A high tacit assumption that the intake of re- REs will make organisms less resistant sources is fixed. This would mean that to environmental challenges, but en- when REp is high the amount of re- vironmental factors determine whether sources available for maintenance is higher mortality results or not, i .e. low. But the intake of resources may high REs may lead to high mortality. be increased tremendously during the In our opinion REs can replace REp breeding season in birds. Hence high

50 ORNIs FENNICA Vol. 56, 1979

REa or REp need not imply a decrease assumption is that organisms can adapt in the amount of resources available in various directions to an almost un- for adult survival. limited extent. But the whole animal kingdom manifests strong correlations Realization of trade-off. Repro- between, for example, body size and duction may lead to the death of an other measures important in life his- adult in three different ways : tory (see Blueweiss et al . 1978) . Such (1) Reproduction drains resources so correlations may indicate that there much that REs increases to dangerous- are perhaps only one or a few very ly high values and a higher mortality general laws, which determine what results during the breeding season or can be combined in a single individ- immediately after it. This is the classic ual. basis for trade-off in the theory of life Longevity is a trait of life history histories. But REs can obviously be re- which is strictly determined by other gulated by the parent. We can predict traits and can occur only in certain that those birds will manage better - combinations. Sacher (1978) de- have a greater probability of bridging monstrated that mammalian longevity gaps in time - that can limit REa is explained well (r2 = 0.85) by the when REs threatens of increase too ratios brain size/body size and body much, if adult survival is high and temperature/metabolic rate. Further he juvenile survival low. The central found that the higher longevity of question thus is: what is limited when small birds when compared with small resources are in short supply, invest- mammals having a corresponding ce- ments in reproduction or survival? If phalization coefficient and metabolic investments in reproduction are limit- rate, agrees nicely with the mammal- ed, high REs will not be observed to ian equation due to the higher body cause trade-off between fecundity and temperature in birds. Sacher concluded survival. that "the fact that mammalian long- (2) Although not increasing REs to evity increases by only one path, in an a dangerous level, REa may make invariable association with four other adults susceptible to predation and constitutional parameters, clearly im- accidents. Here the adult has less plies that the set of genetic mechan- control than in the previous case. But isms regulating these constitutional two lines for adaptation exist: a lower characters is a tightly linked system REa and/or a better design for sur- that all mammals have in common". vival. Both may decrease potential fecundity. Because the same conclusion is obviously (3) Designs demanded by high fe- also true in birds, our attempts to understand variation in life history cundity may decrease adult survival even outside the breeding season. For traits may suffer severely from lack of understanding of the innate instance, brilliant colours, which are restrict- important in pairing, may help pre- ions. We simply do not know how much the dators to locate such individuals more lower fecundity of long- easily than birds with duller colouring. lived organisms results from environmental influences and how much from What is the degree of freedom in internal limitations. evolution of fecundity and longevity? Stearns (1976, 1977), especially, has In most evolutionary analyses the tacit emphasized the role played by design E. Haukioja & T. Hakala : Avian clutch size and life span 51 constraints in hindering the arrival of creases the importance of sensory abil- predicted optimum points. We agree ities and nervous development. Due to with this but at the same time wish to such intercorrelated features birds are point out that design constraints may potentially long-lived and have a high also be caused by natural selection. probability of surviving till the next For instance, if a trait favouring lon- breeding season - adult mortality evity is easily evolved again and rates are seldom higher than 0.6 and again, and if an organism with this may be much lower. Parental care, trait less often bridges gaps in time which is made possible by high nerv- than an alternative type with a trait ous development, permits a large an- favouring fecundity, design constraints nual REa to be distributed over a long which hinder the birth of long-lived period so that REs does not increase types will probably evolve . Converse- to high values at any one time . Par- ly, design constraints which make high ental care also provides a highly pre- fecundity impossible may evolve if in- dictable environment for the young dividuals bearing them are more suc- and, consequently, these are adapted cessful in bridging gaps in time than to take advantage of it. If an adult, individuals with high fecundity. especially a female, succumbs during As regards avian reproduction, Case the breeding attempt, the whole brood (1978) called attention to the large will probably perish, too. This in- minimum size of bird eggs as com- creases the probability that the adult pared with the eggs of reptiles of cor- will die without leaving any young. responding size . He concluded that it Hence, in our opinion, no solutions in is the development of the nervous which REs will threaten adult survival system that determines the minimum are likely to evolve in long-lived size of bird eggs . Thus the potential birds . This means that trade-off be- number of eggs which can be formed tween fecundity and survival due to from a certain REa is not high. An- high REs cannot be a characteristic other factor contributing to the same feature in birds. This does not elimin- direction is the habit of birds to lay, ate trade-off due to predation and at the most, one egg per day. This is accidents. obviously necessary to maintain flight Empirical testing of trade-off be- ability, but it also limits the size of a tween avian fecundity and survival is clutch, because the interval between complicated by the phenotypic plastic- the first and the last egg cannot be ity of clutch size, too. It is not too high increased indefinitely. A factor re- an REa as such which leads to higher stricting clutch size still further is the adult mortality, but more probably too incubation capacity of the parents (e.g. high an REa under particular environ- Anderson 1976) . mental conditions . Trade-off might be more clear in exceptional years than Trade-off in birds. Birds have a in normal years, but this is hard to well-developed nervous system and prove. But it is clear that the longer high metabolic rate. Besides they are the potential life span in adults, the flexible in their reactions due to their more likely they will be to meet bad high nervous development. Their years, and, if they are not able to mobility enables them to select and predict bad years (see Hirschfield & move to a suitable environment (see Tinkle 1975), the less they can afford Wolsky & Wolsky 1976), which in- to endanger survival. 52 ORNIs FENNICA VOI . 56, 1979 E. H,

Pitfalls in empirical testing of avian ly ready to rear their natural brood ing life history theory may not be able to adapt to the de- con( mands of an artificially increased fled To our knowledge the material necess- brood (Wynne-Edwards 1964) . fact ary for testing general theories of life (4) If parents belonging to a potent- datz histories does not exist, since the re- ially long-lived species cannot predict mus levant parameters (adult mortality due the availability of resources during the soul to breeding and the remaining adult breeding season, their best tactic for old( mortality, juvenile mortality, clutch bridging gaps in time is obviously to wea size, available resources) have not been lay smaller clutches than they can rear by measured in several populations- in any under good conditions. If in a good to species of birds. But certain important year they are able to rear extra young, 196 points have been studied, especially this is of limited value as a proof of ed on two fronts: the correlation between general theories. exa the number of young and their sur- (5) When egg numbers are mani- lati vival, and the correlation between pulated, the survival of the adults is les~ brood and clutch size and adult sur- seldom measured at the same time. ing vival rates. The best way to demonstrate poss- (Ri Clutch and brood sizes have been ible trade-off between fecundity and ma manipulated in several species of birds adult survival would probably be to the to find out whether they have the decrease the size of a clutch or brood, bre monitor the l largest clutches and broods that they and to the survival of ing are able to rear. Results from experi- parents. If they survive better, this ments where extra young or eggs have can be taken as evidence of trade-off. na been added to nests have suggested But even such a result must be in- sid that natural brood sizes are the same spected with caution. Kluyver (1966) is or smaller than the most productive artificially halved the production of it the Great-Tit major, of brood size (see Klomp 1970) . The young by Parus wl latter conclusion, especially, is un- which improved both adult survival certain due to the following potential and juvenile survival. But the goal of of error: the study was to examine regulation du sources of ur (1) The fate of the young can usual- of density and hence the whole popu- m, ly be followed only up to their leaving lation was treated. Thus the decisive the nest. However, clutch size-depend- factor may have been decrease in ent differences in the mortality of the density rather than trade-off between young may become evident just at the fecundity and survival. A more in- D time they become independent (Hauki- formative test would be to manipulate oja 1970) . a small proportion of broods, so that T (2) The probability that an egg will the total density of the population was m produce a fledgling may depend on not significantly affected. However, w the order in which it is laid in the one difficulty remains : if birds with r( clutch (e.g. Nisbet 1973, Antikainen low reproductive success are more apt p; 1978) . If an egg chosen randomly to change nesting sites, their move- 91 from another clutch is added to an ments may mask the trade-off . ir experimental clutch, it may be more It is difficult to obtain reliable data p likely to produce a fledgling than a on survival rates e.g. for correlation a further egg laid by the female. analyses. Potential life spans can, at e (3) Parents which are physiological- least in theory, be determined by rear- E. Haukioja & T. Hakala : Avian clutch size and life span 53 ing birds in captivity under optimal cundity and survival are surprisingly conditions . Actual survival rates, re- scanty in the literature (see Stearns flecting the influence of environmental 1976, Calow 1977) . Although such factors, are easier to obtain. Ringing trade-off is the most probable reason data provide abundant material, but for the evolution of low reproductive must be used with caution. Common rates and for the failure of more sources of error are loss of rings in fecund types to replace the present older age classes due to ring metal ones, we are somewhat sceptical about wearing away, and bias introduced the possibility of verifying this con- by greater vulnerability of young birds clusion empirically, especially among to hunting by man (see Haukioja potentially long-lived species of birds. 1969) . More difficulties are encounter- If adults do not invest in reproduction ed when ringing files are used to more than remains after their needs examine adult mortality rates in re- for maintenance have been satisfied, lation to breeding. To us it seems use- trade-off between fecundity and sur- less to compare ringing recoveries dur- vival is not easy to demonstrate. ing and outside breeding seasons Potentially short-lived species are (Ricklefs 1977), because recovery rates obviously more likely to give positive may be different at different times of results. A further complication is that the year, and for other reasons than parents which are able to rear the breeding. largest broods, for example experienc- Birds are excellent objects for study- ed ones, are the most likely to do so, ing the heritability of clutch size in and the survival of such individuals natural environments . However, be- need not to be threatened at all. sides heritability, phenotypic plasticity As regards the classic explanations is also of the utmost importance, and of low reproductive rates offered by it can be revealed by detailed studies Lack and Wynn-Edwards, we must of individually marked birds. Actually point out that both concentrated on what should be known is heritability one side of the coin. Lack's explana- of susceptibility to environmentally in- tion has proved to be very successful duced variation in clutch size. Unfort- for clutch sizes in populations follow- unately, the chances of obtaining such ing the same design for longevity, but material are very slight. in our opinion he overemphasized the role of fecundity and basically had a narrower view of the whole problem Discussion than Wynne-Edwards. Unfortunately, however, the latter applied selection The original theory of evolution by above the individual as an explana- natural selection contains nothing tion. We do not see any difficulties in which disagrees with evolution of low explaining the same traits - e.g. those reproductive rates, longevity or itero- preventing overexploitation - on the parity. The obscurity surrounding the basis of the benefit of the individual. question comes largely from traditions The measure of success, however, in other fields of biology, which have should not be increased frequency as produced models valid in their origin- such but continuity of generations . At al fields but not always applicable in the same time we can discard the un- evolutionary ecology. realistic conditions for the evolution Examples of trade-off between fe- of low reproductive rates demanded 54 ORNIS FENNICA Vol. 56, 1979 by group selection : existence of small luotu lähinnä biologian teoreettisten mallien populations which be- kehittelyn yhteydessä, sovellettaessa muiden semi-isolated alojen teorioita suoraan ekologisiin probleemoi- come extinct after destroying their re- hin. Luonnollisesti syyt ja ympäristötekijät, sources through overpopulation . The jotka ovat johtaneet alhaisten pesyekokojen same end result can be achieved if kehittymiseen ovat paljolti selvittämättä. longevity is a better tactic in bridging Ornitologian alalta ei vielä mistään lintu- lajista ole selvitetty useasta eri populaatiosta gaps in time than fecundity and if the kaikkia niitä parametreja (esim. pesyekoko, sen goal of organisms is simply to bridge vaihtelu, nuorten kuolevuus, pesinnästä johtuva gaps in time. aikuiskuolevuus, muu aikuiskuolevuus, saata- villa olevat resurssit), jotka olisi tunnettava Acknowledgements . We wish to thank Martti yleisten elinkiertoteorioiden testaamiseksi. Soikkeli, Esa Lehikoinen and Risto Lemmetyi- nen for comments on an earlier draft of this paper. Juha Tuomi called our attention to the motto of this paper and Anna Damström kind- References ly checked the language, for which we express our gratitude. ANDERSSON, M. 1976 : Clutch size in the Long- tailed Skua Stercorarius longicaudus: some field experiments. - Ibis 118 :586- 588. Selostus: Lintujen pesyekoon ja eliniän ANDERSSON, M. 1978 : Natural selection of off- välisestä suhteesta spring numbers: some possible inter- generation effects. - Amer . Natur. 112 : Negatiivinen korrelaatio lintujen pesyekoon ja 762-766. eliniän välillä selitetään tavallisesti siten, että ANTIKAINEN, E. 1978 : The breeding adaptation linnun keskittäessä resurssejaan lisääntymistoi- of the Jackdaw Corvus monedula L. in mintoihin, niitä jää vähemmän aikuisen omaan Finland. - Savonia 2 :1-45 . ylläpitoon . Näin voidaan traditionaalisen kel- BLUEWEISS, L., H. FOX, V. KUDZMA, D. NAKA- poisuuskäsitteen (fitness) pohjalta selittää, mik- SHIMA, R. PETERS & S. SAMS 1978 : Re- si linnuilla tavataan myös erittäin pieniä pesye- lationships between body size and some kokoja . Kuitenkaan kelpoisuus evolutiivisen life history parameters. - Oecologia menestyksen mittana ei kirjoittajien käsityksen (Berl.) 37 :257-272 . mukaan kerro koko totuutta, kun tarkastellaan CALOW, P. 1977 : Ecology, evolution and miksi tietyt elinkiertojen piirteet ovat kehitty- energetics : a study in metabolic adapta- neet suhteessa tiettyihin ekologisiin ympäristö- tion . - Adv. Ecol . Res. 10 :1-62. tekijöihin. Kelpoisuuden mittana evolutiivisessa CASE, T. J. 1978 : Endothermy and parental ekologiassa käytetään usein suoraviivaisesti po- care in the terrestrial vertebrates. - pulaatiogenetiikasta ja demografiasta tuttuja Amer . Natur. 112 :861-874 . populaation kasvunopeuden kuvaajia, esimer- CHARNov, E. L. & J. R. KREBS 1973 : On kiksi populaation sisäistä kasvunopeutta, johon clutch size and fitness. - Ibis 116 :217- lisääntymiskapasiteetti luonnollisesti vaikuttaa 219. nopeuttavasti. Pitkän eliniän merkityksen ym- CODY, M. L. 1966 : A general theory of clutch märtäminen tulee kuitenkin vaikeaksi, koska se size . - Evolution 20 :174-184 . ei juuri nosta populaation sisäistä kasvuno- DARWIN, C. 1859 : On the origin of species by peutta . Kirjoitus rakentuu käsityksen varaan, means of natural selection or the pre- jonka mukaan organismin menetyksen mitta servation of favoured races in the struggle tietyllä hetkellä on vain sen olemassaolo. Ts . for life . - Oxford . kaikki sellaiset ominaisuudet ovat edullisia, GOODMAN, D. 1974 : Natural selection and a jotka yksilöiden kohtaamissa ympäristötilan- cost ceiling on reproductive effort . - teissa minimoivat sukupolvien ketjun katkea- Amer . Natur. 108 :247-268 . mistodennäköisyyttä. Korkea populaation si- v. HAARTMAN, L. 1954 : Der Trauerfliegen- säinen kasvunopeus osoittaa yhden taktiikan schniipper . III . Die Nahrungsbiologie . - ko. ongelman ratkaisemiseksi, pitkä elinikä toi- Acta Zool. Fennica 83 :1-96. sen. Tältä pohjalta voidaan pitkäikäisyyttä pi- v. HAARTMAN, L. 1971 : Population dynamics . tää yhtä taseveroisena kelpoisuuden osatekijänä In D. S. FARNER & J. R. KING (eds .) : kuin lisääntymistehokkuuttakin. Avian biology, Vol. 1, pp . 391-459. New Alhaisen lisääntymiskapasiteetin kehittyminen York and London . ei siis sinänsä ole ongelmallista, ongelmat on HAUKIOJA, E. 1969 : Mortality rates of some

E. Haukioja & T. Hakala : Avian clutch size and life span 55

Finnish Passerines. - Ornis Fennica 46 : PERKINS, C. 1965 : Population fluctuations and 13-21 . clutch-size in the Great Tit, Parus major HAUKIOJA, E. 1970 : Clutch size of the Reed L. - J. Anim. Ecol . 34 :601-647 . Bunting Emberiza schoeniclus . - Ornis RICKLEFs, R. E. 1977 : On the evolution of Fennica 47 :101-135 . reproductive strategies in birds: repro- HIRSHFIELD, M. F. & D. W. TINKLE 1975 : ductive effort . - Amer. Natur. 111 :453- Natural selection and the evolution or re- 478. productive effort. - Proc. Nat. Acad . Sci. SACHER, G. A. 1978 : Longevity and aging in USA 72 :2227--2231 . vertebrate evolution . - BioScience 28 : HUSSELL, D. J. T. 1972 : Factors affecting 497-501 . clutch size in Arctic passerines . - Ecol . SKUTcH, A. F. 1949 : Do tropical birds rear as Monogr . 42 :317-364 . many young as they can nourish? - Ibis KLOMP, H. 1970 : The determination of clutch 91 :430-455 . size in birds. A review. - Ardea 58:1- SNOW, D. W. 1958 : The breeding of the 124. Blackbird Turdus merula at Oxford . - KLUYVER, H. N. 1963 : The determination of Ibis 100:1-30. reproductive rates in Paridae . - Proc. STEARNS, S. C. 1976 : Life history tactics : a XIII Int . Ornithol. Congr., p. 706-716 . review of the ideas. - Quart. Rew . Biol . KLUYVER, H. N. 1966 : Regulation of a bird 51 :3-47. population . - Ostrich 38, Suppl. 6 :389- STEARNS, S. C . 1977 : The evolution of life 396. history traits : A critique of the theory and LACK, D . 1947 : The significance of clutch- a review of the data . - Ann. Rev. Ecol . size. Parts I-II. - Ibis 89 :302-352 . Syst . 8 :145-171 . LACK, D. 1948 : The significance of clutch- Tuomi, J. & E. HAUKIOJA 1979 : An analysis size. Part III . - Ibis 90 :25-45 . of natural selection in models of life his- LACK, D. 1954 : The natural regulation of tory theory. - Savonia 3 (in press) . animal numbers. - Oxford . WILLIAMS, G. C. 1966 : Natural selection, the MACARTHUR, R. H. & E. O. WILSON 1967 : cost of reproduction, and a refinement of The theory of island biogeography - Lack's principle . - Amer. Natur. 100 : Princeton, N. J. 687-690. MAYNARD SMITH, J. 1977 : The limitations of WOLSKY, M. & A. WOLSKY 1976 : The me- evolution theory . - In R. DUNCAN & M. chanism of evolution : a new look at old WESTON-SMITH (eds .) : The encyclopedia ideas. Contributions to human develop- of ignorance, 235-242 . Oxford . ment . 4. - Basel. MOUNTFORD, M. D. 1973 : The significance of WYNNE-EDWARDS, V. C. 1962 : Animal dis- clutch size. - In M. S. BARTLETT & R. persion in relation to social behaviour. - H. HIORNS (eds .) : The mathematical Edinburgh and London. theory of the dynamics of biological popu- WYNNE-EDWARDS, V. C. 1964 : Significance of lations. New York . clutch-size in Swift and Grouse .-Nature MURPHY, G. I. 1968 : Pattern in life history 203 :99 . and the environment . - Amer. Natur. 102 :391-403 . NISBET, I. C. T. 1973 : Courtship-feeding, egg- size and breeding success in Common Terns. - Nature 241 :141--142 . Received February 1979