Trade-offs represent the costs paid in the currency of fitness when a beneficial change in one trait is linked to a detrimental change in another. If there were no trade-offs, then selection would drive all traits correlated with fitness to limits imposed by history and design. However, we find that many life-history traits are maintained well within those limits. Therefore, trade-offs must exist. Trade-offs have played a central role in the development of life-history theory, from Gadgil & Bossert (1970), Charnov & Krebs (1973), Schaffer (1972, 1974a, b) and Bell (1980) on to the present. They have been measured through field observations (e.g. Clutton-Brock, Guinness & Albon, 1982, 1983), through experimental manipulations in laboratory (e.g. Partridge & Farquhar, 1981) and field (e.g. Askenmo, 1979), through -phenotypic correlations in the laboratory (e.g. Bell, 1984a, b) and through genetic correlations (e.g. Rose & Charlesworth, 1981a, b), to mention only a few of the more prominent studies. They have been reviewed by Stearns (1976, 1977), Bell (1980), Charlesworth (1980), Warner (1984), Reznick (1985), Partridge & Harvey (1985, 1988) and most thoroughly by Bell & Koufopanou (1986). In addition, the methods used to measure trade-offs have been the subject of criticism (Tuomi, Hakala & Haukioja, 1983; Partridge, 1987) and controversy (Reznick, Perry & Travis, 1986; Bell, 1986). The most prominent life-history trade-off involves the cost of reproduction. It has two major components, costs paid in survival and costs paid in future reproduction. Two approaches to analysing those costs were suggested by Williams: genetic costs represented by antagonistic pleiotropy (Williams, 1957) and phenotypic costs represented by negative correlations between current reproductive effort and future survival and reproduction (Williams, 1966a, b). A third, physiological approach to trade-offs has been developed by Hirshfield & Tinkle (1974) and Calow (1979), among many others (cf. Townsend & Calow, 1981). In this extensive discussion, a few points have not always received the attention they deserve: (1) That trade-offs can be measured and analysed at the level of the genotype, the phenotype and what lies between (intermediate structure) is well known and uncontroversial but it has not always been emphasized that each of those levels makes an essential contribution to our understanding. It is not a question of either genetic correlations or phenotypic correlations or physiological trade-offs but of how such measurements combine to deliver information about potential evolutionary responses. A study conducted at just one of these levels is likely to be of as little use as the information on the nature of the elephant delivered by one blind man holding its tail. (2) One can draw a useful distinction between intraindividual trade-offs for example, between the reproductive effort made by a female in one season and the probability that she will survive to the next season and intergenerational trade-offs for example, between a female's reproductive effort and the probability that her offspring will survive to the next season. Intraindividual tradeoffs (and only some of them) have received most attention but intergenerational trade-offs, which are arguably just as important, have been relatively ignored. They deserve more attention. (3) The genetic structure of a population, in particular the genetic variance-covariance matrix for a set of important life-history traits, reflects the very recent past, describes the present and predicts the near-term future. There is no logical or direct way to use the current genetic structure of a population to infer the trade-offs that constrained the past approach to the current state even if they occurred as recently as a few tens of generations ago (J. Travis, personal communication). (4) Our understanding of a trade-off can be described as first order (slope known), second order (curvature known) or third order (all details, including interaction effects, known). In a few cases we have reliable information about firstorder effects. In no case known to me do we have reliable information on second-order effects, which are important in the theory (e.g. Schaffer, 1974a). Measurement of third-order effects, however desirable (Pease & Bull, 1988), remains a matter for future research.

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Trade-offs in life-history evolution

1. Energetics of reproduction in birds is reviewed with the question in mind how the parent adjusts its effort in relation to prevailing environmental conditions in order to maximize the output of young in its lifetime. Emphasis is on proximate controls, rather than ultimate factors measurable in terms of adult survival and recruitment of young. 2. The decision to breed or not to breed is clearly related to body condition of the female, presumably because of the implications this has for survival. 3. Laying date and clutch size are likewise under the influence of female condition and can hence be modified by experiments involving supplementary feeding. Natural variation in these features may often be related to territory quality. 4. How the bird decides whether or not to commence a second brood is not clear, but in the Great Tit the habitat-related difference in incidence of second broods is functionally understandable when survival probabilities of birds at different times are considered. 5. A distinction is made between a "capital" and "income" model for translatting rates of change of female body condition into appropriate decisions on laying date and clutch size and experiments are suggested that discriminate between the two. 6. Lack's view that brood size is in an evolutionary sense adjusted in order to balance food requirement and foraging capacity of the parents is accepted, and growth rates in nidicolous birds are analysed to ascertain if a finer adjustment exists superimposed on the integer steps of brood adjustment. Critical for this analysis are groups of birds where broods of one are common, since only in these circumstances is growth adjustment the only strategy open to the parents. In common with other animals, growth rate is related to mature body size but within a category of adult weight clear examples can be found for retardation of growth rate in pelecaniform and charadriiform species with singleton broods. 7. Since daily energy requirement is related to nestling size and growth rate, retardation of growth is explicable as a strategy only in terms of reducing the daily commitment of the parents, not reducing the total cost of producing a nestling. 8. An additional economy in growth is to reduce the contribution of fat to the nestling body. 9. Implied in Lack's view of brood size is a limitation of parental foraging capacity, and the last section of the paper is devoted to exploration of the proximate factors delimiting what Royama terms the optimal working capacity of parents feeding young. Observations of parent starlings confronted with manipulated brood size suggest a limit on the time that can be devoted to energetically extravagant flight activity, rather than a shortage of absolute time. Beyond the limit to which stressed parents can be made to fly, body weight declines. 10. Preliminary data on energy metabolized daily by parents confronted with large broods conforms to the simplified view that parental effort on a sustained basis equates to energy mobilization equivalent to 4 B.M.R. units and it is suggested that this level of energy expenditure represents a proximal decision substrate for determining the optimal working capacity of the parent. 11. The paper ends with a plea for more research on the proximate controls of avian reproduction, and calls attention to the central importance of the protein bank to parental body condition.

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The prudent parent: Energetic adjustments in avian breeding

Food limitation is an important issue in ecology because it can influence life history traits, population sizes, and community structure (through effects of competition). Work at the level of populations and communities has led to arguments that food limitation and competition are more important in winter than during the breeding season (e.g. 1, 2, 7, 8, 83, 85, 211, 234). In fact, it is commonly argued that food is superabundant during the breeding season (7, 8, 161, 213, 246, 250, 251, 308-311). However, such arguments are based on indirect rather than direct evidence of the effects of food on reproduction and survival (fitness) (149). Direct evidence for food limitation and competition in winter exists when survival is affected, and some experimental evidence suggests such effects (e.g. 75, 113, 122, 132). However, if current or future reproductive success is limited by food, then food limitation will also exist during the breeding season. Reproductive ecologists historically have argued that food limits reproductive success (e.g. 13, 297). Yet, even this school has included recent arguments against food limitation. For instance, Ettinger & King (77) think that perching time of birds commonly reflects loafing time because birds set their clutch and brood sizes based on years and periods of stringent (low food) conditions. However, perching time may not reflect loafing but rather an important time commitment to reproductive success (T. Martin, unpublished ms.) Thus, the status of food limitation in breeding birds is not clear. Here, I review evidence for food limitation in the context of life history theory because it provides a fundamental framework from which to interpret

Food as a limit on breeding birds: a life-history perspective

Food limitation is generally thought to underlie much of the variation in life history traits of birds. I examined variation and covariation of life history traits of 123 North American Passeriformes and Piciformes in relation to nest sites, nest predation, and foraging sites to examine the possible roles of these ecological factors in life history evolution of birds. Annual fecundity was strongly inversely related to adult survivaI, even when phylogenetic effects were controlled. Only a little of the variation in fecundity and survival was related to foraging sites, whereas these traits varied strongly among nest sites. Interspecific differences in nest predation were correlated with much of the variation in life history traits among nest sites, although energy trade-offs with covarying traits also may account for some variation. For example, increased nest predation is associated with a shortened nestling period and both are associated with more broods per year, but number of broods is inversely correlated with clutch size, possibly due to an energy trade-off. Number of broods was much more strongly correlated with annual fecundity and adult survival among species than was clutch size, suggesting that clutch size may not be the primary fecundity trait on which selection is acting. Ultimately, food limitation may cause trade-offs between annual fecundity and adult survival, but differences among species in tecundity and adult survival may not be explained by differences in food abundance and instead represent differing tactics for partitioning similar levels of food limitation. Variation in fecundity and adult survival is more clearly organized by nest sites and more closely correlated with nest predation; species that use nest sites with greater nest predation have shorter nestling periods and more broods, yielding higher fecundity, which in turn is associated with reduced adult survival. Fecundity also varied with migratory tendencies; short-distance migrants had more broods and greater fecundity than did neotropical migrants and residents using similar nest sites. HowevEr, migratory tendencies and habitat use were confounded, making separation of these two effects difficult. Nonetheless, the conventional view that neotropical migrants have fewer broods than residents was not supported when nest site effects were controlled

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Avian Life History Evolution in Relation to Nest Sites, Nest Predation, and Food

Egg size is a widely-studied trait and yet the causes and consequences of variation in this trait remain poorly understood. Egg size varies greatly within many avian species, with the largest egg in a population generally being at least 50% bigger, and sometimes twice as large, as the smallest. Generally, approximately 70% of the variation in egg mass is due to variation between rather than within clutches, although there are some cases of extreme intra-clutch egg-size variation. Despite the large amount of variation in egg size between females, this trait is highly consistent within individuals between breeding attempts; the repeatability of egg size is generally above 0.6 and tends to be higher than that of clutch size or laying date. Heritability estimates also tend to be much higher for egg size (> 0.5) than for clutch size or laying date (< 0.5). As expected, given the high repeatability and heritability of egg size, supplemental food had no statistically significant effect on this trait in 18 out of 28 (64%) studies. Where dietary supplements do increase egg size, the effect is never more than 13% of the control values and is generally much less. Similarly, ambient temperature during egg formation generally explains less than 15% of the variation in egg size. In short, egg size appears to be a characteristic of individual females, and yet the traits of a female that determine egg size are not clear. Although egg size often increases with female age (17 out of 37 studies), the change in egg size is generally less than 10%. Female mass and size rarely explain more than 20% of the variation in egg size within species. A female's egg size is not consistently related to other aspects of reproductive performance such as clutch size, laying date, or the pair's ability to rear young. Physiological characteristics of the female (e.g. endogenous protein stores, oviduct mass, rate of protein uptake by ovarian follicles) show more promise as potential determinants of egg size. With regards to the consequences of egg-size variation for offspring fitness, egg size is often correlated with offspring mass and size within the first week after hatching, but the evidence for more long-lasting effects on chick growth and survival is equivocal. In other oviparous vertebrates, the magnitude of egg-size variation within populations is often as great or greater than that observed within avian populations. Although there are much fewer estimates of the repeatability of egg size in other taxa, the available evidence suggests that egg size may be more flexible within individuals. Furthermore, in non-avian species (particularly fish and turtles), it is more common for female mass or size to explain a substantial proportion of the variation in egg size. Further research into the physiological basis of egg-size variation is needed to shed light on both the proximate and ultimate causes of intraspecific variation in this trait in birds.

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Avian egg size: variation within species and inflexibility within individuals

(1) Trade-offs between brood size and offspring growth and survival were manipulating the size of 221 great tit Parus major L. broods between 1983 (2) Nestling mass, wing length and tarsus length were inversely related to Hatching date also affected nestling growth, but its effect differed between Intrabrood variability in nestling size increased with brood size. (3) Nestling survival was inversely related to brood size. Small nestlings suffered mortality than large ones. The difference in nestling survival rate between categories was too small to equalize their productivity. (4) Fledgling survival to the autumn and to the following breeding season inversely related to brood size; fledglings from reduced broods survived better from control broods, which in turn survived better than fledglings from enlarged This resulted in the most productive category being the control broods. The fledglings increased with their size as nestlings and decreased with their hatching (5) The sex-ratio among independent fledglings was affected by brood size tion; proportionally more males survived in enlarged than in reduced broods. (6) Dispersal distance of juveniles was not affected by brood size, size hatching date, whereas it was affected by sex. The effects of starvation competition on fledgling survival and dispersal (Less)

The trade-off between offspring number and quality in the great tit Parus major

Advancement of laying of great tits by the provision of food

Brood size in the Great Tit (Parus major) was manipulated (increased, decreased, or unchanged) when nestlings were 5 days old. Both the frequency of second clutches and the interbrood interval were affected. The number of nestlings and hatching date, but not nestling and female mass, differed between first broods followed and not followed by second clutches. Hatching date and the number of nestlings in the first brood explained most of the variation in interbrood interval, whereas female mass did not contribute. Feeding first-brood nestlings and fledglings is an energy-demanding process, and the female may have to allocate resources to brood-feeding at the expense of reproductive de- velopment. Furthermore, a larger brood requires a longer period of feeding than a smaller brood. These circumstances probably explain why the size of the first brood affects the timing of the second clutch. Female condition and food depletion of the territory do not seem to be important. Because late second clutches have a lower probability of fledgling survival than do earlier ones-and consequently are of lower value from the female's standpoint-a large first clutch may delay laying to the point that a second clutch is not worthwhile. We conclude that a female's decision whether to lay a second clutch is a strategic one based on the value of the second clutch; a female that "decides" to lay a second clutch starts as quickly as possible. (Less)

Effect of experimentally altered brood size on frequency and timing of second clutches in the great tit

We studied the relative contribution of each sex and total effort expended in feeding nestlings in the great tit Parus major in relation to artificially altered brood size. A recent model suggests that feeding frequency should reflect the optimal trade-off between parental and fledgling survival, the former being negatively, the latter positively, influenced by high feeding frequencies. In both sexes weight loss was linearly related to feeding frequency. Since fledgling survival increases with nestling weight, the conditions of this model are fulfilled. However, in contrast to the predictions of the model, the total feeding frequency for both sexes combined did not differ between control and enlarged broods, but was lower for reduced ones. This outcome was not the result of a physiologically related inability of the parents to increase their delivery rate. Instead, we suggest that parents with enlarged broods could not find sufficient amounts of prey large enough to be economically worth transporting to the nest. Differences in brood-provisioning rates between the sexes may arise because costs and benefits of feeding nestlings may differ. Females lost more weight than males during the nesting period, but maintained a relatively higher weight during the incubation period. The relationship between weight loss and feeding frequency was similar for both sexes. Male and female brood-feeding frequency was related to brood size in a similar way. This is discussed in light of the great tit's mating system and the fact that the great tit is facultatively double-brooded.

https://www.jstor.org/stable/pdfplus/4600175.pdf

Feeding frequency and parental division of labour in the double-brooded great tit Parus major: Effects of manipulating brood size

One can distinguish two main hypotheses concerning the timing of breeding in temperate nidicolous birds: (1) the female starts laying eggs in response to some external cue(s) in such a way that maximum nestling demand coincides with the period of highest food abundance (Lack 1954); (2) laying females are foodconstrained and cannot start laying until they can accumulate enough energy to form eggs (Perrins 1970). The latter hypothesis was based on the observation that, while young Great Tits (Parus major) and Manx Shearwaters (Puffinus puffinus) from early broods survived better than those from late broods, clutches were often started later than would be optimal with regard to the food peak (Perrins 1965, 1966). A third hypothesis, "the food stability hypothesis" (Bryant 1975), is a variant of Lack's original hypothesis. The critical period is that of incubation rather than the nestling period, and females are thought to defer laying until the risk of encountering poor feeding conditions during incubation is minimal. It follows from Perrins' food limitation hypothesis that experimental provisioning of adequate food should result in an earlier start of laying. This has been supported in a number of studies on different species of birds (for reviews see Arcese and Smith 1988, Daan et al. 1988, Boutin 1990). The European Starling (Sturnus vulgaris) is one of the earliest migrants to return to southern Sweden in spring, arriving in late February or early March. More or less immediately on arrival, prospective nest-holes are visited and defended against other Starlings. Still laying does not start until the last days of April or, usually, the first week of May (median dates in 14 years prior to 1989 were between 29 April and 10 May; Killander and Karlsson, unpubl.). This makes the Starling an appropriate species on which to test the food limitation hypothesis; this paper presents the results of experiments in which Starlings were supplied with extra food prior to laying.

https://www.jstor.org/stable/info/1369440

Hans Källander

Papers

The trade-off between offspring number and quality in the great tit Parus major

Advancement of laying of great tits by the provision of food

Effect of experimentally altered brood size on frequency and timing of second clutches in the great tit

Feeding frequency and parental division of labour in the double-brooded great tit Parus major: Effects of manipulating brood size

Supplemental Food and Laying Date in the European Starling