Clutch Size In Birds Research Articles

The Insurance Hypothesis and the Theory of Clutch Size in Birds and in Invertebrates

The Insurance Hypothesis and the Theory of Clutch Size in Birds and in Invertebrates

Components of uncertainty in clutch-size optimization

Environmental uncertainty can be both a cause and consequence of chance variation in many of the phenotypic factors associated with the control of clutch size in birds. When such uncertainty inflates or otherwise influences the variance associated with expected reproductive success for any genotype, it will also influence the resulting phenotypic optima. Random variation that affects the evolution of clutch size optima explicitly may occur both within (intra-) and across (inter-) generations. Examples of intra-generational uncertainty could include chance variation in: (1) the quality and quantity of offspring, (2) parental quality, and (3) temporal resources like food. Inter-generational uncertainty would include chance variation in demographic and population characters. With respect to clutch (or litter) size, almost all forms of uncertainty tend to favor an optimum (genetic) strategy with a clutch that is smaller than the clutch associated with the apparent or actual maximal fitness of an individual parent. The overall effect of all the components of uncertainty can be evaluated through the integration of all this phenotypic variation: however each step of the integration is a conditional expectation of each component. Therefore, a single factor analysis may indicate a false optimum, and an integrated analysis of all components is necessary to evaluate the importance of their individual and joint effects on the adaptive evolution of clutch size.

The costs of reproduction in tree swallows (Tachycineta bicolor)

We investigated the effect of brood size on nestling growth and survival, parental survival, and future fecundity in tree swallows (Tachycineta bicolor) over a 4-year period (1987–1990) in an effort to understand whether reproductive trade-offs limit clutch size in birds. In addition to examining naturally varying brood sizes in a population on Kent Island, New Brunswick, Canada, we experimentally modified brood sizes, increasing or decreasing the reproductive burdens of females by two offspring. Unlike previous studies, broods of the same females were enlarged or reduced in up to 3 successive years in a search for evidence of cumulative costs of reproduction that might go undetected by a single brood manipulation. Neither observation nor experiment supported the existence of a trade-off between offspring quality and quantity, in contrast with the predictions of life-history theory. Nestling wing length, mass, and tarsus length were unrelated to brood size. Although differences between means were in the direction predicted, few differences were statistically significant, despite large sample sizes. Nestlings from small broods were no more likely to return as breeding adults than nestlings from large broods, but return rates of both groups were very low. Parental return rates were also independent of brood size, and there was no evidence of a negative effect of brood size on future fecundity (laying date, clutch size). Reproductive success, nestling size, and survival did not differ between treatments for females whose broods were manipulated in successive years. Within the range of brood sizes observed in this study, the life-history costs of feeding one or two additional nestlings in tree swallows appear to be slight and cannot explain observed clutch sizes. Costs not measured in this study, such as the production of eggs or postfledging parental care, may be more important in limiting clutch size in birds.

Adaptation: Statistics and a Null Model for Estimating Phylogenetic Effects

-Tests of adaptive explanations are often critically confounded by phylogenetic heritage. In this paper we propose statistics and a null model for estimating phylogenetic effects in comparative data. We apply a model-independent measure of autocorrelation (Moran's I) for estimating whether cross-taxonomic trait variation is related to phylogeny. We develop a phylogenetic correlogram for assessing how autocorrelation varies with patristic distance and for judging the appropriateness and effectiveness of an autoregressive model. We then revise Cheverud et al.'s (1985, Evolution, 39:1335-1351) autocorrelational model to incorporate greater flexibility in the relation between trait variation and phylogenetic distance. Finally, we analyze various comparative data sets (body weight in carnivores, clutch size in birds) and phylogenies (morphological, molecular) to illustrate some of the complications that may arise from using an autoregressive model and to explore the effects of different weighting matrices in adjusting for these complications. Although our approach has limitations, it is both effective in partitioning trait variation into adaptive and phylogenetic components and flexible in adjusting to peculiarities in taxonomic distribution. [Phylogenetic effects; phylogenetic correlation; autoregressive models; comparative methods.] The comparative method is commonly used to investigate adaptation. A researcher examines the attributes of a number of species. Statistical analyses of these data are then used to formulate and test adaptive hypotheses of life history, morphology, physiology, demography, and behavior (e.g., Clutton-Brock and Harvey, 1977; Damuth, 1981; Gittleman and Harvey, 1982; Harvey and Clutton-Brock, 1985; Gittleman, 1986a, b; Huey and Bennett, 1987). If traits are analyzed across a broad range of independently derived taxa, the resulting adaptive explanations may be quite robust (Clutton-Brock and Harvey, 1984; Huey and Bennett, 1986; Gittleman, 1989). If, however, the data reflect a highly structured phylogeny (with little statistical independence), results may be misleading (Felsenstein, 1985). To neglect phylogeny is to invite type I and type II errors (see Fig. 1). A number of techniques have been developed for removing the effects of phylogeny (see reviews in Huey, 1987; Pagel and Harvey, 1988; Gittleman, 1989; Burghardt and Gittleman, 1990). Some of these techniques are better suited for particular variables or certain evolutionary questions, and all possess limitations. Nominal or categorical data (e.g., mating system: monogamy, polygamy) may be analyzed by evaluating the agreement between the variation in a trait and an accepted phylogeny (Dobson, 1985; Greene, 1986) or by using outgroup comparisons to identify evolutionary transitions among traits (Gittleman, 1981; Ridley, 1983). For quantitative data, there are several strategies. One may avoid spurious correlation by averaging over closely related species, thereby reducing the degrees of freedom and significance of the correlation. Alternatively, one may transform the data so that phylogenetically disparate groups appear on a common scale. Even within this general framework there are several methods for evaluating the association between the ordinal or continuous values of a trait and phylogeny: (1) Nested analysis of variance partitions the total variation in a continuous character among various taxonomic levels. By selecting the taxonomic level that accounts for the greatest proportion of the total variance as the appropriate level for analysis, this method attempts to control for bias from low-level clades that are both uniform and speciesrich (Harvey and Mace, 1982; Harvey and

Directional Selection and Clutch Size in Birds

Directional Selection and Clutch Size in Birds

Cost of reproduction and covariation of life history traits in birds

Evolution of life history traits can be studied at two different levels: (1) current selection processes, including trade-offs in life history traits in natural populations as revealed by observations or, preferably, exieriments; and (2) patterns of variation in life history traits with each other and with ecology among extant species. Selection is not evolution, but selection pressures must have caused evolutionary change and led to current patterns of life history traits. These problems are exemplified by recent research on clutch size in birds.

THE EVOLUTION OF CLUTCH SIZE. II. A TEST OF THE MURRAY-NOLAN EQUATION.

We test the Murray-Nolan equation for calculating clutch sizes of birds with demographic data from a long-term study of the Florida Scrub Jay (Aphelocoma c. coerulescens). The predicted value, 3.43, is very close to the observed clutch size, 3.33. This result provides further support for the equation's being an adequate description of the relationship among the factors affecting the clutch size of birds.

Selection on Clutch Size in Birds

In bird populations, it is commonly observed that (1) clutch size is positively correlated with the number of surviving young a clutch produces and (2) the most productive clutch size is higher than the average clutch size. We review hypotheses that have been proposed to account for these observations. We then show that the patterns are inevitable if females in different health or nutritional states vary in both the number of eggs they lay and the number of chicks they raise, for which there is good empirical evidence. A persistent selection differential for clutch size is expected at evolutionary equilibrium, even if clutch size is heritable.

THE EVOLUTION OF CLUTCH SIZE. I. AN EQUATION FOR PREDICTING CLUTCH SIZE.

We derive an equation for calculating the clutch sizes of birds and other long-lived animals from Murray's (1979) theory on the evolution of clutch size. For the Prairie Warbler (Dendroica discolor) in Indiana, this equation predicts an average clutch size of 3.49, less than half an egg smaller than the recorded average clutch size of 3.89. We attribute the discrepancy to sampling error and suggest that the equation satisfactorily identifies the important factors affecting the evolution of clutch size. The success of the equation in predicting clutch size of the Prairie Warbler provides additional support for Murray's theory on the evolution of clutch size.

Clutch Size in Birds: A Predation Perspective

A simple model of clutch size in nidicolous birds is developed in which the cost of reproduction is the risk of predation to both the parent and its dependent young. An analysis of the model shows that (1) conventional ideas of food limitation, though sufficient, are not necessary for existence of an optimal clutch size; (2) predation as the sole cost of reproduction is adequate for the existence of an optimal clutch size; and (3) one need not expect a clutch—size—dependent physiological cost to a parent (leading to increased mortality) to be a major determinant of clutch size. In addition, several experimental and observational results that purport to demonstrate food limitation in breeding birds are also consistent with the idea of predation risk as the major cost of reproduction. Current ideas of clutch size determination and the costs of reproduction are considered in light of the above results, and possible syntheses are suggested. A predation—risk perspective may offer considerable insight into the evolution of clutch size in birds.

Temporal Components of Reproductive Variability in Eastern Kingbirds (Tyrannus Tyrannus)

The influences of weather and food supply on Eastern Kingbird (Tyrannus tyrannus) reproduction were examined using data on annual (1980-1983) and seasonal variation in timing of breeding, clutch size, and egg mass in a population of kingbirds breeding in eastern Kansas, USA. Timing of breeding differed significantly among years and appeared to be related to insect abundance. Clutch size declined significantly with breeding date in all years, but differed significantly only between 1980 and 1981. A severe drought in 1980 likely depressed food availability, whereas an emergence of periodical cicadas in 1981 provided a superabundant food source. Across all years, however, clutch size varied inversely with the mean breeding date of the population, whereas average egg mass varied directly with food availability. Maximum annual differences in mean clutch and egg size were small compared to variation in food availability. I examined 10 hypotheses that have been proposed to explain seasonal changes of clutch size in birds. All but three were rejected as possible explanations for kingbirds. The tendency of older females to lay both earlier and larger clutches is probably responsible for most of the seasonal decline of clutch size, but reduced probability of survival for offspring fledged late in the breeding season and avoidance of stress in adults just prior to migration may also be contributing factors. Reduced clutch size late in the breeding period should permit females to hatch young sooner, to raise them faster, and to fledge them at higher masses. These features should increase survival of both juveniles and adults since postfledging care is long in kingbirds.

REPRODUCTIVE COST, AGE-SPECIFIC SURVIVAL AND A COMPARISON OF THE REPRODUCTIVE STRATEGY IN TWO EUROPEAN TITS (GENUS PARUS).

Theoretical analyses of optimal reproductive rates usually assume a trade-off between offspring production and parental survival. This study verified a survival cost for willow tit males; nonbreeding males survived better than males attending a brood. Theory also predicts a smaller clutch size in birds that are less successful in transforming reproductive investments into mature offspring. As predicted, we found that crested tits, suffering a higher nest predation rate, laid smaller clutches than willow tits. The generally lower survival rate of willow tit adults may largely be attributed to their higher reproductive commitment (larger willow tit clutch size), because no significant interspecific survival difference remained between nonbreeding males. Finally, in willow tits we found a positive correlation between average clutch size and juvenile survival rate (density-dependent) the ensuing year, suggesting that willow tits may adjust clutch size in response to changing survival prospects for their young by using the breeding density as a cue.

Evolution of Clutch Size in Birds: Adaptive Variation in Relation to Territory Quality

Reproductive output from enlarged or reduced magpie broods showed that each female generally lays a clutch of optimal size. This size varies considerably between females. Approximately 85 percent of the within-years variation in clutch size was associated with differences between territories. Colonial bird species, lacking individual foraging territories, have a smaller clutch size variation than territorial species.

Laying Dates, Clutch Size and Egg Weight of Captive Mallards

This has led to several major reviews on the possible determinants of clutch size in birds (Cody 1971, Klomp 1970, von Haartman 1971). Waterfowl have posed particular problems because the chicks are precocial and do not require parental feeding, a factor that correlates with clutch size in many altricial species (Lack 1968a). Most hypotheses concerning the evolution of clutch size in waterfowl have implied that food supply for the female prior to laying and/or for the young is both a proximate and an ultimate determinant (Bengston 1971, Johnsgard 1973, Lack 1967, 1968a, 1968b, Ryder 1970). We report the results of an experiment in which single pairs of Mallards (Anas platyrhynchos) were held in identical breeding compartments and were thereby subjected to the same environmental cues for breeding. We determined laying dates, clutch sizes and egg weights for the same pairs of birds in 1973, 1974 and 1975. The responses of the birds in terms of the three parameters of nesting were considered to closely reflect interactions between genotype and environment. They should provide bases for evaluating proximate environmental effects in wild populations.

The nest as a factor determining clutch-size in tropical birds

Discussions of the evolution of clutch-size in birds have largely ignored the physical characteristics of nests. In the tropics, the size and structure of nests have evolved under the influence of intense predation by nest-predators. One result of selection for inconspicuousness has been a reduction in nest size, in some cases to an extreme degree. It is argued that reduction in nest size has been an important factor limiting clutch-size, and, more generally, that the evolution of clutch-size cannot be fully understood without considering the dimensions and other physical properties of nests.

Components of Avian Breeding Productivity

Numbers of nestlings fledged per pair per season were calculated for 35 species of passerine birds in four localities from data on length of breeding season, clutch size, nesting mortality, and length of nest cycle. Mean numbers of nestlings fledged was 6.42 for 12 woodland and edge species in Kansas, 4.33 for 8 species in lower Sonoran desert habitat in Arizona, 2.36 for 6 species in humid forest and second-growth habitats in Costa Rica, and 4.80 for 9 species in a desert scrub habitat in Ecuador. The data were analyzed by multiple correlation and stepwise multiple regression to determine the contribution of the variables to variation in the number of young fledged from one species to another in each locality. Variations in season length, clutch size, and breeding success were important components in variation in numbers of young fledged in Kansas, but length of nest cycle, being positively correlated with both clutch size and nest success, had the strongest correlation with the number of young fledged. In Arizona and Costa Rica, variation in breeding success held the key to predicting variation in number of young fledged, whereas in Ecuador season length and clutch size were the critical variables. The analyses suggest that data on production of young and the factors that cause production to vary from one species to another would add immensely to our understanding of the population ecology of birds.-Department of Biology, University of Pennsylvania, Philadelphia, Pennsylvania 19174. Accepted 1 July 1975. Ornithologists have pondered the significance of clutch size in birds for nearly three decades without fully resolving the factors responsible for its variation among species and regions. Although Skutch (1949) linked the evolution of clutch size to adult mortality and population turnover rate, population biologists have only re- cently placed clutch size in a demographic context. For example, not until 1961 did Drury point out that the production of young by a pair of birds during a breeding season depends on the number of broods raised as well as clutch size. Cody (1971) and Ricklefs (1973) treated avian demography more fully and demonstrated the use of life table analyses to compare species. Although breeding biology and mortality have been investigated in scores of de- tailed field studies, complete life tables have been constructed for only a handful of species. Of the statistics required to complete a life table, the annual production of young is one of the most difficult to obtain. Observation of individually marked birds yields relatively few data per hour of effort (for example see Nice 1937, Snow 1958). Furthermore, estimates of mean annual productivity obtained from direct observa- tion have large probable errors (wide confidence limits) owing to the great variation between individuals. Annual productivity can be calculated indirectly from clutch size, breeding success, nest cycle length, and length of breeding season (Ricklefs 1970). These data are more readily obtained than direct measures of breeding pro- ductivity. Furthermore, data from different studies may be combined to calculate the annual production of a species. In this paper we calculate annual production (fledglings per pair of adults) for various passerine birds in two temperate and two tropical localities. The data are

An Explanation of Egg-Weight Variation in the Lesser Snow Goose

Lesser snow goose (Chen caerulescens) eggs were weighed at the McConnell River, N.W.T., in 1972 and 1973. Mean egg weight was the same in different size clutches. Within each clutch size there was great variability in egg weights; the lightest eggs weighed only 60 percent as much as the heaviest. This variation was greater than that reported for other waterfowl species. The egg-weight variation resulted in overlap between the weight of the heaviest clutches of a particular size and the lightest clutches of the next larger size. Hence, the nutrient commitment to a particular clutch size was not the same for all females. An hypothesis is presented to explain the constancy of mean egg weight among clutch sizes and the great variation in egg weights within each clutch size. We assume that there is a heritable component in egg weight of the lesser snow goose and that heavier eggs enhance gosling survival under energetically poor conditions. Thus, we argue that the mean egg weight is adapted to the average environmental conditions at hatch and propose that a wide range of egg-weight genotypes survives because of the great annual variation in environmental conditions at hatch. J. WILDL. MANAGE. 40(4):729-734 An important factor in the evolution of clutch size in birds that do not feed their young is the size of egg that provides adequate nutrient reserves for the newly hatched young (Lack 1967, 1968a:233-234, Ryder 1970, Klomp 1970). This optimum egg size generally is presumed to be relatively constant in each species of the Anatidae (Lack 1967). During a study of the reproductive biology of the lesser snow goose .(including white and blue color phases), data on clutch size and egg weight were obtained (Ankney 1974:97). Here, using egg weight as an index of the amount of nutrients allocated to individual eggs, we evaluate the possible significance of eggweight variation in the lesser snow goose. We are grateful to C. D. MacInnes, whose help and ideas were the impetus for the study from which these data resulted. L. Patterson provided valuable field assistance. We thank D. M. Scott, J. S. Millar, H. Boyd, and C. D. MacInnes for critically reviewing earlier drafts-of this paper. Financial support was provided by the Canadian Wildlife Service, National Research Council of Canada, and The University of Western Ontario. MATERIALS AND METHODS We weighed 1,457 eggs from 366 clutches from the snow goose nesting colony at the mouth of the McConnell River, Northwest Territories (60'50'N, 94025'W), in 1972 (C.D.A.) and 1973 (A.R.B.). In both years we used a spring scale (Welch Scientific Co., Chicago) to weigh individual eggs at the nest. The scale had a 250-g capacity calibrated in 2-g intervals; we used a known weight to check its accuracy periodically. Weights were recorded to the nearest gram. Clutch weight was determined by summing egg weights. In both years we weighed only eggs from nests (on a transect) that had a known history. In 1972, eggs were weighed when the clutch was complete; in 1973, eggs were weighed the day, or the day after, they were laid. This between-year difference in technique may have increased the variability of the data. Eggs lose weight after being laid, although nearly all weight loss occurs after incubation begins (Romanoff and Romanoff 1949:378) and the lesser snow goose does not begin incubation until the clutch is complete (Cooch 1958:42). Also, mean egg weight was not different in the J. Wildl. Manage. 40 (4):1976 729 This content downloaded from 157.55.39.127 on Tue, 28 Jun 2016 07:17:55 UTC All use subject to http://about.jstor.org/terms 730 EGG WEIGHTS IN LESSER SNOW GEESE Ankney and Bisset Table 1. The mean egg weights and clutch weights (g) within each clutch size and the percentage overlap between clutch weights of different sizes in the lesser snow goose at the McConnell River, N.W.T., 1971 and 1972.

The Clutch Size and Numbers of Eggs of Brown-Headed Cowbirds: Effects of Latitude and Breeding Season

Geographic variation in reproduction in certain bird species has been explained in several ways. The larger clutch size at higher latitudes in several species of passerines in the northern regions of the Old World and New World (Lack 1947-48, 1968, Klomp 1970, Hussell 1972) has sometimes been accounted for by the extra hours of daylight for feeding activity by the parents at the higher latitudes (Lack 1947-48) and by the extra hazards faced by adult birds in more northern populations or by higher mortality of eggs and nestlings in the tropics (Skutch 1967). The short breeding season available to birds of the higher latitudes also may help explain the large clutch size of birds in those areas. With time for rearing no more than one brood, one adult may be likely to leave more offspring than another if it lays a large clutch. In more tropical areas the long nesting season, with repeated opportunities for rearing young e.g., Willis 1974) may provide an ecological setting where the long-term breeding success of a pair of birds is higher if little parental investment goes into a single nesting effort (Williams 1966). The numbers of eggs laid in a season and the number in each set or clutch within a season may vary in parasitic birds as well as in birds that rear their own young. Such variation may help provide a general explanation for the trends in the nesting passerines. Parasitic birds such as the cowbirds provide a natural experiment to test the generality of some of the explanations of variation in clutch size, particularly those explanations that are based on the concept that family size in birds has a selective history shaped mainly by the amount of food the parents can provide to their young. The brood parasites lay eggs in the nests of other species that rear their young. As they do not feed their own young, they may be expected to lay more eggs than their nesting relatives, and so to show a different pattern of geographic variation in breeding effort. To test the effectiveness of these general explanations in forecasting the reproductive variation of a parasitic bird, I sampled populations of Brown-headed Cowbirds (Molothrus ater) in Oklahoma, California, and Michigan. These areas differ in (a) latitude, (b) length of the breeding season, and (c) number of years the population has existed. The predictions were as follows: (1) if clutch is imite in birds by the amount of food the arents can find for their brood, then the n rth rn (Michigan) cowbirds should not hav lar er clutches than cowbirds in Oklahoma or California. (2) If clutch size is smaller than the number of young that parents usually can feed because long-term seasonal reproductive success is adversely affected by a big early reproductive effort (Williams 1966), then the northern (Michiga ) cowbirds should have larger clutches because they have a shorter breeding season, and hence should be less restrained in their

The Ecological Significance of Clutch Size in the South African Gannet (Sula capensis (Lichtenstein)

Lack (1954) suggests that clutch size in birds is regulated to enable the maximum possible number of chicks to survive to breeding age. In the South African gannet, Sula (bassana) capensis (Lichtenstein), the most dangerous phase of the life history is soon after birds fledge (leave the nesting area). These birds have to learn to fly and fish, apparently without aid from adults. Possibly because of this a high percentage die during the first few weeks of independence. If Lack is correct, it is probable that the body weight of chicks when they fledge has some relationship to their chances of post-fledging survival; evolutionary selection pressures will probably have acted to ensure that the maximum possible number of young leave the nest at the optimum weight for survival. One possible adaptation, that might aid chicks to survive after fledging, could be the regulation of clutch size to ensure that the chicks hatched could be reared to an optimum fledging weight. In addition, or alternatively, the fledging period (time spent in the nest) could be lengthened and chick's growth slowed down. This would spread the parent's feeding burden over a longer period and make the rearing of a chick less strenuous. The chick would probably benefit by receiving a greater total amount of food. If evolution had proceeded in a different direction, it could have led to parents feeding their young after fledging, until they learned to fish. However, at present, this would conflict with the juveniles' behaviour of migrating to the tropics as soon as they can fly. In this paper the chief problem investigated is the significance of the gannet's clutch size. This was investigated at Lambert's Bay, South Africa (320 05' S, 180 18' E) in the following ways. Firstly, I tried to see if birds ever laid two-egg clutches. Secondly, adults were tested to see if they could hatch two eggs and rear two young, and the breeding success of these birds was compared with that of birds with one egg. Thirdly, a comparison was made between the rates of development and the weights at fledging of chicks from single-chick broods and two-chick broods (twins). The twins had been experimentally created in most cases by donating a second egg of equal age to several marked nests; similar experiments had previously been undertaken by several workers with other species, including Nelson (1964) who worked with the North Atlantic gannet (S. (bassana) bassana). Fourthly, the post-fledging survival rates of juveniles were analysed to see which fledging weight seemed to be optimum. The relationship between date of fledging and post-fledging survival, was tested by analysing recovery and recapture data. Possible detrimental effects on parents rearing two chicks were also investigated.

Clutch Size in Birds: Outcome of Opposing Predator and Prey Adaptations

A model is proposed to explain clutch size in birds as the outcome of the interaction between predatory adaptations of birds to increase their feeding efficiency and adaptations of their food resources to avoid predation. Variations in clutch size are consistent with the model. A modification that incorporates the seasonality of food resources is also discussed.