Effect Of Brood Size Research Articles

Body Condition and Brood Abandonment in Common Eiders Breeding in the High Arctic

Abstract Post-hatch brood amalgamation is common in many waterfowl species and especially prevalent in the Common Eider (Somateria mollissima). Studies in the temperate region have shown that brood abandonment is influenced both by female body condition and brood size. In this study we tested how these two factors influenced the probability of brood desertion in eiders breeding under harsh conditions in the high Arctic (78N). We found that females in poor body condition were less likely to care for their young, but there was no effects of brood size. We speculate that the differences between southern and northern populations of eiders may be due to different environmental conditions.

Short-term fluctuations in cellular immunity of tree swallows feeding nestlings.

We examined cellular immunity of adult tree swallows feeding nestlings under variable weather conditions. Birds received an injection of phytohaemagglutinin (PHA), which causes a local swelling, reflecting the strength of T-cell-mediated immunocompetence. There was a negative relationship between the immune response and the number of nestlings in the brood (range 3-6young) which suggests that parental effort suppresses the immune function. However, there was also a strong effect of ambient temperature and food abundance (aerial insects) on immune response. Parents that received the PHA injection during cold weather and at low food abundance showed a suppressed immune response compared to birds treated during more favourable conditions. They also lost more body mass during the 24h inoculation period, and their offspring showed reduced growth. When controlling for ambient temperature and food abundance in a multivariate analysis, there was no longer any significant effect of brood size on the parents' immune response. Three of 39 pairs deserted their broods after PHA injection. All three desertions took place when the mean ambient temperature fell below 13°C. The PHA response is known to have both heritable and environmental components; our study emphasizes its condition-dependency. Previous studies of other passerine birds have shown that high levels of parental effort may have an immunosuppressive effect. Our study indicates that weather conditions may override the effects of natural variation in parental effort, and that the PHA response is particularly influenced by short-term fluctuations in energy balance.

Food limitation explains most clutch size variation in the Nazca booby

Summary Natural selection is expected to optimize clutch size, but intrapopulation variation is maintained in many bird species. The Nazca booby provides a simple system in which to investigate clutch size evolution because clutch size and brood size are decoupled due to obligate siblicide. The indirect effect of brood size on clutch size evolution can therefore be eliminated. In Nazca boobies, second eggs provide insurance against the failure of the first egg or early death of the first hatchling, but approximately half of all females lay only one egg. We tested the hypothesis that one‐egg clutches result from food limitation by providing female Nazca boobies with supplemental food. A higher proportion of supplemented females produced two‐egg clutches than did control females. Supplemented females produced larger second‐laid eggs than did control females, but not first‐laid eggs. Laying date and laying interval were not affected. Comparisons of clutch size and egg volume between years indicated that the supplemental feeding experiment was not conducted in a year with a poor natural food supply. Thus supplemented females produced larger clutch sizes despite apparently normal natural food levels. This experiment nearly completes our understanding of clutch size variation in the Nazca booby, and indicates that food limitation and the costs of egg‐laying should be considered carefully in studies of clutch size evolution.

Physiological and biochemical correlates of brood size and energy expenditure in tree swallows.

Intra-population variation in many fitness-related traits (e.g. clutch size) is often attributed to variation in individual parental quality. One possible component of quality is the level at which each individual can expend energy while provisioning dependent young. We used breeding tree swallows (Tachycineta bicolor) to test whether adults with large, natural-sized broods and/or nestlings in good nutritional condition had relatively high daily energy expenditures (DEEs). Adults with high DEEs were predicted to have large internal organs and high metabolic capacities. We first measured the growth rate of nestlings in natural broods of five, six and seven over a 4-day period and then measured parental DEE using doubly labelled water. Adults were then dissected for analyses of body composition and to determine maximum enzyme activities in the pectoral muscle. Although the total mass gain of large broods was greater than that of small broods, parental DEE was independent of brood size. We hypothesize that adults matched their clutch size (and consequently, brood size) to their individual foraging efficiencies. When statistically controlling for the effects of brood size, in one of two years there was a positive correlation between DEE and brood mass. This suggests that among individuals rearing the same-sized broods there were reproductive benefits of a relatively high DEE. There was no correlation between either brood size or DEE and the mass of any internal organ or the metabolic capacity of the pectoral muscle.

EVOLUTION OF CLUTCH SIZE IN THE EASTERN KINGBIRD: TESTS OF ALTERNATIVE HYPOTHESES

David Lack proposed that parental feeding ability ultimately limited clutch size in bird species in which the young were dependent upon their parents for food. However, many species can raise broods that are larger than their normal clutch size. Based on nine years of experimental results from an individually marked population of Eastern Kingbirds (Tyrannus tyrannus) breeding in central New York (USA), I test six hypotheses that have been proposed as explanations for why birds fail to lay larger, seemingly more productive clutch sizes. I modified brood sizes by adding or removing 1–2 nestlings when broods were 1–3 d old and then documented the effects of brood size and manipulated brood size on nestling size and survivorship, offspring recruitment, adult survival, and future adult reproduction. Most first clutches of the season held three eggs (62% of 503 clutches), but the proportion of young to fledge did not vary with brood size (1–5 young), and as a result, broods of five were the most productive. Lack's basic food-limitation model was thus rejected. Although nestling mass and ninth-primary length at fledging declined with brood size, offspring survival during the immediate 10–12 d period after fledging was unrelated to nestling mass or lengths of the tarsus or ninth primary. The findings that the underweight young in broods of four and five did not suffer disproportionate mortality and that they were just as likely to appear as recruits in future years led to a rejection of the extended version of Lack's food-limitation model. Comparisons of annual variation in the relationship between productivity and brood size showed that productivity increased with brood size in eight of nine years (significant in six years). Thus, high temporal stochastic variation in conditions for rearing young (the “bad-years” hypothesis) is unlikely to explain the relatively small clutch size of kingbirds. Predictions of two other hypotheses that predict asymmetrically low survivorship of young in large broods (the “cliff-edge” and “brood-parasitism” hypotheses) were also rejected. On the other hand, evidence suggested that females individualize clutch size such that each female lays a clutch that matches her individual feeding ability. Although fledgling production was not adversely affected by experimental increases in brood size, most enlarged broods lost young during the 10–12 d immediately after fledging. Thus, enlarged broods ultimately produced no more independent young than did control broods that began with the same number of eggs. Fledgling deaths were not related to nestling mass or size, and recruitment was independent of manipulations. Survival and fecundity costs of reproduction also existed for females. Male survival (68%) was independent of the number of young that had been raised (0–5 young), and future breeding efforts were not compromised by elevated effort in the past year. However, females that raised broods of five were less likely to return to breed in the following year (42%) than were females that raised 2–4 young (62%). Among the survivors, females that raised enlarged broods in the preceding year also experienced more hatching failure and fledged fewer young than females that raised reduced broods in the preceding year. I suggest that costs of reproduction probably set the ultimate limit to clutch size in Eastern Kingbirds. I did not test the hypothesis that high rates of nest predation favor the evolution of small clutch size, but given that predators destroyed ∼50% of nests each year, it is also likely that nest predation has contributed to the evolution of the current clutch size of kingbirds. Whether a female produces a clutch of three or four eggs is probably determined by individual differences in parental ability, which may be related to either intrinsic properties of the female or territory quality.

The benefit of large broods in barnacle geese: a study using natural and experimental manipulations

1. In precocial birds, where the young feed themselves, the costs and benefits of brood size are still poorly understood. An experimental manipulation of brood size was employed to examine the effects of brood size on both parents and young in a wild population of barnacle geese [Branta leucopsis (Bechstein)] during brood‐rearing on Svalbard. 2. Social dominance of the family unit, the amount of vigilance behaviour of the parents, the growth of the goslings in the family unit and an index of body condition for female parents during moult were all positively correlated with brood size. 3. When brood size changed as a result of natural events (i.e. predation or adoption) or experimental manipulation, rates of dominance, parental vigilance, gosling growth and female parent condition changed in a similar direction to the observed relation between the variable and brood size in unchanged broods. 4. After fledging, the fast‐growing goslings in large broods survived better during autumn migration, while there was no apparent net cost in survival or next‐year breeding for the parents. 5. Via a direct effect of brood size on dominance of the family unit, large broods were beneficial for both parent and young in a situation where there was strong intraspecific competition for the available food resources. 6. This study provides a clear demonstration of a causal relationship between brood size and various components of both gosling and adult fitness and is of direct relevance to the phenomenon of adoption and the evolution of brood size in this species.

Parental investment and the management of an endangered penguin

The yellow-eyed penguin, Megadyptes antipodes, is considered one of the most threatened species of penguin in the world. Research has indicated that yellow-eyed penguin parents will reduce their overall investment in offspring to match the available food supply, but they rarely exhibit brood reduction. Increased adult mortality under adverse conditions has led to the suggestion that yellow-eyed penguins incur a cost of reproduction in `poor food' years and that such a cost could be avoided by managers relieving them of the burden of raising one of their two chicks. We examined the effects of artificially reducing brood size in yellow-eyed penguins to determine: (1) how reproductive costs may be expressed, and (2) whether brood reduction has the potential to relieve such costs. Our results indicate that yellow-eyed penguins adjust their level of parental effort according to brood size, at least under favourable conditions of food supply. Moreover, parents of single-chick broods were in better condition and guarded their offspring for longer periods than those raising twins. Nevertheless, we found no significant effects of reducing brood size on adult survival or the frequency with which juveniles were resighted. However, if adult condition is sensitive to the effects of brood size this outcome may have significant implications for their ability to survive post-breeding moult in years when food availability is low. Overall, the study emphasises the value of approaching conservation issues from within a theoretical framework.

Erratum: Sex differences in parental care in the common swift (Apus apus): effect of brood size and nestling age

Erratum: Sex differences in parental care in the common swift (<i>Apus apus</i>): effect of brood size and nestling age

The price of eggs: increased investment in egg production reduces the offspring rearing capacity of parents

Understanding the selective pressures shaping the number of offspring per breeding event is a key area in the study of life-history strategies. However, in species with parental care, costs incurred in offspring production, rather than rearing, have been largely ignored in both theoretical and empirical studies until relatively recently. Furthermore, the few experimental studies that have manipulated production costs have not yet teased apart effects that operate via the parental phenotype from effects on the quality of the resulting young. To examine whether increased egg production influences parental brood rearing capacity independently of effects operating via egg quality, we experimentally increased egg production in gulls and then examined their capacity to rear a control clutch. We found that the capacity of parents to rear the control brood was substantially reduced solely as a consequence of having themselves produced one extra egg. The paradox that, in many species, parents apparently aim for fewer young per breeding event than the experimentally and theoretically demonstrated optimum, has partly arisen from the failure to take into account the constraints imposed by production costs.

Sex differences in parental care in the common swift (Apus apus): effect of brood size and nestling age

We analysed sex differences in parental behaviour in a Mediterranean population of common swifts (Apus apus). Females attended the nest and fed the chicks at a higher rate than males and were more in contact with the nestlings during the first days after hatching, while no sex differences were found in other parameters measured. For larger broods no quantitative sex differences were observed in meal-delivery and brooding time, while for small broods the rate of feeding by males was lower than the rate of feeding by females, and males attended the nest less than females. No sex differences were observed at the early nestling stage, but males attended the nest less than females later in the nestling stage. No extra-pair breeding activity was observed. These results suggest that males reduce parental care when the energy requirements of the chicks decrease, while females do not.

Helping-at-the-Nest and Group Size in the Arabian Babbler Turdoides squamiceps

Wright, J. 1998. Helping-at-the-nest and group size in the Arabian Babbler Turdoides squamiceps. - J. Avian Biol. 29: 105-112. The effect of group size on the productivity of communally-breeding bird species has for many years been linked with the various adaptive explanations for helping-at-thenest. This study examines the effect of group size on group provisioning effort and group productivity in the Arabian Babbler Turdoides squamiceps, a species in which helping behaviour is reported to have limited impact on parental reproduction, and has instead been uniquely proposed to operate as a signal within the group to gain social prestige. Contrary to these earlier reports, the present study showed a strong positive and linear effect of group size upon the total number of provisioning visits to the nest and the number of fledglings produced. Larger groups produced larger numbers of young, although individual fledging mass did not decrease with this increase in brood size. Fewer faecal sacs were removed per visit in nests with larger broods, and this was the only variable for which an initial positive effect of group size could be explained by the potentially confounding effect of brood size. In addition, individual provisioning effort was reduced in larger groups, both in terms of number of nest visits and the size of prey items delivered per visit. Although these data were collected over only three breeding seasons, the group sizes and breeding conditions involved are broadly representative of this population during the previous 25 years of records. These results therefore suggest that, under the majority of circumstances, helping-at-the-nest in Arabian Babblers has a positive and linear effect on the reproductive success of parents, as well as substantially reducing the provisioning effort required of other group members. Groups were nearly always made up of closely related individuals, and so these data are consistent with kin selection. Helpers gain indirect fitness benefits from the increased reproductive success of nests, as well as potentially reducing the survival costs of chick feeding for adult relatives. The possibility of direct fitness benefits from helping, especially regarding pseudoreciprocity and adaptive group augmentation, is also discussed with reference to the evolution of helping behaviour and group-living in the Arabian Babbler.

Parental-care responses by yellow warblers (Dendroica petechia) to simultaneous manipulations of food abundance and brood size

Theoretical models view biparental care as a state of equilibrium that can be maintained only when the amount of parental care provided by one parent depends on the amount provided by its mate. According to these models, biparental care results when a decrease in the contribution of one parent is partially, but not completely, compensated for by an increase in the contribution of the other parent. Furthermore, this equilibrium state can only be stable if any other external factor affects male and female effort equally. We used yellow warblers (Dendroica petechia) to examine whether changes in brood size and food abundance affect the parental contributions of the sexes equally. Supplemental food did not affect parental care by either sex, but brood size did. Both males and females provided more to larger broods, and in large broods only, their provisioning rates increased with nestling age. Parental effort per nestling was similar in the two sexes, being higher for smaller broods and increasing with nestling age. Based on brood biomass, parental effort was greater for smaller broods, and decreased with nestling age in females only. Therefore, in agreement with current models of the maintenance of biparental care, the effects of brood size and nestling age on parental care did not differ significantly between the sexes. Nonetheless, data from other species and theory indicate that the costs and benefits of providing parental care differ between the sexes, so it is unlikely that biparental care can be maintained solely by a partial compensation response.

Erratum: Sex differences in parental care in the common swift (&lt;i&gt;Apus apus&lt;/i&gt;): effect of brood size and nestling age

Erratum: Sex differences in parental care in the common swift (&lt;i&gt;Apus apus&lt;/i&gt;): effect of brood size and nestling age

A model for the evolution of reproductive skew without reproductive suppression

Reproductive skew is a measure of the way breeding is distributed among the members of an animal society or group. Up to now, explanations of patterns of skew have been limited to one particular model, which assumes that a single dominant has full control over the distribution of subordinate reproduction. If this control is incomplete or absent, however, unsanctioned breeding by subordinate females will increase the total number of young produced. Here I present a new model for the evolution of skew that considers the effect of brood size on the inclusive fitness of dominants and subordinates. By augmenting brood size, a subordinate female reduces the per capita fitness of a dominant's offspring, so the net benefits of producing young are lower for related subordinates. I consider the stable level of skew when both dominant and subordinate attempt to maximize their inclusive fitness under two conditions: (1) when the dominant is unable to anticipate that a subordinate will add to her brood; and (2) the dominant does anticipate subordinate reproduction and can respond by adjusting her own brood size. In the first case, the model predicts that reproductive skew will increase with relatedness between breeders, because related subordinates are selected to add fewer young to the dominant's brood. In the second case, the dominant's optimal response to the presence of a second breeder exaggerates the relationship between relatedness and skew: dominants should produce more young when breeding with related compared with unrelated subordinates.

Sex differences in parental care in the common swift (&lt;i&gt;Apus apus&lt;/i&gt;): effect of brood size and nestling age

Sex differences in parental care in the common swift (&lt;i&gt;Apus apus&lt;/i&gt;): effect of brood size and nestling age

Long-Term Effects of Brood Size Manipulation on Morphological Development and Sex-Specific Mortality of Offspring

1. Little is known about long-term effects of brood size on fitness components of offspring. This is unfortunate because such information is needed to predict optimal brood size. Furthermore, ontogenetic circumstances are potentially important in explaining the large individual differences in lifetime reproductive success documented in many species. 2. In a laboratory study the long-term effect of brood size manipulation on mortality and morphological development of offspring was investigated. Young zebra finches Taeniopygia guttata were reared in small or large broods. Young were exchanged in such a way that natural siblings from different rearing conditions could be compared. 3. Manipulated brood size affected offspring morphology permanently (measurements were taken up to the age of 12 months). Individuals reared in small broods were heavier, had longer tarsi and wings, higher beaks, were in better condition, and males had redder beaks. The experiment did not affect beak length or female beak redness. Individuals from large broods had caught up on wing length and males from large broods on beak redness by 6 months of age, which may reflect priority of investment in traits important to fitness. 4. Mortality of offspring raised in large broods was higher both before and after independence. After independence the effect of manipulated brood size on mortality was sex specific; females were most likely to die. This suggests that the optimal sex ratio of offspring may depend on brood size. 5. The effect of brood size on mortality after independence was probably partly mediated by size and condition during the nestling phase and possibly by peer aggression among adult offspring. Sex-linked mortality could not be explained by a sex difference in morphological development. Why females are more vulnerable remains to be discovered.

Common goldeneyes adjust maternal effort in relation to prior brood success and not current brood size

Parental investment theory predicts that parental effort should be related to the reproductive value of the current brood. This depends on both the number of young and the survival prospects of each of them. Thus parents may provide more care to larger broods either because of (1) the direct effect of brood size per se on reproductive value (the “brood size” hypothesis) or because (2) past mortality, reflected in current brood size, predicts future mortality of the brood and hence its reproductive value (the “brood success” hypothesis). Earlier studies have not attempted to distinguish between these alternatives. We tested the hypotheses in the precocial, nidifugous common goldeneye Bucephala clangula, a species with uniparental female care. Maternal effort was measured as the time spent by the female in rearing the brood. We found that brood size itself is not associated with maternal effort, but that females modify their maternal effort according to the mortality already experienced by the brood, supporting the prediction of the brood success hypothesis. We also found that brood mortality varied considerably between broods and that previous mortality predicts future mortality within broods, basic assumptions of the brood success hypothesis.

The effect of larval aggregation behaviour on larval growth of the spruce bark beetle Dendroctonus micans

1. The great spruce bark beetle Dendroctonus micans is a primary pest of spruce in Europe. It is unusual among Eurasian scolytids in that apparently healthy trees are attacked by solitary adults, but larvae feed en masse, in response to a larval aggregation pheromone.2. The effect of brood size on larval growth was determined in experiments on detached bark, logs and living trees. A positive relationship was found between brood size and larval growth in experiments started with either eggs or larvae up to fifth instar. The relationship appeared to be independent of the effects of both preformed and induced defences in bark.3. No evidence was found to support the hypothesis that larvae feeding in groups spend a greater proportion of time feeding. Addition of resin to egg chambers in detached bark pieces resulted in high mortality of first‐instar larvae. A possible role for larval aggregation in minimizing the effects of host defences is discussed.

Does Brood Size Manipulation Affect Later Competitive Behaviour of Offspring?

AbstractData from several field experiments support the existence of a trade-off between number and quality of offspring. However, long term effects of brood size on fitness related traits of offspring have been a relatively neglected area of research. In a laboratory experiment the effect of manipulated brood size on subsequent competitive ability of adult offspring was investigated. Zebra finches, Taeniopygia guttata, were reared in small or large broods and young were exchanged in such a way that natural siblings from different rearing conditions could be compared. Competitive behaviour was assessed in two different contexts: competition for food (both sexes tested) and competition for mates (only males tested). There was no significant difference between males from small and large broods in number of succesfull attacks (after which the other male moved away) during male-male aggressive interactions provoked by the presentation of a female in an adjacent cage. Nor did brood size affect latency to eat, time spent eating or success at displacing the other bird from the feeder during food competition tests. The results thus suggest consistently that later competitive ability of offspring is not affected by brood size in this species.

Costs and benefits of parental care in eastern kingbirds

We manipulated brood sizes of eastern kingbirds (Tjrannus tyrannus) to measure the costs and benefits of parental care and to test whether kingbirds showed evidence of individual optimization of reproductive effort. We found that the number of feeding trips (trips/h) increased and that per capita feeding rates (trips/nestling/h) declined as brood size increased. The decline in per capita feeding rates was mostly due to high feeding rate* to broods of one: parents made roughly equal number of trips to feed each nestling in broods of two to five. Nonetheless, nestling mass declined with brood size, probably because large broods were fed more small prey. Nesding condition (mass adjusted for structural size) differed only between broods of one and five. After controlling for effects of brood size, feeding rates had no supplementary influence on either nesding size or condition, but productivity and feeding rate were positively and significantly related. Adult male condition did not vary with brood size, manipulated brood size, or total feeding rate, but declined as the pair's per capita feeding rates increased. In addition, males that returned to breed were in better condition before leaving for migration than those that failed to return. Female condition tended to decline, and the probability of returning to breed dropped when broods were enlarged. However, female condition was independent of the probability of returning. Our results show that high feeding rates were costly, but that they carried benefits (greater productivity). Some evidence for individual optimization of reproductive effort existed: variability in nestling and adult female condition were better explained by changes in brood* size than by the actual number of young in the nest. However, most evidence supported die alternative that increased brood size was equally costly for all birds.