Clutch Size Manipulation Research Articles

Extra Nestlings That Are Condemned to Die Increase Reproductive Success in Hoopoes.

AbstractThe adaptive value of routinely laying more eggs than can be successfully fledged has intrigued evolutionary biologists for decades. Extra eggs could, for instance, be adaptive as insurance against hatching failures. Moreover, because recent literature demonstrates that sibling cannibalism is frequent in the Eurasian hoopoe (Upupa epops), producing extra offspring that may be cannibalized by older siblings might also be adaptive in birds. Here, directed to explore this possibility in hoopoes, we performed a food supplementation experiment during the laying period and a clutch size manipulation during the hatching stage. We found that females with the food supplement laid on average one more egg than control females and that the addition of a close-to-hatch egg at the end of the hatching period increased the intensity of sibling cannibalism and enhanced fledging success in hoopoe nests. Because none of the extra nestlings from the experimental extra eggs survived until fledging, these results strongly suggest that hoopoes obtain fitness advantages by using temporarily abundant resources to produce additional nestlings that will be cannibalized. These results therefore suppose the first experimental demonstration of the nutritive adaptive function of laying extra eggs in vertebrates with parental care.

Hatching failure is greater in altricial bird species with cavity nests and large clutches

AbstractMortality rates are high for most avian species during early life stages, forming a critical source of natural selection that helps shape the diversity of avian life-history traits. We investigated hatching failure (i.e., non-predatory embryonic mortality, excluding abandoned or damaged eggs) and found significant variation among passerine species. Failure rates ranged from 1.0% to 12.7%, and species with cavity nests and larger clutches experienced greater rates of hatching failure. While past research has focused on the direct effects of predation on the nestling and fledgling stages, little is known about how predation may indirectly influence other sources of mortality such as hatching failure. We investigated the influence of nest predation risk and other factors on variation in hatching failure among 14 free-living grassland and shrubland songbird species. Across all species, 7.7% of 1,667 eggs failed to hatch. We found little evidence that variation in nest temperature influenced rates of hatching failure within and among species, although species with larger clutch sizes had more variable nest temperatures. Dissection of failed eggs revealed that most hatching failures occurred before or shortly after the onset of development; however, there was no difference between cavity and open-cup nesters in the rate of early-stage mortality. Our findings suggest there may be tradeoffs to having a large clutch, with a benefit of rearing more young at the cost of greater hatching failure, possibly due to delayed onset of incubation, poor incubation behavior, or inability to incubate large clutches. Additionally, as larger clutches are often laid in cavity nests, which have a relatively low predation risk, this may outweigh the costs of increased hatching failure. More experimental approaches, such as clutch size manipulations and egg-specific incubation behavior are needed to provide greater insight into factors driving variation in hatching failure across species.

Brood size manipulation reveals cost on the size of the second clutch in the earwig Anisolabis maritime

Abstract There may be a trade-off between the duration of parental care and future reproductive success. Traditionally, studies about the cost of parental care have included the removal of the parent. However, producing a secondary clutch after the failure of the first one is a compensatory behaviour that occurs in cases of brood failure. In this study, attempts were made to detect the cost of maternal care in the earwig, Anisolabis maritima (Dermaptera: Anisolabididae) by either extending the period of care or increasing the brood size to prevent compensation through the brood’s success. The results indicated that manipulation did not change the inter-clutch interval, although my previous study revealed shortening of these intervals after the removal of the clutch in this species. In this study, decreased clutch size manipulation increased the size of the following clutch.

Gull chicks grow faster but lose telomeres when prenatal cues mismatch the real presence of sibling competitors.

During embryonic life, individuals should adjust their phenotype to the conditions that they will encounter after birth, including the social environment, if they have access to (social) cues that allow them to forecast future conditions. In birds, evidence indicates that embryos are sensitive to cues from clutch mates, but whether embryos adjust their development to cope with the expected level of sibling competition has not hitherto been investigated. To tackle this question, we performed a 'match versus mismatch' experimental design where we manipulated the presence of clutch mates (i.e. clutch size manipulation) and the real (postnatal) level of sibling competition (i.e. brood size manipulation) in the yellow-legged gull (Larus michahellis). We provide evidence that the prenatal cues of sibling presence induced developmental changes (such as epigenetic profiles) that had programming effects on chick begging behaviour and growth trajectories after hatching. While receiving mismatching information favoured chick begging and growth, this came at the cost of reduced antioxidant defences and a premature loss of telomeres. Our findings highlight the role of the prenatal social environment in developmental plasticity and suggest that telomere attrition may be an important physiological cost of phenotype-environment mismatch.

Wild zebra finches do not use social information from conspecific reproductive success for nest site choice and clutch size decisions

Information about the quality of local habitat can greatly help to improve an individual’s decision-making and, ultimately, its fitness. Nevertheless, little is known about the mechanisms and significance of information use in reproductive decisions, especially in unpredictable environments. We tested the hypothesis that perceived breeding success of conspecifics serves as a cue for habitat quality and hence influences breeding decisions (nest site choice and clutch size), using the zebra finch (Taeniopygia guttata) as a model species. Zebra finches breed opportunistically in the unpredictable, arid zone of Australia. They often inspect the nests of conspecifics, potentially to prospect on conspecific reproductive success, i.e., to collect social information. We conducted a clutch and brood size manipulation to experimentally create the perception of high and low quality areas. In six areas, clutch sizes of almost 300 zebra finch nests were either all increased (N = 3 areas) or reduced (N = 3 areas) throughout one breeding season. The number of breeding pairs and sizes of newly laid clutches were not significantly affected by the manipulated reproductive success of the areas. Thus, zebra finches did not use social cues for their reproductive decisions, which contrasts with findings of species in temperate zones, and could be an adaptation to the high unpredictability of their habitat. Even the personal experience of rebreeding birds did not directly affect their clutch size. Our study suggests that zebra finches employ a high level of opportunism as a key strategy for reproduction. Further, this is the first study to our knowledge using an experimental approach in the wild to demonstrate that decision-making in unpredictable natural environments might differ from decision-making in temperate environments with seasonal breeding. Social information can help to optimize the behavior of animals. Birds in temperate climates with seasonality use breeding success of others to predict where they should breed. However, very little is known about information use in less predictable environments. In a field experiment, we created a patchy environment by increasing and decreasing brood sizes of wild zebra finches to test if social information is also used in unpredictable conditions. We found no evidence that zebra finches in the Australian outback use social information from their conspecifics when deciding on nest site and clutch size. They probably gather personal information on environmental parameters and the current availability of resources, which might be more reliable than social information.

The effect of directional wind components on survival of Little Penguins <i>Eudyptula minor</i>

We live in an age of increased awareness of climate change and its potential eects on our ecosystems. Here we look at the eect of one aspect of climate, directional wind components, on the survival of Little Penguins Eudyptula minor on Phillip Island in southeastern Australia, using mark-recapture data gathered over a 42 year period since 1968. We apply biologically realistic age structures for the survival and recapture probabilities, and use mean seasonal wind magnitudes from the four cardinal compass directions as covariates in our modelling of the survival probability. Results indicate that rst year survival is most aected by southerly winds in the winter prior to the chick’s

Costs of incubation in Common Terns Sterna hirundo: replication and extension of a clutch‐enlargement experiment

Manipulation of clutch size during incubation has been used in a number of studies to investigate life‐history costs of incubation. We increased or decreased clutches of Common Terns Sterna hirundo during incubation and measured subsequent chick growth and productivity. Our results provide little support for the hypothesis that costs of incubation in Common Terns are substantially affected by the number of eggs incubated.

Parental Investment Strategies Determined by Expected Benefits

The purpose of this study was to determine whether parental investment decisions are made on the basis of cumulative past investment or the prospects of expected benefits minus costs. Through clutch size manipulations at nests of Redwinged Blackbirds, Agelaius phaeniceus, we obtained ♂♂ with equal past investment but varying expected benefits. Parental investment was measured by intensity of nest defense against a dummy predator at the nest. Results indicate expected benefits minus costs, rather than cumulative investment, determine parental investment decisions.

Follicle ablation increases offspring size in a lizard with a low clutch frequency

AbstractStudies of lizards and snakes have shown that an experimental reduction of offspring number sometimes, but not always, results in an increase in offspring size. We applied the “follicle ablation” technique to an oviparous lacertid lizard (Eremias argus) to test the hypothesis that offspring size can be easily altered by manipulating clutch size in species with a low clutch frequency. Our manipulation of clutch size had the effect of inducing variation in egg size in the first post-surgical clutch, with follicle-ablated females producing fewer larger eggs than did controls. Follicle-ablated females produced a second post-surgical clutch as normally as did controls. The proportional amount of resources allocated to reproduction did not shift seasonally in E. argus, but females normally switched from producing a larger number of smaller eggs early in the breeding season to a smaller number of larger eggs later in the season. Females used in this study never produce more than two clutches per breeding season. Therefore, our data validate the hypothesis tested. Our data also provide an inference that maximization of reproductive success could be achieved in females of E. argus by diverting a larger enough, rather than an extraordinarily high, fraction of the available energy to individual offspring in single reproductive episodes.

Recent Advances in the Behavioral Ecology of Tropical Birds

Tropical birds offer unique opportunities to test ecological and evolutionary theory because their life history traits are so diverse and different from temperate zone models upon which most empirical studies are based. We review recent studies on the behavioral ecology of tropical birds, studies that explore new advances in this field. Life histories and their evolution remain the focus of research on tropical birds. Clutch size manipulations in two species showed that food limitation does not explain small clutch size. In antbirds, enlarged clutches decreased post-fledging survival whereas in thrushes, enlarged broods were costly due to high nest predation. Small clutches may be favored via different ultimate selective forces and shared underlying tradeoffs between the immune, metabolic, and endocrine systems in the body may account for the commonly observed ‘slow pace of life’ in tropical birds. The physiological tradeoff between testosterone and immunocompetence may explain the evolution of low testosterone levels in tropical passerines where adult survival is paramount. In contrast to life history theory, few studies have explored temperate-tropical differences in territoriality, mating systems, and song function. The idea that low breeding synchrony in tropical birds is associated with low levels of extra-pair fertilizations was supported by several new paternity studies conducted on tropical passerines. Seasonally breeding tropical birds have higher testosterone levels than tropical birds with prolonged breeding seasons, although it is unclear if this pattern is driven by mating systems per se or selection from pathogens. Recent work on relations between pair members in permanently paired tropical passerines focuses on the question of mate defense versus territorial defense and the extent of cooperation versus selfish interests in inter-sexual relations.

Metabolic rate of nocturnal incubation in female great tits,Parus major, in relation to clutch size measured in a natural environment

To study the energetic costs of incubation in relation to clutch size, clutch sizes were manipulated and the metabolic rate of female great tits, Parus major (Linnaeus), during nocturnal incubation (MR(inc)) was measured using mobile oxygen analysers. Individuals were measured on consecutive nights while incubating their own or manipulated clutches. The experiment was performed under field conditions in order to place possible effects of clutch size manipulation within the context of other factors explaining variation in MR(inc). Females spent more energy when incubating enlarged clutches as compared with controls (6-10% more energy for three additional eggs) but did not spend significantly less energy when incubating reduced clutches. MR(inc) was strongly negatively related to ambient temperature. The effect of clutch enlargement is consistent with previous studies whereas the absence of an effect of clutch reduction is not. The small effect of clutch enlargement on MR(inc) highlights the need for further studies to include measurements of daily energy expenditure in order to judge how important energy expenditure can be in explaining fitness consequences of incubating experimentally enlarged clutches.

A trade‐off between clutch size and incubation efficiency in the Barn Swallow Hirundo rustica

Summary To investigate reproductive costs associated specifically with incubation for a single‐sex intermittent incubator, clutches of the Barn Swallow (Hirundo rustica Linnaeus) were manipulated in successive trios, involving reduced (n − 2), control (n) and enlarged (n + 2) clutch sizes (n is the natural clutch). All manipulated clutches were within the natural range. Manipulations were made shortly after clutch completion and reversed immediately prior to hatching, so the costs of incubation were examined in isolation while the demands of egg‐laying and nestling rearing were maintained at natural levels. The efficiency of incubation was dependent on clutch size: the period from laying to hatch was shortest for those incubating reduced (R), and longest for birds incubating enlarged (E) clutches (R: 14·8 ± 0·1 days, n = 28; control (C): 15·3 ± 0·3, n = 24; E: 15·6 ± 0·2 days, n = 28). Also, hatching success was higher amongst R (92%) than either C (85%) or E (81%) clutches. There was no evidence of an incubation cost due to clutch size in terms of parental body condition, provisioning of nestlings, or in the time interval before a second clutch. No difference was found in female nest attendance behaviour or in the decline in egg density (which could reflect water loss) according to our clutch manipulations, indicating that other factors probably underlie the effects observed. It was shown that the number of eggs being incubated affects components of fitness in Barn Swallows, independently of laying and chick‐rearing costs. Intraseasonal costs of incubation included prolonged egg development and reduced hatching success among larger clutches but did not affect nestling growth or interclutch intervals. We suggest that an inability to maintain optimal temperature for egg development is likely to underlie the observed responses to clutch size manipulations.

Variable energetic responses to clutch size manipulations in white-throated dippersCinclus cinclus

We used the doubly-labelled water technique to measure daily energy expenditure (DEE) of a free-living uniparental incubator, the white-throated dipper Cinclus cinclus, in Scotland. DEE was 205 ± 8 (s.e.m.) kJ d -1 for 17 females incubating their natural clutch sizes, equivalent to 3.2 ± 0.1 x basal metabolic rate (BMR). To investigate the influence of clutch size on the energy budget, we measured the DEE of 14 females with clutches increased or reduced by a single egg. Birds with reduced clutch sizes had an energy expenditure with a mean and variance that did not differ from those of birds with unmanipulated clutches. Enlarging the clutch led to an increase in energy expenditure to over 4 x BMR for some individuals but not for others, resulting in greater variance in energy expenditure for birds with enlarged clutches. Individual variation in energy expenditure could not be fully explained by environmental conditions, by patterns of behaviour or clutch size. Incubating females received a maximum of only 4 kJ d-' (2% of DEE) from provisioning by the male, and mobilised up to 6 kJ d -1 (3% of DEE) from reserves. Females spent 2.9 ± 0.2 h (n = 20) away from the nest each day, so a foraging rate of 95 kJ h -1 was required during incubation recesses to balance DEE. This 'required foraging rate' is double previous estimates of the maximum rates of energy acquisition for birds of this size. We suggest that the greater likelihood of a raised energy expenditure associated with larger clutches, combined with the difficulties in maintaining energy supplies, may constitute a constraint on avian clutch size.

Reproductive effort and blood parasites of breeding pied flycatchers: the need to control for interannual variation and initial health state

The association between reproductive effort, as assessed by experimentally altered clutch size, and prevalence of blood parasite infections was studied during two breeding seasons in the pied flycatcher (Ficedula hypoleuca). Only in the first study year (1998), enlarged clutches resulted in more fledged young and birds presumably responded to the intended experimental increase in reproductive effort by increasing their provisioning rates. Only on this study year, female body mass was negatively affected by the clutch size manipulation. For both sexes, no significant effect of experimental treatment on the prevalence of blood parasite infections (Trypanosoma spp., Haemoproteus balmorali) on day 13 after chick hatching was found in both study years. However, only in the first study year, when the intended increase in parental reproductive effort was achieved, we found a higher within female seasonal increase in Trypanosoma spp. prevalence for birds with enlarged clutches compared to those with control and reduced clutches. In this year, the Trypanosoma spp. prevalence was significantly higher than in the second study year. These results suggest that more than one year has to be covered, and that the knowledge of the premanipulative health status of parents can facilitate detection of the proposed trade‐off between reproductive effort and immunocompetence.

Inter‐sexual differences in the immune response of Eurasian kestrel nestlings under food shortage

When resources are limited, parents should decide the optimal number, size, and sex of progeny, and offspring should decide the optimal allocation of resources to different costly functions, such as growth and immunity. We manipulated clutch sizes of Eurasian kestrels by one egg to estimate possible cumulative effects of incubation and chick rearing costs on parental body condition, feeding effort, and offspring viability. No obvious effects of clutch size manipulations on feeding effort were found while feeding effort was adjusted to the original clutch size. Enlarged and control nests suffered from higher nestling mortality than reduced nests, and chicks of the enlarged group were in poorer body condition than chicks of the reduced group. Controlling for body mass, male chicks exhibited lower cell‐mediated immunity assessed by a cutaneous hypersensitivity response than females, but only in treatments suffering from food restrictions, as indicated by chick starvation. These novel results reveal inter‐sexual differences in physiological strategies in early life, suggesting sex‐related differences in susceptibility to disease and consequently in survival prospects of offspring.

A cost of maternal care in the dung beetle Onthophagus taurus?

Abstract Parental care theory assumes that investment in current offspring will trade against future investment. A number of field studies on birds have used clutch size manipulations to demonstrate a survival cost to chick rearing. However, such studies do not account for costs accrued during earlier stages of reproduction because not all aspects of reproductive effort are manipulated by varying the number of nestlings. In this study, we investigate the effect of reproductive effort on female survival in the dung beetle, Onthophagus taurus. By experimentally manipulating mating status and dung availability, we demonstrate that virgin females survive longer than mated females and that the survival of mated females was negatively associated with the number of brood masses produced. Using a novel manipulation of the mating system, we separated the effects of egg production and maternal care on female survival. Previously, we have shown that females provisioning with the assistance of a major male provide relatively less care than unassisted females. However, paternal assistance did not alter the number of brood masses produced and hence the amount of reproductive effort that was allocated to egg production. Therefore, our finding that female survival was increased when receiving paternal assistance provides, to our knowledge, the first definitive evidence that maternal care reduces female lifespan. These results are of major importance to theoretical models on the evolution of parental care.

Factors influencing the evolution of clutch size in a gregarious insect parasitoid

Summary 1. In this study we examined the clutch size of Hyssopus pallidus (Askew) (Hymenoptera, Eulophidae), a gregarious ectoparasitoid of codling moth (Lepidoptera, Tortricidae) larvae that is characterized by a host handling time of 2 days approximately. 2. Clutch size increased asymptotically with host larval size (fresh weight) to a maximum of 32 eggs. A single male developed from each clutch, brood survival was 83% and pupal size of female offspring was 0·45 mg, all independent of clutch size. 3. A comparison of the response of both experienced and naïve adult parasitoids to varying host larval size, indicated that experience was not necessary for H. pallidus to be able to accurately estimate host size. 4. Clutch size increased by 50% for medium size host larvae (30–40 mg) and 83% for large host larvae (60–76 mg) when the encounter rate was reduced from one host every second day to one every seventh day. A significantly lower clutch size was produced on the last day at the highest encounter rate for large host larvae, suggesting exhaustion of the egg supply for H. pallidus. 5. Manipulation of parasitoid clutch size on constant size host larvae did not influence sex ratio (single male) or brood survival (93%), but produced a linear trade‐off between clutch size and the size (fresh weight) of female pupae. Longevity, lifetime fecundity, total number of hosts attacked, and clutch size all increased linearly with the adult size (female pupal weight). 6. The observed clutch size in newly emerged H. pallidus is consistent with a strategy of maximizing lifetime fitness gain rather than the gain from individual hosts (Lack clutch size) despite a significant investment in the time required to attack each host.

Costs of incubation and immunocompetence in the collared flycatcher.

This paper investigates the costs of incubation in terms of reduced reproductive success and investigates whether incubation competes with immune function for resources. I performed a clutch size manipulation experiment in which two eggs were either removed from or added to the nests of collared flycatchers, Ficedula albicollis, for 1week during incubation and subsequently returned to their original nests before hatching. To induce immune response, the females were challenged with sheep red blood cells. While the duration of incubation, hatching success and fledgling number did not differ between experimental groups, fledgling condition was significantly lower in broods that had been enlarged during incubation. Neither the females' condition nor their ability to respond to a novel antigen differed between treatments. The relationship between antibody production and female condition was significantly positive, but only among females incubating reduced clutches. I conclude that the costs of incubation in the collared flycatcher are not negligible and are manifested only at the chick-rearing phase.

Reproductive effort and T-lymphocyte cell-mediated immunocompetence in female pied flycatchers Ficedula hypoleuca

The physiological mechanism underlying the cost of reproduction may consist of immunodepression caused by increased parental effort. Here, we report effects of experimental manipulation of clutch size on T-lymphocyte cell-mediated immune response in female pied flycatchers, Ficedula hypoleuca. Parents with reduced broods provisioned at lower rates than those caring for control and enlarged broods three days after hatching. Parents caring for enlarged broods provisioned nests at higher rates 13 days after chick hatching than those feeding control and reduced broods. Females with enlarged broods weighed less than females with control or reduced broods. No effect of experimental treatment on nestling mass and size was found. The response to the injection of phytohaemagglutinin in the wing-web of females decreased with increasing brood size and with increasing provisioning rate when the chicks were three days old, when controlling for the negative effect of female mass on response. The T-lymphocyte cell-mediated response decreased from the reduced to the control, and from this to the enlarged group, when controlling for female mass. This effect of experimental manipulation of clutch size was significant and consistent with a trade-off between maternal effort and immunocompetence.

Survival costs of chick rearing in black‐legged kittiwakes

1. We tested for costs of chick rearing in the black‐legged kittiwake Rissa tridactyla (Linnaeus) by removing entire clutches from 149 of 405 randomly selected nests, in which one or both mates was colour‐banded. After the manipulation, we monitored adult nest attendance and body condition at unmanipulated and manipulated nests, and measured the survival and fecundity of these adults the following year. 2. Late in the chick‐rearing period, adults from unmanipulated nests (i.e. with chicks) went on significantly longer foraging trips, and were significantly lighter for their size, than adults from manipulated nests (i.e. without chicks). 3. Adults from unmanipulated nests also survived to the following nesting season at a significantly lower rate than those from the manipulated nests (0·898 vs. 0·953), suggesting that attempting to raise chicks can reduce life expectancy by 55%. 4. There was a tendency for adults from nests that were unmanipulated in year one to have lower reproductive success in year two, primarily because of reduced fledging success, and a higher incidence of non‐breeding. 5. These findings suggest that mass loss in kittiwakes during chick rearing may not be adaptive. Raising chicks can lead to reproductive costs, and the causal mechanism appears to be a reduction in body condition. 6. We compare our results with previous brood (or clutch) size manipulation experiments that have measured adult body condition, survival and/or future fecundity. Although the empirical evidence suggests that long‐lived species are more likely to experience survival costs than short‐lived species, we believe the opposite may be true. We suggest that shifting the experimental protocol of cost of reproduction studies from brood enlargements (an approach taken in most prior studies) to brood reductions will provide more accurate quantifications of naturally occurring costs. 7. The cost of reproduction is one mechanism proposed to explain the reduced survival rates reported for kittiwake populations in the North Atlantic relative to those in the North Pacific ocean. Oceanographic data, however, suggest that lower food availability may limit survival of kittiwakes in the North Atlantic where a deeper mixed layer and reduced primary production combine to make conditions less favourable for this seabird during the winter months.