Variation In Clutch Size Research Articles

Sensory control of clutch size in the Common Swift Apus apus

Clutch size varies among individuals in most bird species. A widespread assumption is that such variation results from variable timing in the disruption of ovarian follicular growth that brings, with a few days' lag, egg‐laying to an end. Currently, there is empirical evidence that this is the case in Blue Tits Cyanistes caeruleus but not in Zebra Finches Taeniopygia guttata, in which the timing of follicular disruption has been shown to be invariant. Here, I investigate clutch size regulation of Common Swifts Apus apus. Using experimental egg removal, I show that, assuming the female gets enough egg contact, the determination of clutch size occurs at noon solar time on the day the first egg of a clutch is laid. In spite of individual variation in clutch size, there was no hint of any variability in the timing of clutch‐size determination. In this species therefore, the timing of the signal disrupting follicular growth appears invariant. The physiological mechanism that controls clutch size is discussed, including the existence of an endogenous circadian clock and potential zeitgeber, the developmental range of clutch size for the species, the sensory nature of the input that triggers follicular disruption, and the stress of laying extra eggs.

Exploring patterns of variation in clutch size–density reaction norms in a wild passerine bird

Negative density dependence of clutch size is a ubiquitous characteristic of avian populations and is partly due to within-individual phenotypic plasticity. Yet, very little is known about the extent to which individuals differ in their degree of phenotypic plasticity, whether such variation has a genetic basis and whether level of plasticity can thus evolve in response to selection. Using 18years of data of a Dutch great tit population (Parus major), we show that females reduced clutch size with increasing population density (slopes of the reaction norms), differed strongly in their average clutch size (elevations of the reaction norms) at the population-mean density and that the latter variation was partly heritable. In contrast, we could not detect individual variation in phenotypic plasticity ('I×E'). Level of plasticity is thus not likely to evolve in response to selection in this population. Observed clutch sizes deviated more from the estimated individual reaction norms in certain years and densities, implying that the within-individual between-year variance (so-called residual variance) of clutch size was heterogeneous with respect to these factors. Given the observational nature of this study, experimental manipulation of density is now warranted to confirm the causality of the observed density effects. Our analyses demonstrate that failure to acknowledge this heterogeneity would have inflated the estimate of 'I×E' and led to misinterpretation of the data. This paper thereby emphasizes the fact that heterogeneity in residuals can provide biologically insightful information about the ecological processes underlying the data.

Genomic dissection of variation in clutch size and egg mass in a wild great tit (Parus major) population

Clutch size and egg mass are life history traits that have been extensively studied in wild bird populations, as life history theory predicts a negative trade-off between them, either at the phenotypic or at the genetic level. Here, we analyse the genomic architecture of these heritable traits in a wild great tit (Parus major) population, using three marker-based approaches - chromosome partitioning, quantitative trait locus (QTL) mapping and a genome-wide association study (GWAS). The variance explained by each great tit chromosome scales with predicted chromosome size, no location in the genome contains genome-wide significant QTL, and no individual SNPs are associated with a large proportion of phenotypic variation, all of which may suggest that variation in both traits is due to many loci of small effect, located across the genome. There is no evidence that any regions of the genome contribute significantly to both traits, which combined with a small, nonsignificant negative genetic covariance between the traits, suggests the absence of genetic constraints on the independent evolution of these traits. Our findings support the hypothesis that variation in life history traits in natural populations is likely to be determined by many loci of small effect spread throughout the genome, which are subject to continued input of variation by mutation and migration, although we cannot exclude the possibility of an additional input of major effect genes influencing either trait.

COEVOLUTIONARY FEEDBACKS BETWEEN FAMILY INTERACTIONS AND LIFE HISTORY

Families with parental care show a parent-offspring conflict over the amount of parental investment. To date, the resolution of this conflict was modeled as being driven by either purely within-brood or between-brood competition. In reality the partitioning of parental resources within- versus between-broods is an evolving life history trait, which can be affected by parent-offspring interactions. This coevolutionary feedback between life history and family interactions may influence the evolutionary process and outcome of parent-offspring coadaptation. We used a genetic framework for a simulation model where we allowed parental parity to coevolve with traits that determine parental investment. The model included unlinked loci for clutch size, parental sensitivity, baseline provisioning, and offspring begging. The simulation showed that tight coadaptation of parent and offspring traits only occurred in iteroparous outcomes whereas semelparous outcomes were characterized by weak coadaptation. When genetic variation in clutch size was unrestricted in the ancestral population, semelparity and maximal begging with poor coadaptation evolved throughout. Conversely, when genetic variation was limited to iteroparous conditions, and/or when parental sensitivity was treated as an evolutionarily fixed sensory bias, coadapted outcomes were more likely. Our findings show the influence of a feedback between parity, coadaptation, and conflict on the evolution of parent-offspring interactions.

One size fits all: Eurasian lynx females share a common optimal litter size

Lack proposed that the average clutch size of altricial species should be determined by the average maximum number of young the parents can raise such that all females in a given population should share a common optimal clutch size. Support for this model remains equivocal and recent studies have suggested that intra-population variation in clutch size is adaptive because each female has its own optimal clutch size associated with its intrinsic ability to raise offspring. Although Lack litter size and condition-dependent litter size are presented as two competing models, both are based on the concept of individual optimization. We propose a unified optimal litter size model (called 'adaptive litter size') and identify a set of conditions under which a common vs. a state-dependent optimal litter size should be observed. We test whether females of Eurasian lynx (Lynx lynx) have a common optimal litter size, or whether they adjust their litter size according to their state. We used a detailed individual-based data set collected from contrasting populations of Eurasian lynx in Scandinavia. Observed reproductive patterns in female lynx provide strong support for the existence of a common optimal litter size. Litter size did not vary according to female body mass or reproductive category, or among contrasted populations and years. A litter size of 2 was associated with a higher fitness than both smaller and larger litters, and thus corresponded to the 'adaptive litter size' for female lynx. We suggest that the reproductive pattern of female lynx might correspond to a risk avoidance tactic common to all individuals, which has evolved in response to strong environmental constraints generated by a highly unpredictable food supply during lactation.

Trajectories of Reproductive Effort inColeomegilla maculataandHippodamia convergens(Coleoptera: Coccinellidae) Respond to Variation in Both Income and Capital

Lady beetles such as Coleomegilla maculata DeGeer and Hippodamia convergens Guerin-Meneville have been categorized as income breeders because egg maturation depends on resources available during reproduction. However, capital resources acquired during larval feeding influence reproductive success via effects on adult body size and other traits. We produced three sizes of beetles by varying larval access to food and subjected mated females to alternating periods of food surplus and deficit. Both species exhibited temporal variation in clutch size that reflected changes in income, the amplitude of fluctuations being greater in H. convergens. Egg mass in C. maculata appeared to increase as a fixed function of oviposition sequence but also was affected by maternal body size, whereas H. convergens egg mass fluctuated more in response to food supply. Fertility was largely unaffected by income fluctuation but appeared constrained by capital, especially in H. convergens. Dynamic changes in reproductive effort (daily mass of eggs/female mass at emergence) mirrored changes in clutch size, the amplitude of fluctuations being greater in H. convergens, the species with higher reproductive effort. Generally, large females compensated for income fluctuations better than small females, likely because of their greater capital and possibly an ability to consume larger meals when food was available. The greater sensitivity to income observed in H. convergens likely reflects a higher degree of aphid-specificity compared with the more generalist C. maculata because effective exploitation of ephemeral aphid outbreaks would favor rapid reproductive responses to changes in food supply.

Origin of evolutionary change in avian clutch size

Why different bird species lay different numbers of eggs is a question that has long been associated with factors external to the organism, that is, factors which operate on inherited variation in clutch size through the action of natural selection. Yet, while external factors are important, the extent of what is evolutionarily possible rests with the mechanisms developed by birds for clutch-size control. Hitherto neglected, these mechanisms generate factors internal to the organism that are central to the origin of evolutionary change. They are related to the fact that a species-specific range of clutch size arises from the differential survival of pre-ovulatory follicles undergoing growth when the signal causing egg laying to end reaches the ovary. Herein, I examine three internal factors that, together with external factors, could impact the evolution of avian clutch size. Each factor acts by changing either the number of pre-ovulatory follicles present in the ovary at the time of follicular disruption or the timing of this event. These changes to clutch size can be explained by the concept of heterochrony. In light of this, the role of phenotypic plasticity and genes determining clutch size is discussed. Finally, to account for the origin of evolutionary change in clutch size, I detail an hypothesis involving a process similar to Waddington's theory of genetic assimilation.

Breeding together: modeling synchrony in productivity in a seabird community

With environmental conditions changing rapidly, there is a need to move beyond single-species models and consider how communities respond to environmental drivers. We present a modeling approach that allows estimation of multispecies synchrony in productivity, or its components, and the contribution of environmental covariates as synchronizing and desynchronizing agents. We apply the model to long-term breeding success data for five seabird species at a North Atlantic colony. Our Bayesian analysis reveals varying degrees of synchrony in overall productivity, with a common signal indicating a significant decline in productivity between 1986 and 2009. Productivity in seabirds reflects conditions in the marine ecosystem so the estimated synchronous component is a useful indicator of local marine environment health. For the two species for which we have most data, the environmental contribution to overall productivity synchrony is driven principally by effects operating at the chick stage rather than during incubation. Our results emphasize the importance of studying together species that coexist in a community. The framework, which accommodates interspecific clutch-size variation, is readily applicable to any species assemblage in any ecosystem where long-term productivity data are available.

Breeding Difficulty of Marsh Crocodile (Crocodylus Palustris, Lesson, 1831) in Safari Park of Bangladesh

An assessment of Marsh crocodile's (Crocodylus palustris LESSON, 1831) habitat, its feasibility in supporting the total population, feeding, hatching difficulty and variations in egg and clutch size of Marsh crocodile were carried out in Bangabandhu Sheikh Mujib Safari Park for one year (July 2010 to June 2011). Major problem identified was the breeding failure, i.e. till now no hatching were found. Mainly two local weather variables (average temperature and humidity) were identified as the most obstructing parameters. Higher temperature (average <TEX>$37.03^{\circ}C$</TEX>) and Lower Humidity (average 75.33%) in comparison with standard range of incubation temperature and humidity for Marsh crocodile were seems to be vital reason for the failure of hatching. Understanding of the identified problem of this paper can direct the wildlife conservationist and the policy maker to take proper action for conserving the Marsh Crocodile in Bangladesh.

Overlap of Female Reproductive Cycles Explains Shortened Interclutch Interval in a Lizard with Invariant Clutch Size (Squamata: Gekkonidae:Paroedura picta)

Variable clutch size is unambiguously an ancestral state in reptiles. Only several lizard lineages have evolved so-called invariant clutch size, where all females lay just one or two eggs per clutch. This mode of reproduction is characteristic for geckos. In some gecko lineages, decreased fecundity in a single clutch is compensated by conspicuous shortening of interclutch intervals. The proximate mechanism of high clutch frequency in these geckos is not known. Here, we document that three subsequently laid clutches develop simultaneously in females of the Madagascar ground gecko (Paroedura picta). The extremely short interclutch intervals in this species-even as short as a week-thus could be attributed to the overlap of female reproductive cycles. Such overlap should be associated with altered female hormonal cycles. Based on measurements of hormonal levels, we suggest that cycles of estradiol and progesterone during reproductive cycles of females in P. picta are largely independent. Thus, in contrast to the presumable ancestral reptile state, higher levels of progesterone do not seem to interfere with vitellogenesis in this species. We discuss potential consequences of this derived mode of reproduction, such as possible simultaneous maternal transfer of nutrients and other yolk components to several subsequent clutches.

Intraspecific variation in reproductive traits of burrowing owls

Reviews of hatching asynchrony in birds recommended more studies on intraspecific variation in the extent of hatching asynchrony. We examined intraspecific variation in clutch size, laying chronology, onset of incubation, incubation period, and hatching asynchrony in burrowing owls (Athene cunicularia) in the Imperial Valley of California. Mean clutch size was 7.4 eggs and owls averaged 0.5 eggs laid per day. Females varied considerably in laying interval and onset of incubation (range = 1st to 9th egg in the clutch). The mean incubation period was 21.9 days. Hatching interval also varied greatly among females (\( \overline{x} \) = 0.8, range 0.1–2.0 days between successively hatched eggs). Past burrowing owl studies have largely overlooked the substantial intraspecific variation in these traits or have reported estimates that differ from ours. Future studies designed to identify the environmental factors that explain the large intraspecific variation in these traits will likely provide insights into the constraints on local abundance.

The Breeding Biology of the Northern Pygmy-Owl: Do the Smallest of the Small Have an Advantage?

We explored the breeding biology of the Northern Pygmy-Owl (Glaucidium gnoma) from 2007 to 2009 in a forested reserve in Portland, Oregon. Large body size is often assumed to give animals reproductive advantages, and we tested whether body size affected timing of breeding and examined variation in diet, breeding date, clutch size, and reproductive success to explore whether the presumed benefits of large body size are evident in this species. The average size of 13 clutches was 5.8, and nest success was high (92%); 22 successful nests fledged an average of 5.2 young. Dates of first laying varied over an 18- or 19-day period in each year and averaged 3 weeks earlier in 2008 than in 2009. Early breeders laid larger clutches. After the effect of breeding site was controlled for, small rather than large female pygmy-owls bred earliest. The same pattern of small size was evident in all years. Diet varied by year in regard to whether mammals (2008) or birds (2007 and 2009) were the dominant prey taken, and the owls bred earlier in 2008 occurred when mammals dominated their diet. Although small females tended to breed earlier in all years, it was only in 2009 when pairs bred very late and birds dominated their diet that small females also laid larger clutches and fledged more young. These data suggest that the merits of small or large body size may vary with stages of the breeding cycle and ecological conditions.

Clutch Investment in the Chilean Swallow (Tachycineta meyeni) Shifts with Time of Breeding and Position in the Sequence of Laying

Within a population, the sizes of eggs and clutches vary, and the combination of both determines a female's reproductive investment. We investigated females' investment in clutches of the Chilean Swallow (Tachycineta meyeni) at the southern limit of its range, where it breeds in an extreme climate. We analyzed variation in clutch, egg and yolk size in relation to the female's condition, date of laying, and position in the sequence of laying. As predicted by energy/nutrient constraints, clutch size and yolk size decreased over the breeding season, though egg mass increased. Females' investment strategy differed with clutch size: their investment in eggs increased with each successive egg in clutches of 4 and 5 eggs but decreased with each successive egg in clutches of 3 eggs. Eggs that were heavier for their size were more likely to hatch than eggs light for their size. Females may make a strategic decision, with high investment in eggs laid early resulting in a short sequence (small clutch) and delayed investment resulting in a long sequence (large clutch). Alternatively, the shift in clutch investment may simply be a consequence of physiology. Better estimates of female body condition are needed for these options to be distinguished.

Widespread reproductive variation in North American turtles: temperature, egg size and optimality

Theory predicts the existence of an optimal offspring size that balances the trade-off between offspring fitness and offspring number. However, in wild populations of many species, egg size can still vary from year to year for unknown reasons. Here, we hypothesize that among-year variation in population mean egg size of freshwater turtles is partly a consequence of their gonadal sensitivity to seasonal temperatures, a physiological mechanism which principally functions to synchronize reproduction with a favorable time of year. As part of this process, among-year variation in seasonal temperatures modifies the extent of egg follicle development, and this may translate into variation in mean egg size among years (both at the individual and population level). To test this hypothesis, we applied an information-theoretic approach to model relationships between mean egg mass and the temperatures experienced during discrete periods of follicular development in wild populations of three turtle species (Chrysemys picta, Chelydra serpentina, Glyptemys insculpta) over 12 consecutive years. Because follicular development occurs in the fall for C. serpentina and G. insculpta, whereas it occurs both in the fall and spring for C. picta, we expected only fall temperatures would explain egg size variation in C. serpentina and G. insculpta, whereas both fall and spring temperatures would correlate with egg size variation in C. picta. These predictions were upheld. We then compared among-year variation in within-female egg and clutch sizes of each species in order to evaluate whether such variation might still be consistent with some tenets of optimal egg size theory. In all three species, we found that clutch sizes vary more than egg sizes in spite of temperature-induced egg size variation, and this pattern of relatively high clutch-size variation matches theoretical predictions. Future work should explore the roles of direct and indirect (i.e., nutritional) influences of temperature on egg size in natural settings.

Breeding Ecology of the Pink-Billed Lark,Spizocorys conirostris, in an Agricultural Landscape in South Africa

We studied the breeding ecology, parental care and nesting success of the pink-billed lark, Spizocorys conirostris, in an agricultural landscape in the Limpopo Province of South Africa, from October 2008 to October 2010. The results are compared with data in Birdlife South Africa's Nest Record Card Scheme. Nesting was observed between February and May, with peak activity in April and May. Fifty-three nests were found and the nest structure, dimensions and microhabitat characteristics are described. Egg mass and dimensions showed no regional variation but there was evidence of geographical variation in clutch size. There was no significant difference (P ≥ 0.05) in the rate of food delivery by males and females, but the results suggest there may be inter-sexual differences in the type of food delivered to nestlings. Invertebrates comprised two-thirds of the diet of nestlings while seed made up the remainder. The growth and development of nestlings are described for the first time. The incubation period was 12 days and the nestlings fledged after 10–13 days. Breeding success was very low, averaging 12.8% over the two years of this study.

Breeding ecology of the pink-billed lark, Spizocorys conirostris, in an agricultural landscape in South Africa

We studied the breeding ecology, parental care and nesting success of the pink-billed lark, Spizocorys conirostris, in an agricultural landscape in the Limpopo Province of South Africa, from October 2008 to October 2010. The results are compared with data in Birdlife South Africa’s Nest Record Card Scheme. Nesting was observed between February and May, with peak activity in April and May. Fifty-three nests were found and the nest structure, dimensions and microhabitat characteristics are described. Egg mass and dimensions showed no regional variation but there was evidence of geographical variation in clutch size. There was no significant difference (P ⩾ 0.05) in the rate of food delivery by males and females, but the results suggest there may be inter-sexual differences in the type of food delivered to nestlings. Invertebrates comprised two-thirds of the diet of nestlings while seed made up the remainder. The growth and development of nestlings are described for the first time. The incubation period was 12 days and the nestlings fledged after 10–13 days. Breeding success was very low, averaging 12.8% over the two years of this study.

Clutch Size Variation in Egg-BroodingStefania

Among the numerous and varied reproductive modes observed in anurans, some of the most uncommon are found in hemiphractids. For example, the hemiphractid genus Stefania is one of the rare anuran taxa in which females brood their eggs openly upon their backs with the embryos undergoing direct development. Much remains to be learned about reproductive patterns in Stefania and other hemiphractids. Here, using museum specimens, literature records, and specimens recently collected by us, we present data on clutch size variation and the relationship between clutch size and female snout-vent length throughout Stefania. We found a statistically significant correlation between clutch size and female snout-vent length for Stefania (46 individuals, 8 of 19 species) and for three of the four species examined individually (S. evansi, S. ginesi, and S. satelles). Only one species, S. ayangannae, showed no correlation between these traits although this result may be due to low sampling rather than the correlation being absent.

Optimal egg size in a suboptimal environment: reproductive ecology of female Sonora mud turtles (Kinosternon sonoriense) in central Arizona, USA

We studied the reproductive ecology of female Sonora mud turtles (Kinosternon sonoriense) at Montezuma Well, a chemically-challenging natural wetland in central Arizona, USA. Females matured between 115.5 and 125 mm carapace length (CL) and 36-54% produced eggs each year. Eggs were detected in X-radiographs from 23 April-28 September (2007-2008) and the highest proportion (56%) of adult females with eggs occurred in June and July. Clutch frequency was rarely more than once per year. Clutch size was weakly correlated with body size, ranged from 1-8 (mean = 4.96) and did not differ significantly between years. X-ray egg width ranged from 17.8-21.7 mm (mean 19.4 mm) and varied more among clutches than within. Mean X-ray egg width of a clutch did not vary significantly with CL of females, although X-ray pelvic aperture width increased with CL. We observed no evidence of a morphological constraint on egg width. In addition, greater variation in clutch size, relative to egg width, suggests that egg size is optimized in this hydrologically stable but chemically-challenging habitat. We suggest that the diversity of architectures exhibited by the turtle pelvis, and their associated lack of correspondence to taxonomic or behavioral groupings, explains some of the variation observed in egg size of turtles.

The role of male nest building in post-mating sexual selection in the monogamous red-necked grebe

AbstractIn monogamous birds, early male parental effort, such as nest building, may serve as a post-mating sexually-selected display allowing female assessment of male quality. We examined the functional significance of male nest building and the potential role of nest size as a sexually-selected signal in the red-necked grebe (Podiceps grisegena), a species with high mate fidelity. Time-activity budgets showed that no behaviour was performed exclusively by one sex in the pre-laying period, but males spent significantly more time nest building and were more often involved in aggressive intra- and interspecific interactions. Nest building in pairs attempting a second brood was also performed predominantly by males. Greater participation in nest construction by males allowed females to allocate more time to self-maintenance activities in the period prior to egg-laying. The positive relationship found between the relative contribution of males to nest building and later to brood provisioning indicates that male nest building is an honest indicator of future paternal effort. Males obtained copulations solicited by females proportionally to the time spent on nest building, and the extent of male participation in nest construction was of importance for explaining variation in clutch size. Nest size itself is not very likely to be sexually selected in red-necked grebes, as it was found to depend on nest site conditions such as water depth and exposure to wave action. We suggest that greater investment of males in energetically demanding pre-laying activities is functionally similar to post-mating courtship feeding; it constitutes males’ indirect contribution to clutch production and may help to negotiate the relative investment each sex makes in the different stages of the breeding cycle. The results support the idea that, in monogamous birds, naturally selected male characters related to parental care may evolve into important sexual signals to females, although not into extreme displays.

Geographic variation in avian clutch size and nest predation risk along a productivity gradient in South Africa

Geographic variation in avian clutch size is thought to be related to the variation in nest predation rate and food availability. We studied predation on artificial ground nests along a large-scale geographic gradient in South Africa characterised by increasing productivity from the deserts in the west to humid savannas in the east, and calculated mean clutch sizes of birds occurring in atlas quadrates surrounding our study sites. Clutch sizes generally increased with increasing productivity and seasonality. The least productive desert site was characterised also by the highest predation rate, whereas all the other sites located in savannas revealed much lower and more or less constant predation rate. We found no evidence for relationship between nest predation rates and clutch sizes of ground-nesting birds. This indicates that food availability is the major factor responsible for geographical variation in bird clutch sizes across South Africa, though high predation rate might also contribute to low clutch size in least productive arid environments.