The relationship between geographic variation in clutch size and migration pattern in the Yellow Wagtail

Clutch size in the Yellow Wagtail increases by about one egg per 19° of latitude throughout Europe, but at a given latitude larger clutches are laid in western than in eastern Europe. In eastern Asia there is evidence of a reverse latitudinal cline, in which clutch size is smaller at higher latitudes, and considerably smaller than in Europe at equivalent latitudes. These trends suggest that smaller clutches are laid by populations with access to extensive wintering areas in east Africa and south-east Asia, whereas larger clutches are laid by populations with more restricted wintering areas in west Africa and India. Higher winter mortality in the latter populations may reduce breeding density, leading to less competition for resources and allowing larger clutches to be produced.     

FACTORS AFFECTING THE EGG SIZE OF RED-BILLED GULLS LARUS NOVAEHOLLANDIAE SCOPULINUS

The factors influencing the egg size of the Red-billed Gull Larus novaehollandiae scopulinus were studied at Kaikoura, New Zealand, between 1964 and 1972. In two- and three-egg clutches there was a trend for the eggs to become smaller in the sequence of laying. Length, breadth and volume of eggs of one-, two- and three-egg clutches declined significantly as the season progressed. The size of eggs from single-egg clutches tended to be smaller than eggs from two-egg clutches laid at the same time. There were correlations between the proportions of one-egg and of three-egg clutches being laid at a given period and the mean egg volume of two-egg clutches. When the mean egg volume of two-egg clutches increased there was a corresponding increase in the proportion of two- and three-egg clutches laid. When the mean egg volume of two-egg clutches decreased there was an increase in the proportion of single-egg clutches laid. The egg size of the Red-billed Gull showed no direct correlation with the abundance or availability of food; the largest eggs were produced early in the season when food was in short supply. In spite of an increase in the food supply in the middle of the breeding season, birds laying at this time produced smaller eggs than birds which laid earlier in the season. However, early breeders which relayed at the peak in food abundance on average produced a larger replacement clutch than originals laid early in the season. It is suggested that the birds nesting early in the season are able to produce the largest eggs because they are the most efficient foragers for food, and those which nest later in the season produce smaller eggs, even at peak food abundance, because of their inefficiency or inexperience. Early breeders laying replacement clutches tended to lay larger eggs and larger clutches than birds which are producing their first clutches at the same time. Two-year-old females laid eggs which were significantly shorter than older aged birds while the breadth and volume of the egg increased with the age of the female up to the fifth year. There was a trend for females to lay larger eggs when mated with older rather than younger males. No statistical differences in egg size were detected between females changing or retaining the partner of the previous season. Female body weight and egg volume were positively correlated in females weighing less than 275 g but not for heavier females. It is suggested that the seasonal decline in egg size and clutch size results from a decrease in the availability of food and the ability of the individual to exploit the resource.     

Encountering competitors reduces clutch size and increases offspring size in a parasitoid with female-female fighting

Understanding the size of clutches produced by only one parent may require a game-theoretic approach: clutch size may affect offspring fitness in terms of future competitive ability. If larger clutches generate smaller offspring and larger adults are more successful in acquiring and retaining resources, clutch size optima should be reduced when the probability of future competitive encounters is higher. We test this using Goniozus nephantidis, a gregarious parasitoid wasp in which the assumption of size-dependent resource acquisition is met via female-female contests for hosts. As predicted, smaller clutches are produced by mothers experiencing competition, due to fewer eggs being matured and to a reduced proportion of matured eggs being laid. As assumed, smaller clutches generate fewer but larger offspring. We believe this is the first direct evidence for pre-ovipositional and game-theoretic clutch size adjustment in response to an intergenerational fitness effect when clutches are produced by a single individual.     

Intraclutch egg-size variation and offspring survival in the Kentish Plover Charadrius alexandrinus

We studied the consequences of intraclutch egg-size variation in Kentish Plovers Charadrius alexandrinus in southern Spain to test the hypothesis that females allocate resources preferentially to eggs with the greatest survival potential. Second eggs were larger and had a greater hatching success than the other eggs in a clutch. However, we did not find unequivocal support for the differential allocation hypothesis, because egg failures according to laying order were not due to the causes predicted by the hypothesis, and the change in egg-size with laying order was not consistent between successive clutches of renesting females. This suggests that nutrient availability during egg formation could act as a proximate factor affecting egg-size independently of laying order. Larger eggs produced heavier chicks and, within broods, heavier chicks were most likely to be recruited into the breeding population. By laying clutches with a larger mean egg volume in replacement clutches, females took longer to lay again after failure of the first clutch. Thus, opposing selective forces may act to increase and decrease egg volume. Although a trade-off between egg-size and clutch-size has not been found in precocial birds, our results suggest that there may be a trade-off between egg-size and fecundity, since an increase in the size of eggs within clutches may limit the time remaining within the season to lay second or replacement clutches.     

Multiple paternity and breeding system in the gopher tortoise, Gopherus polyphemus

Little is known about the reproductive behaviors and the actual outcomes of mating attempts in the gopher tortoise (Gopherus polyphemus). We examined the mating system and reproductive behaviors of a population of gopher tortoises in central Florida. Using microsatellite markers, we assigned fathers to the offspring of seven clutches and determined that multiple fathers were present in two of the seven clutches examined. We found that gopher tortoises exhibited a promiscuous mating system with larger males fertilizing the majority of clutches. The advantage of larger males over smaller males in fertilizing females may be a result of larger males winning access to females in aggressive bouts with other males or larger males may be more attractive to females. Clutches produced by larger females tended to be sired by a single male, whereas clutches of smaller females tended to be sired by multiple males.     

Effects of Life Histories on Genome Size Variation in Squamata

Genome size changes significantly among taxonomic levels, and this variation is often related to the patterns shaped by the phylogeny, life histories and ecological factors. However, there are mixed evidences on the main factors affecting molecular evolution in animals.In this study, we used phylogenetic comparative analysis to investigate the evolutionary rate of genome size and the relationships between genome size and life histories(i.e.,hatchling mass, clutch size, clutches per year, age at sexual maturity, lifespan and body mass) among 199 squamata species. Our results showed that the evolutionary rate of genome size in Lacertilia was significantly faster than Serpentes. Moreover, we also found that larger species showed larger hatchling mass, more clutches per year and clutch size and longer lifespan. However, genome size was negatively associated with clutch size and clutches per year, but not associated with body mass we looked at.The findings suggest that larger species do not possess the evolution of large genomes in squamata.     

Seasonal shifts in clutch size and egg size in the side-blotched lizard,Uta stansburiana Baird and Girard

Summary There is evidence that the side-blotched lizard, Uta stansburiana, and some other organisms of temperate latitudes produce fewer and larger eggs as the reproductive season progresses. There are at least two models that could explain this phenomenon.Proponents of the parental investment model claim that females are selected to increase egg size, at the cost of clutch size, late in the season in order to produce larger and competitively superior hatchlings at a time when food for hatchlings is in low supply and when juvenile density is high. In this model the selective agent is relative scarcity of food available to hatchlings late in the reproductive season, and the adaptive response is production of larger offspring.The alternative explanation (bet-hedging model) proposed in this paper is based on the view that the amount of food available to females for the production of late-season clutches is unpredictable, and that selection has favored conservatively small clutches in the late season to insure that each egg is at least minimally provisioned. Smaller clutches, which occur most frequently late in the season, are more likely to consist of larger eggs, compared to larger clutches, for two reasons. Firstly, unlike birds, oviparous lizards cannot alter parental investment after their eggs are deposited, and therefore, in cases of fractional optimal clutch size, the next lower integral clutch size is selected with the remaining reproductive energy allocated to increased egg size. With other factors constant, eggs of smaller clutches will increase more in size than eggs of larger clutches when excess energy is divided among the eggs of a clutch. Secondly, unanticipated energy that may become available for reproduction during energy-rich years will similarly increase egg size a greater amount if divided among fewer eggs.     

Clutch size variation in the Nazca booby: a test of the egg quality hypothesis

In obligately siblicidal bird species, aggressive behavior bya dominant chick results in a fixed brood size of one, yetthese species usually show clutch size variation between individuals.Simmons proposed that variation in clutch size in obligatelysiblicidal species is related to a trade-off between egg qualityand egg quantity: some individuals produce a single highly hatchable egg, while others produce two small, lower qualityeggs. We tested the egg quality hypothesis as an explanationfor observed clutch size variation in the Nazca booby (Sulagranti), an obligately siblicidal seabird. We tested the assumptionthat egg volume is positively correlated with hatchabilityand the prediction that eggs from one-egg clutches are largerthan eggs from two-egg clutches. We did not find a positive relationship between egg volume and hatchability in this species.Eggs from two-egg clutches were either equivalent in volumeor larger than eggs from one-egg clutches. Thus, the egg qualityhypothesis was rejected as an explanation for clutch size variationin the Nazca booby. Instead, two-egg clutches appear to befavored because of the insurance value of the second-laid egg,while one-egg clutches result from food limitation.     

Body size and sexual size dimorphism in calanoid copepods

Patterns of sexual size dimorphism and body size in calanoid copepods are examined. We hypothesize that favorable conditions for development will result in large body size and high sexual size dimorphism among populations of a given species and that differences in this allometric relationship among species is governed by the male's role in insemination. We confirm that there is a greater advantage to large female size, normally the larger sex, when compared to males, hence leading to selection for developmental patterns favoring high size dimorphism. Individuals from populations of four centropagid copepod species were measured; other sizes were obtained from published sources. In the four species we examined, the relationships between prosome length and both clutch size and the ability to produce multiple clutches with one insemination were determined. Results show a trend toward hyperallometry in all centropagid species examined: sexual size dimorphism increases with increasing size. Large females produce larger clutches and more additional clutches on one insemination. That hyperallometry is not observed in diaptomid copepods may result from the greater role the male plays in reproduction. Males are needed for each clutch produced, hence the selective pressure to be larger is greater than that in the centropagidae.     

The moult of British thrushes and chats

Incubated clutches of the Mandarin Duck were larger earlier in the breeding season. At all nests there were days on which a single egg, and days on which no egg was added to a clutch; days on which more than one egg was added to a clutch were less frequent. Larger clutches had proportionately more days on which two or more eggs were added; the proportion of days on which no eggs were added was unrelated to clutch size. Larger clutches had a relatively shorter laying period than expected, but a longer incubation period than smaller clutches. Unhatched eggs resulted mainly from embryonic mortality, which was concentrated in the period just prior to pipping. Similar proportions of unhatched eggs resulted from eggs laid into clutches before and after the start of incubation. Larger clutches did not have proportionately more unhatched eggs. As all clutches had at least one day on which no eggs were laid, it is proposed that the proportion of nests parasitized was greater than that estimated from nests with more than one egg laid on any one day.     

Offspring fitness and individual optimization of clutch size

Within-year variation in clutch size has been claimed to be an adaptation to variation in the individual capacity to raise offspring. We tested this hypothesis by manipulating brood size to one common size, and predicted that if clutch size is individually optimized, then birds with originally large clutches have a higher fitness than birds with originally small clutches. No evidence was found that fitness was related to the original clutch size, and in this population clutch size is thus not related to the parental capacity to raise offspring. However, offspring from larger original clutches recruited better than their nest mates that came from smaller original clutches. This suggests that early maternal or genetic variation in viability is related to clutch size.     

Scaling of gelatinous clutches: effects of siblings' competition for oxygen on clutch size and parental investment per offspring

Theories on the evolution of clutch size are primarily influenced by examples from terrestrial animals, yet most animal phyla occur exclusively in water. Oxygen has a lower diffusion coefficient and lower solubility in water than in air, and siblings in aquatic clutches often compete for oxygen. Mitigating this competition could affect allocation of resources to offspring. Gelatinous clutches are common in aquatic habitats and have evolved multiple times in many phyla. We hypothesized that spacing of embryos by gel enhances delivery of oxygen but that gel is organically costly. A model of diffusion predicts that clutch thickness should scale inversely with the square root of embryo concentration, indicating a need to reduce embryo concentration (and increase gel volume) disproportionately with increasing clutch thickness. For embryos in artificial clutches constructed with agarose gel, development was faster in clutches with more gel per embryo, as predicted. For natural gelatinous clutches of gastropods, thick clutches had disproportionately larger volumes of gel and disproportionately more organic material invested in gel relative to embryos. Thus, for aquatic gelatinous clutches, requirements for oxygen supply can affect trade-offs involving clutch thickness and parental investment per offspring: resources are diverted to gel, and the proportion diverted increases with clutch thickness.     

Egg-size variation in the bluethroat (Luscinia s. svecica): constraints and adaptation

We examined inter- and intra-clutch egg-size variation in the bluethroat (Luscinia s. svecica), an open-nesting passerine breeding in the sub-alpine region in southern Norway. By removing first clutches shortly after egg-laying, we induced laying of a repeat clutch. Females significantly reduced the number of eggs from the first to the second nesting attempt, but increased mean egg size. Females in good condition laid significantly larger eggs than those in poor condition. Consistent with predictions of the brood survival hypothesis, assuming an adaptive investment in last eggs to ensure survival of all eggs in the clutch, we found that the size of the last eggs in first clutches was generally larger than the mean egg size of the clutch, and that the relative size of the last egg increased with clutch size. However, a large last egg reflected a general increase in egg size throughout the laying sequence rather than a specific investment in the last egg only. Egg size was not significantly influenced by sex or paternity (within-pair versus extra-pair) of the embryo. In repeat clutches the last egg was not consistently larger than the mean for the clutch. We conclude that female bluethroats face resource limitations during egg formation early in the season, and that the patterns of increase in egg size with laying order for first clutches, and from first to repeat clutches, can largely be explained by proximate constraints on egg formation.     

Parental age and reproduction in the Marsh Tit Paras palustris

In the Marsh Tit Parus palustris , breeding performance improved with the age of both the male and the female parent—older birds produced larger clutches earlier in the season. The effect of male age, however, was mainly due to a positive association with the age of the female. When the effect of laying date was accounted for, there was no additional effect of female age on clutch-size. However, pairs with previous breeding experience laid larger clutches than newly formed pairs of the same age. There was some evidence that young females that laid late in the season were less likely to survive to breed again, leading to an age effect on reproductive performance. There was no evidence that individual birds laid earlier or produced larger clutches when they became older.     

Sequential Polygyny During Egg Attendance is Rare in a Tree Frog and Does not Increase Male Fitness

Sequential polygyny is a reproductive strategy that allows males to continue to mate and compensates for the loss of future breeding opportunities incurred by parental care (i.e. egg attendance). Using the frog Kurixalus eiffengeri, we tested predictions that (1) attending males fathered two, overlapping clutches; and (2) that double clutching leads to improved offspring numbers. Using five microsatellite DNA markers, we genotyped 15 pairs of overlapping clutches, which differed slightly in developmental stage at a single egg‐laying site. Parentage analyses showed at least 12 of 15 pairs of overlapping egg clutches were sired by the attending male mated with different females, providing the first genetic evidence to support an earlier prediction that attending males sired both egg clutches. Field surveys found a low incidence of overlapping clutches (4.9% of 263 egg‐occupied stumps), suggesting sequential polygyny is uncommon. Stumps with multiple clutches contained significantly more eggs than stumps with single clutches but hatched similar number of tadpoles. Results suggest that continuous calling that attracts females during egg attendance is a reproductive tactic that maximizes mating opportunities. However, adoption of the sequential polygyny tactic may only result in marginal fitness gains for males that are traded off against average higher egg mortality in larger egg clutches.     

Clutch predation in great tinamous Tinamus major and implications for the evolution of egg color

Egg camouflage has been found to reduce predation in several ground‐nesting species. Therefore, the evolution of eggs that lack camouflage in ground nesting birds is puzzling. Even though clutch predation in the tropics is high, tinamous are the only tropical ground‐nesting birds that do not build a nest and do not lay cryptic eggs. I studied predation of great tinamou clutches in a lowland tropical forest and found that risk of predation was higher during incubation when the eggs are covered by the parent, than during laying when they are exposed, suggesting that predators primarily use cues from the incubating males to locate the clutch and not cues from the eggs. Clutch size had no effect on predation rate, even though larger clutches are more conspicuous to a human observer. Predation by visual cues is likely reduced during incubation by the camouflaged plumage and high nest attendance of males. If most predators use cues from the incubating male and not the eggs to locate clutches, then conspicuous egg color may have evolved in great tinamous as an intra‐specific signal. I evaluate hypotheses that may explain the maintenance of conspicuous egg color in tinamous.     

The influence of age on laying date,clutch size,and egg size of the white-fronted tern,Sterna striata

Abstract

The age at first breeding and the influence of age on laying date, clutch size, and egg size of white-fronted terns were studied at the Kaikoura Peninsula, New Zealand, between 1971 and 1976. Of the 134 banded birds recovered breeding at Kaikoura, 63% were marked as nestlings at Kaikoura; the remainder came from colonies within 104 km. The bird is extremely capricious in its choice of nesting locality, and there are indications that it is not consistently philopatric. A small number bred as 3-year-olds, but the majority did not commence breeding until after they were 6 years old. Most pairs (73%) were of partners with an age difference of 1 year or less. Laying date and egg size varied with the age of the parent, but clutch size showed no significant change in relation to age. Mean egg volume did not vary between one-egg and two-egg clutches, but in two-egg clutches the first egg laid was significantly the larger in length, breadth, and volume. Single-egg clutches were the most common, but as the season progressed the proportion of two-egg clutches increased. There was no significant seasonal change in egg size.     

Clutch size limitation in waders: experimental test in redshank Tringa totanus

Several hypotheses have been raised to explain the upper limit of clutch size at four eggs in waders (suborder Charadrii), which may play an important role in the evolution of the variety of mating and parental care systems in this group. Experimental tests of the hypotheses have produced conflicting results. It was recently suggested that the combined effects of several incubation costs of a larger clutch suffice to limit its size to four eggs in this group. Here we test the incubation-limitation hypothesis in a field experiment, in redshank Tringa totanus. We created five-egg clutches by adding one egg from another nest to a just completed four-egg clutch. Four-egg control clutches were created by replacing one of the eggs by an egg from another nest. All egg removals, additions and replacements were done before incubation started. Incubation time in five-egg clutches increased by 1 day to 24.3ǂ.23 days, compared to 23.3ǂ.32 days in four-egg clutches. Egg hatchability and nest predation rates did not differ significantly between treatments. On average five-egg clutches produced one extra chick at hatching (4.5ǂ.26 chicks) compared to four-egg clutches (3.5ǂ.27 chicks). Also when several additional costs from incubating enlarged clutches are added, redshanks by laying a fifth egg would on average increase their reproductive success at hatching by an estimated 22%. The incubation-limitation hypothesis therefore is clearly rejected in this species. Possible mechanisms behind the four-egg clutch limit in waders and ways of testing the alternatives are discussed.     

Mating preference of female zebrafish, Danio rerio, in relation to male dominance

Mating success tends to be skewed toward dominant males, thoughfemale mate preferences may not always correlate with male dominance.In this study, we investigated the mating preferences of femalezebrafish, Danio rerio, in the absence of male–male competition.We paired females sequentially with males of known dominancerank, using a nested, repeated measures design, with egg productionas a measure of female mate preference. We predicted that femaleswould spawn more frequently and produce larger clutches whenpaired with males of higher dominance rank. We found significantdifferences among females in the size of clutches produced andamong males in the size of clutches received, but these differenceswere independent of male dominance rank. Male body size wasnot related to either dominance rank or clutch size received.These results indicate that females vary clutch size in relationto the males with which they are paired but that they do notfavor dominant males. Thus, male competition may normally overridefemale mate preference in zebrafish.     

CLUTCH-SIZE, INCUBATION AND HATCHING SUCCESS IN THE HOUSE SPARROW AND TREE SPARROW PASSER SPP. AT OXFORD

Clutch-size, incubation and hatching success were studied in P. domesticus and P. montanus in 1961 and 1963–64 at Oxford. The most frequent clutch-size was four eggs in P. domesticus and five eggs in P. montanus . With one exception, colonies of P. domesticus showed no significant annual or local variations in its mean clutch-size; in P. montanus , however, there were significant annual variations in the mean clutch-size. Both species showed a seasonal increase followed by a decrease in their mean clutch-sizes.
Partial incubation occurred during the laying period of the clutch; sufficient incubation for continuous development of the embryo was apparently achieved when the last egg had been laid in clutches of two and three eggs in P. domesticus and in clutches of four eggs in P. montanus , but when the penultimate egg had been laid in larger clutches of both species. On average, hatching in P. domesticus occurred more or less synchronously in all eggs in clutches of two and three eggs, and in all eggs except the last one laid in larger clutches; the last egg in the larger clutches hatched up to a day after the others. It is suggested that this pattern of hatching was brought about by the pattern of incubation during the laying period.
P. domesticus had a lower hatching success than P. montanus , probably because fewer of its eggs were fertile.