Effect of nesting environment on incubation temperature and hatching success of Morelet's crocodile (Crocodylus moreletii) in an urban lake of Southeastern Mexico

Incubation temperature is an important aspect in terms of biological performance among crocodiles, and several controlled experiments have demonstrated a significant relationship between incubation temperature, success in hatching and survival of hatchlings. However, a few studies have tested these relationships in the wild. The objective of this study was to determine the relationship of nest characteristics and environment (hatch year, nest basal area and height, clutch size, distance to shore line, and vegetation cover), to incubation temperature and hatching success among Morelet's crocodile (Crocodylus moreletii). The study was carried out during the nesting seasons of Morelet's crocodile, from 2007 to 2009 in the Laguna de Las Ilusiones, an urban lake located in Villahermosa, Tabasco, Mexico. We physically characterized 18 nests and inserted a temperature data logger in each nest chamber. At the end of the nesting season and prior to hatching, we recovered the crocodile eggs and data loggers and calculated hatching success, under laboratory conditions. We related the environmental variables of the nest with the mean and fluctuation (standard deviation) of nest temperature, using linear models. We also related the environmental variables affecting the nest, to mean nest temperature and fluctuation in incubation temperature and to hatching success, using linear models. Although we found differences in incubation temperature between nests, mean incubation temperature did not differ between years, but there were differences in nest thermal fluctuation between years. The mean incubation temperature for 11 nests (61.1%) was lower than the suggested Female–Male pivotal temperature (producing 50% of each sex) for this species, and all hatchlings obtained were males. There were no differences in clutch size between years, but hatching success varied. Our study indicates that hatching success depends on certain environmental variables and nest conditions to which the eggs are subjected, including season, nest size and clutch size. We also discuss the importance of the fluctuation of incubation temperature on hatching success and sex determination.     

Clutch size manipulation, hatching success and offspring phenotype in the ball python (Python regius)

In a diverse array of avian and mammalian species, experimental manipulations of clutch size have tested the hypothesis that natural selection should adjust numbers of neonates produced so as to maximize the number of viable offspring at the end of the period of parental care. Reptiles have not been studied in this respect, probably because they rarely display parental care. However, females of all python species brood their eggs until hatching, but they do not care for their neonates. This feature provides a straightforward way to experimentally increase or reduce clutch size to see whether the mean clutch size observed in nature does indeed maximize hatching success and/or optimize offspring phenotypes. Eggs were removed or added to newly laid clutches of Ball Pythons ( Python regius ) in tropical Africa (nine control clutches, eight with 50% more eggs added, six with 42% of eggs removed). All clutches were brooded by females throughout the 2-month incubation period. Experimental manipulation of clutch-size did not significantly affect the phenotypes (morphology, locomotor ability) of hatchlings, but eggs in 'enlarged' clutches hatched later, and embryos were more likely to die before hatching. This mortality was due to desiccation of the eggs, with females being unable to cover 'enlarged' clutches sufficiently to retard water loss. Our results support the notion of an optimal clutch size, driven by limitations on parental ability to care for the offspring. However, the proximate mechanisms that generate this optimum value differ from those previously described in other kinds of animals. © 2003 The Linnean Society of London . Biological Journal of the Linnean Society 2003, 78 , 263–272.     

Beach erosion and nest site selection by the leatherback sea turtle Dermochelys coriacea (Testudines: Dermochelyidae) and implications for management practices at Playa Gandoca, Costa Rica

Leatherback sea turtles (Dermochelys coriacea) nest on dynamic, erosion-prone beaches. Erosive processes and resulting nest loss have long been presumed to be a hindrance to clutch survival. In order to better understand how leatherbacks cope with unstable nesting beaches, I investigated the role of beach erosion in leatherback nest site selection at Playa Gandoca, Costa Rica. I also examined the potential effect of nest relocation, a conservation strategy in place at Playa Gandoca to prevent nest loss to erosion, on the temperature of incubating clutches. I monitored changes in beach structure as a result of erosion at natural nest sites during the time the nest was laid, as well as in subsequent weeks. To investigate slope as a cue for nest site selection, I measured the slope of the beach where turtles ascended from the sea to nest, as well as the slopes at other random locations on the beach for comparison. I examined temperature differences between natural and relocated nest sites with thermocouples placed in the sand at depths typical of leatherback nests. Nests were distributed non-randomly in a clumped distribution along the length of the beach and laid at locations that were not undergoing erosion. The slope at nest sites was significantly different than at randomly chosen locations on the beach. The sand temperature at nest depths was significantly warmer at natural nest sites than at locations of relocated nests. The findings of this study suggest leatherbacks actively select nest sites that are not undergoing erosive processes, with slope potentially being used as a cue for site selection. The relocation of nests appears to be inadvertently cooling the nest environment. Due to the fact that leatherback clutches undergo temperature-dependent sex determination, the relocation of nests may be producing an unnatural male biasing of hatchlings. The results of this study suggest that the necessity of relocation practices, largely in place to protect nests from erosion, should be reevaluated to ensure the proper conservation of this critically endangered species.     

Maternal behavior and clutch manipulation in the ring-legged earwig (Dermaptera: Carcinophoridae)

The expression and maintenance of maternal behavior in the earwig,Euborellia annulipes, was examined through manipulation of clutch size, age, and species and through observations of interactions between brooding females. Females underwent discrete gonadotrophic cycles culminating in oviposition of first clutches that were highly variable in size. Neither the head capsule width nor the age of the mother was correlated with clutch size. Maternal care extended through embryogenesis and for the week following hatching. Clutch removal significantly shortened the interclutch interval, indicating that the presence of brood inhibited the onset of the second gonadotrophic cycle. Brooding females readily accepted replacement clutches of the same age. Thus, mothers did not appear to distinguish their own eggs from those of other females. Experimental doubling of clutch size did not significantly reduce the proportion hatching or fledging. In contrast, reducing clutch size diminished the percentage successfully fledging. Manipulation of clutch age resulted in reduced hatching/fledging success. Placing two females, each with newly laid clutches, in the same cage usually resulted in egg transfer from the nest of one female to that of the other within 12 h. Nests of females with larger forceps were significantly more likely to contain both clutches. When mothers with first clutches were paired with mothers with third clutches, eggs were more likely to be transferred to the nest of the older female.E. annulipes females with newly laid clutches appeared to accept as replacement clutches eggs of the earwigDoru taeniatum. Alien clutches were maintained for the typical duration of embryogenesis; however, noD. taeniatum hatchlings were observed.     

The role of prolactin in the regulation of clutch size and onset of incubation behavior in the American kestrel

In most bird species, the timing of incubation onset may influence the degree of hatching asynchrony, which, together with variation in clutch size, affects reproductive success. In some domesticated species that usually show no hatching asynchrony, plasma prolactin concentrations in females rise with the onset of incubation and the end of laying, and this rise enhances incubation behavior and may terminate laying. To investigate whether a rise in prolactin during laying is involved in the regulation of clutch size and incubation onset in a species with hatching asynchrony, we measured plasma concentrations of immunoreactive prolactin (ir-prolactin) in laying American kestrels, Falco sparverius, and quantified clutch size and incubation behavior. In a separate study, we administered one of three concentrations of ovine prolactin (o-prolactin) via osmotic pumps implanted in females when egg 2 of a clutch was laid. ir-Prolactin concentrations during laying were higher in small than in large clutches and increased in parallel with the development of incubation behavior. o-Prolactin treatment enhanced incubation behavior, but did not affect clutch size, possibly because the manipulation was performed after clutch size had already been determined. Consistent with studies on domesticated species that show synchronous hatching, our results indicate that rising prolactin during laying enhances the expression of incubation behavior in a species that shows hatching asynchrony. Further studies are necessary to determine whether the relationship between prolactin and clutch size in the American kestrel is one of causation or of mere association.     

Relationship between clutch size,egg volume and hatching success in a Yellow-legged Gull Larus michahellis colony in south-eastern Tunisia

This study aimed to test the hypothesis that clutch size covaries with egg volume and hatching success in the Yellow-legged Gull Larus michahellis. We determined clutch size and egg volume in a sample of 131 nests, and we used the data to check whether egg volume varied among nests according to clutch size, while taking into account the effects of egg laying order. We also estimated hatching success rate and investigated the relationship between hatching success and clutch size. Egg volume varied among clutches according to clutch size, with eggs being larger in three-egg clutches than in two-egg clutches. Moreover, three-egg clutches showed higher daily survival rates, and hence hatching success, than two-egg clutches. Overall, our results suggest that in the Yellow-legged Gull clutch size covaries with egg volume and hatching success, which could possibly reflect an age effect through different mechanisms. Indeed, older females could be hypothesised to exhibit greater breeding performance than younger females because of their higher experience in tapping energy resources for egg formation and defending nests from dangers. Moreover, due to their age, older females are likely to have lower residual reproductive potential and should invest more heavily in current breeding attempts.     

Sex ratios in naturally incubating Macrochelys temminckii nests,a species with type II temperature-dependent sex determination

The alligator snapping turtle, Macrochelys temminckii, exhibits type II temperature-dependent sex determination (TSD), wherein females are produced at high and low incubation temperatures. This TSD pattern is well studied at constant temperatures, but little work has focused on sex ratios in natural nests that experience daily and seasonal temperature fluctuations. We monitored nesting activity of reintroduced Macrochelys temminckii at Tishomingo National Wildlife Refuge in 2010–2011. Nests located prior to predation were excavated to determine clutch size and the eggs were reburied with a temperature data logger to collect nest temperatures. Overall, 24% of nests were protected with wire mesh prior to predation, and the average clutch size in intact nests was 22.4 eggs. Nest predation rates in the study population will likely approach 100% if nest protection efforts do not continue. Temperature profiles were used to compare estimated sex ratios using two methods—mean nest temperature during middle third of incubation and the degree-day model—to actual sex ratios in naturally incubated Macrochelys temminckii nests. The sex ratio in all 2010 recruits was female-biased (91.8% female); 2011 nests did not produce any hatchlings, likely the result of severe drought. The predicted sex ratios based on mean nest temperature and the degree-day model matched actual sex ratios in the warmer nests (0% male), but the degree-day model estimate proved more accurate in the cooler nest. A strongly skewed population sex ratio could become a threat to this reintroduced population if the strongly female-biased sex ratio in 2010 reflects a long-term trend.     

Metabolic heating and the prediction of sex ratios for green turtles (Chelonia mydas)

We compared incubation temperatures in nests (n=32) of the green turtle (Chelonia mydas) on Ascension Island in relation to sand temperatures of control sites at nest depth. Intrabeach thermal variation was low, whereas interbeach thermal variation was high in both control and nest sites. A marked rise in temperature was recorded in nests from 30% to 40% of the way through the incubation period and attributed to metabolic heating. Over the entire incubation period, metabolic heating accounted for a mean rise in temperature of between 0.07 degrees and 2.86 degrees C within nests. During the middle third of incubation, when sex is thought to be determined, this rise in temperature ranged between 0.07 degrees and 2.61 degrees C. Metabolic heating was related to both the number of eggs laid and the total number of hatchlings/embryos produced in a clutch. For 32 clutches in which temperature was recorded, we estimate that metabolic heating accounted for a rise of up to 30% in the proportion of females produced within different clutches. Previous studies have dismissed any effect of metabolic heating on the sex ratio of marine turtle hatchlings. Our results imply that metabolic heating needs to be considered when estimating green turtle hatchling sex ratios.     

The feminizing effect of metabolic heating in Green Turtle (Chelonia mydas) clutches in the eastern Mediterranean

Metabolic heating has been poorly investigated in eastern Mediterranean coastline of Turkey, which host some of the most important Green Turtle (Chelonia mydas) nesting sites in the Mediterranean. We studied the effects of clutch size and embryo numbers on nest temperature and discuss the feminizing effect of metabolic heating. Two test sites were conducted in Sugözü Beaches (Turkey). Data loggers were placed in eight nests with different clutch sizes. Nest temperature was strongly correlated with embryo numbers and metabolic heating produced by embryos was calculated to be 0.019°C per late stage embryo and 0.020°C per hatchling. Metabolic heating was calculated to be 0.6°C in the middle third of the incubation period during which sex is determined. It was estimated that metabolic heating increased 10.4% of female hatchlings. The heat produced by embryos should be taken into consideration while estimating sex ratios indirectly by nest and sand temperatures. Additionally, the metabolic heating value should be known for conservation measures, such as nest relocation, dividing the nest for controlling nest temperature, especially related to climate change.     

Costs of large communal clutches for male and female Greater Rheas Rhea americana

The breeding system of the Greater Rhea Rhea americana is almost unique among birds as it combines harem polygyny and sequential polyandry, with communal egg-laying and uniparental male care. In this species, large communal clutches (more than 30 eggs) are rare and have a lower hatching success than smaller clutches. Here we analyse the proximate causes of hatching failures and the costs of large communal clutches (and therefore the costs of extensive polygyny) for males and females. We evaluated if length of the nesting period, egg viability, egg losses during incubation and male parental activity at the nest were affected by clutch size. We also evaluated if chicks hatched from large clutches have a lower survival during the first 2 months after hatching. Large clutches had longer nesting period and lower hatching success, mainly as a result of bacterial contamination of the eggs and increased hatching asynchrony. In addition, large clutches tended to lose more eggs as a result of accidental breakage or predation. Male activity at the nest and chick survival were not related to clutch size. Low hatching success, nest predation risk and energetic costs associated with large clutches penalize females that join large harems and males that accept additional eggs into the nest.     

Breeding success and the question of clutch size of Northwestern Crows Corvus caurinus

The breeding biology of two island populations of the Northwestern Crow was studied in British Columbia over a period of five years. Both populations laid an average of four eggs per clutch, of which approximately 75% hatched. More young survived to fledge on Mandate than on Mitlenatch Island. The young from nests close (< 100 m) to the beach on Mitlenatch had a higher survival rate than those from further inland. The number of eggs lost or failing to hatch was lowest in clutches of four eggs. There was a non-significant trend for chick survival to be inversely related to clutch size. The net result was that clutches of five eggs produced no more young than did clutches of four eggs. It is suggested that the major limiting factor on clutch size is food availability but the decreased hatching success of any remaining eggs once hatching has started is also a contributing factor.     

Consistent sex ratio bias of individual female dragon lizards

Sex ratio evolution relies on genetic variation in either the phenotypic traits that influence sex ratios or sex-determining mechanisms. However, consistent variation among females in offspring sex ratio is rarely investigated. Here, we show that female painted dragons (Ctenophorus pictus) have highly repeatable sex ratios among clutches within years. A consistent effect of female identity could represent stable phenotypic differences among females or genetic variation in sex-determining mechanisms. Sex ratios were not correlated with female size, body condition or coloration. Furthermore, sex ratios were not influenced by incubation temperature. However, the variation among females resulted in female-biased mean population sex ratios at hatching both within and among years.     

Incubation capacity contributes to constraints on maximal clutch size in Brent Geese Branta bernicla nigricans

Lack ( 1967 ) proposed that clutch size in species with precocial young was determined by nutrients available to females at the time of egg formation; since then others have suggested that regulation of clutch size in these species may be more complex. We tested whether incubation limitation contributes to ultimate constraints on maximal clutch size in Black Brent Geese (Black Brant) Branta bernicla nigricans. Specifically, we investigated the relationship between clutch size and duration of the nesting period (i.e. days between nest initiation and the first pipped egg) and the number of goslings leaving the nest. We used experimental clutch manipulations to assess these questions because they allowed us to create clutches that were larger than the typical maximum of five eggs in this species. We found that the per‐capita probability of egg success (i.e. the probability an egg hatched and the gosling left the nest) declined from 0.81 for two‐egg clutches to 0.50 for seven‐egg clutches. As a result of declining egg success, clutches containing more than five eggs produced, at best, only marginally more offspring. Manipulating clutch size at the beginning of incubation had no effect on the duration of the nesting period, but the nesting period increased with the number of eggs a female laid naturally prior to manipulation, from 25.4 days (95% CI 25.1–25.7) for three‐egg clutches to 27.7 days (95% CI 27.3–28.1) for six‐egg clutches. This delay in hatching may result in reduced gosling growth rates due to declining forage quality during the brood rearing period. Our results suggest that the strong right truncation of Brent clutches, which results in few clutches greater than five, is partially explained by the declining incubation capacity of females as clutch size increases and a delay in hatching with each additional egg laid. As a result, females laying clutches with more than five eggs would typically gain little fitness benefit above that associated with a five‐egg clutch.     

Potential limitations of behavioral plasticity and the role of egg relocation in climate change mitigation for a thermally sensitive endangered species

Anthropogenic climate change is widely considered a major threat to global biodiversity, such that the ability of a species to adapt will determine its likelihood of survival. Egg‐burying reptiles that exhibit temperature‐dependent sex determination, such as critically endangered hawksbill turtles (Eretmochelys imbricata), are particularly vulnerable to changes in thermal regimes because nest temperatures affect offspring sex, fitness, and survival. It is unclear whether hawksbills possess sufficient behavioral plasticity of nesting traits (i.e., redistribution of nesting range, shift in nesting phenology, changes in nest‐site selection, and adjustment of nest depth) to persist within their climatic niche or whether accelerated changes in thermal conditions of nesting beaches will outpace phenotypic adaption and require human intervention. For these reasons, we estimated sex ratios and physical condition of hatchling hawksbills under natural and manipulated conditions and generated and analyzed thermal profiles of hawksbill nest environments within highly threatened mangrove ecosystems at Bahía de Jiquilisco, El Salvador, and Estero Padre Ramos, Nicaragua. Hawksbill clutches protected in situ at both sites incubated at higher temperatures, yielded lower hatching success, produced a higher percentage of female hatchlings, and produced less fit offspring than clutches relocated to hatcheries. We detected cooler sand temperatures in woody vegetation (i.e., coastal forest and small‐scale plantations of fruit trees) and hatcheries than in other monitored nest environments, with higher temperatures at the deeper depth. Our findings indicate that mangrove ecosystems present a number of biophysical (e.g., insular nesting beaches and shallow water table) and human‐induced (e.g., physical barriers and deforestation) constraints that, when coupled with the unique life history of hawksbills in this region, may limit behavioral compensatory responses by the species to projected temperature increases at nesting beaches. We contend that egg relocation can contribute significantly to recovery efforts in a changing climate under appropriate circumstances.     

Progeny sex ratios in a short-lived lizard: seasonal invariance despite sex-specific effects of hatching date on fitness

When fitness returns are sex-specific, selection should favor the facultative adjustment of offspring sex ratios. Seasonal shifts in offspring sex ratios are predicted to be particularly beneficial in short-lived, sexually dimorphic species in which hatching date is linked to adult size, which is related to fitness in a sex-specific fashion. We used four time series of hatching dates and progeny sex ratios in the brown anole (Anolis sagrei), a short-lived lizard with male-biased sexual size dimorphism, to test for such a seasonal shift in progeny sex ratio. In 2 of the 4 years, we also released hatchlings to their natural environment to test for sex-specific effects of hatching date on juvenile survival and adult size. We found that the relationship between hatching date and size the following year was significantly steeper in males than in females, and previous work has shown that adult size is more strongly tied to fitness in males than in females. Based on those results and on further evidence linking hatching date and body size to sex-specific survival and reproductive success, we predicted that sex ratios should shift from male- to female-biased as the breeding season progressed. Contrary to our prediction, we detected no clear seasonal shift in progeny sex ratio. Furthermore, although juvenile survival was correlated with hatching date, this relationship did not consistently differ between the sexes. The observation that progeny sex ratios are seasonally invariant despite several apparent links to adult fitness suggests that the evolution of a seasonal sex-ratio bias is either inherently constrained or requires a stronger selective advantage with respect to juvenile survival.     

Hatching asynchrony that maintains egg viability also reduces brood reduction in a subtropical bird

In birds, hatching failure is pervasive and incurs an energetic and reproductive cost to breeding individuals. The egg viability hypothesis posits that exposure to warm temperatures prior to incubation decreases viability of early laid eggs and predicts that females in warm environments minimize hatching failure by beginning incubation earlier in the laying period, laying smaller clutches, or both. However, beginning incubation prior to clutch completion may incur a cost by increasing hatching asynchrony and possibly brood reduction. We examined whether Florida scrub jays (Aphelocoma coerulescens) began incubation earlier relative to clutch completion when laying larger clutches or when ambient temperatures increased, and whether variation in incubation onset influenced subsequent patterns of hatching asynchrony and brood reduction. We compared these patterns between a suburban and wildland site because site-specific differences in hatching failure match a priori predictions of the egg viability hypothesis. Females at both sites began incubation earlier relative to clutch completion when laying larger clutches and as ambient temperatures increased. Incubation onset was correlated with patterns of hatching asynchrony at both sites; however, brood reduction increased only in the suburbs, where nestling food is limiting, and only during the late nestling period. Hatching asynchrony may be an unintended consequence of beginning incubation early to minimize hatching failure of early laid eggs. Food limitation in the suburbs appears to result in increased brood reduction in large clutches that hatch asynchronously. Therefore, site-specific rates of brood reduction may be a consequence of asynchronous hatching patterns that result from parental effort to minimize hatching failure in first-laid eggs. This illustrates how anthropogenic change, such as urbanization, can lead to loss of fitness when animals use behavioral strategies intended to maximize fitness in natural landscapes.     

ESTABLISHMENT OF WEIGHT HIERARCHIES IN THE BROODS OF HOUSE MARTINS DELICHON URBICA

Nestling birds may differ in size and weight on the first day a clutch is fully hatched, mainly because eggs within clutches hatch over a period of several days. This asynchronous pattern of hatching is usually thought to facilitate brood reduction when the food supply is unpredictably restricted. The purpose of the study reported here was to examine the contribution of egg-weight, clutch-size, hatching spread, food supply and season to weight differences in newly hatched broods of the House Martin. At laying, heavy eggs had a greater moisture and dry weight content than light eggs and immediately before hatching there was a correlation between initial egg-weight and the dry weight of embryo and yolk. Heavier clutches also tended to give rise to heavier hatchlings. There was, however, no correlation of fresh egg-weight with the dry weight of embryos alone and the relative dry weight of embryos in a clutch was dependent on laying sequence. Hatching spread (the number of days between the emergence from the egg of the first and the last hatchling of the clutch) was 0.75 ± 0.46 days for clutches of two and increased with the size of the clutch up to 1.80 ± 0.79 days for clutches of five. When food was scarce during laying, hatching spread was greater. Weight difference in newly hatched broods was correlated with hatching spread and moreover in multivariate analysis was also correlated with periods of food scarcity during laying. It was concluded that all examples of weight hierarchies among hatchlings should not be considered adaptive; in some cases they may be imposed by food scarcity. This can lead to mortality of the runs even if food is plentiful. When the weight hierarchy is not adversely accentuated by food scarcity it may function as previously suggested, to allow brood reduction. Alternatively, particularly among House Martins, it may spread out the peak food needs of individual nestlings thereby spreading the demand on the adults.     

Proximate causes and fitness consequences of hatching failure in lesser kestrels Falco naumanni

Hatching failure is a pervasive phenomenon in birds, but factors affecting hatchability remain poorly understood. We studied proximate causes and fitness consequences of hatching failure in a long‐monitored population of the colonial lesser kestrel Falco naumanni. We investigated whether hatchability was related to clutch characteristics, parental traits, and social or environmental features. Hatching failure represents a cost for the parents in terms of immediate fitness, since it reduced both their number of young fledged and recruits in the breeding population, even when controlling for clutch size. Hatching failure showed a non‐linear relationship with clutch size, clutches of four eggs showing higher levels of hatching success than larger or smaller clutches. Hatchability could therefore play a role in the evolution of optimal clutch size in this species, at least constraining the maximum number of eggs the parents can afford to incubate. Contrary to most studies, the mean volume of the clutch and the individual egg volume were negatively related to hatching failure, indicating that large eggs have thermoregulatory and/or nutritional advantages. Mean daily maximum temperature during incubation affected hatching success negatively, but only for females in poor condition. This result seems to indicate that females are reluctant to jeopardize their own condition, but instead sacrifice incubation effort by paying the costs of a lower hatching success in circumstances of high temperatures. There was no evidence that hatching failure was related to the intrinsic properties of individuals or genetic similarity between the parents as indicated by low repeatabilities of: (1) males that bred with different females, (2) females that bred with different males, and (3) pairs breeding together in different years. Neither colony size nor subpopulation size affected hatchability. All these findings show how hatching failure is simultaneously influenced by several factors acting in a complex way, which could in part explain the apparently conflicting conclusions of empirical or even experimental studies carried out to date.     

Quantity or quality? Determinants of maternal reproductive success in tropical pythons (Liasis fuscus)

A female''s reproductive output (size and number of offspring) may say little about her reproductive success; the ''quality'' of her progeny and the time that they are produced may be more important in this respect. We marked and released 1224 hatchling water pythons (Liasis fuscus) from 116 clutches of laboratory-incubated eggs, in a study site in tropical Australia. Clutches varied widely in the number of eggs, mean offspring sizes, the proportion of eggs that hatched, and recapture rates of offspring. Clutch size was not significantly correlated with the number of recaptured hatchlings in either year, nor was mean offspring size related to recapture probability. Instead, offspring recapture rates varied as a function of the time of hatching. Recapture rates were higher for hatchlings released in a year with high prey availability than in a year with few prey. In addition, recapture rates were higher for earlier-hatching clutches in the year with few prey. The other significant effect on recapture rates involved offspring viability: some clutches had a much higher proportion of surviving offspring than did others, and recapture rates were higher from clutches with higher hatching success (i.e. proportion of eggs hatching). Hence, the usual measures of reproductive output (clutch size and offspring size) offer only a poor index of a female''s reproductive success in this system.     

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.