Adaptation of the Avian Egg to High Altitude

Theoretical predictions and experiments on eggs of domesticatedbirds indicate that the diffusion coefficient of gases is inverselyproportional to barometric pressure. Therefore potentially lethallosses of CO₂ and water vapor from eggs laid at high altitudemight result if the increased tendency of gases to diffuse atreduced barometric pressure were not counteracted in some manner.Limited data from two wild populations indicate that water lossfrom eggs is independent of altitude over a 3000 m elevationalgradient. Four different possibilities are discussed by whichcompensation for increased diffusion of water vapor might beachieved at high elevations 1) a reduction in eggshell conductance(GH₂O) 2) an increase in the initial water content of the eggs3) an increase in shell thickness, and 4) alteration of watervapor pressure in the nest microenvironment or incubation temperatureby variation in parental behavior. Mean GH₂O of eggs of twoprecocial and four altricial species breeding above 2800 m issignificantly reduced below values of related birds breedingat lower elevations, but no change in initial water contentor shell thickness has been observed in such eggs. Observationsof parental behavior in species breeding over wide elevationalgradients have not yet been made. Identification of the mechanismsby which eggshell structure is modified to achieve a reducedGH₂O the environmental cues used by females to determine theelevation of the breeding location and the rapidity with whichshell structuie can be modified awaits further research.     

Physiological and Ecological Aspects of Gas Exchange by Sea Turtle Eggs

The sea turtle clutch of about 100 eggs is buried deeply inthe nesting beach.The eggs exchange respiratory gases with thesurrounding beach as their metabolic activity increases throughoutthe 60 day incubation. The O₂ consumption of individual eggsthroughout incubation is less than that of avian eggs of similarmass; however, this difference may be attributed to the differencein incubation temperature and growth rate. The O₂ consumptionof the sea turtle embryo is sufficiently low and the gas conductanceof the shell sufficiently large that only small gas partialpressure gradients occur across the shell. However, the metabolicintensity of the entire clutch is quite large, and since gasmovement through the beach is restricted, increasing gas partialpressure gradients are established between the center and peripheryof the clutch and between the clutch and surrounding beach.The rate of growth and mortality of the embryos is related torespiratory gas exchange, since maximum growth and hatchlingsuccess appear to occur in respiratory environments similarto those observed in natural nests. Embryonic growth slows andmortality increases in environments in which gas exchange isreduced below naturally occurring levels. Gas exchange considerationsmay influence nest construction, clutch size and incubationtime among sea turtles.     

Megapodes: Recent ideas on origins, adaptations and reproduction

Some birds don't incubate their eggs using the warmth of their bodies. The megapodes have evolved a markedly different method of incubation. By making use of environmental heat, adult megapodes are emancipated from most of the usual constraints to reproductive success associated with parental care. Consequences of this unique incubation method are evident in all aspects of megapode development and behaviour. Recently, renewed interest in these birds from a number of disciplines has led to significant advances, especially in aspects associated with reproduction. Both eggs and embryos exhibit remarkable adaptations to the physiologically stressful incubation environment. The use of various types of incubation heat may also shape the mating systems of the species involved.     

Avian eggshell thickness: scaling and maximum body mass in birds

The avian eggshell represents a highly evolved structure adapted to the physiological requirements of the embryo and the potential fracturing forces it is exposed to during incubation. Given its many roles, it is not surprising that the eggshell is also central to the current hypothesis about maximum avian body mass. Eggshell thickness ( L ) and strength has historically been scaled as a function of initial egg mass (IEM). However, maximum incubator mass (IM) is likely a better indicator of the forces the shell must be selected to withstand during incubation. We compare the results of analyses of L ² (an indicator of shell strength) as a function of IEM and IM. We conclude from IM scaling that megapode and kiwi eggshells are not thin but rather are thicker than expected and in general birds with a clutch size of 1 have thicker shells, and further, that reversed sexual dimorphism in the large, particularly extinct birds may be a strategy to avoid shell breakage during incubation of the largest eggs without creating a shell so thick as to inhibit hatching.     

Gas Exchange of the Avian Egg Time, Structure, and Function

Data are presented for oxygen consumption water loss duringincubation water vapor conductance of the shell and pore numberof avian eggs and the way in which these values relate not onlyto egg mass but also to incubation time. It is proposed thatall these functions are proportional to the product of egg massand rate of development where the latter is defined as the inverseof incubation time. These interrelationships account at theend of incubation for similar O₂ and CO₂ tensions in the airspace of eggs utilization of calories (0.5 kcal g^–1) andwater loss (15 g g^–1)     

How avian incubation behaviour influences egg surface temperatures: relationships with egg position,development and clutch size

While understanding heat exchange between incubating adults and their eggs is central to the study of avian incubation energetics, current theory based on thermal measurements from dummy eggs reveals little about the mechanisms of this heat exchange or behavioural implications for the incubating bird. For example, we know little about how birds distribute their eggs based on temperature differences among egg positions within the nest cup. We studied the great tit Parus major, a species with a large clutch size, to investigate surface cooling rates of individual eggs within the nest cup across a range of ambient temperatures in a field situation. Using state‐of‐the‐art portable infrared imaging and digital photography we tested for associations between egg surface temperature (and rate of cooling) and a combination of egg specific (mass, shape, laying order, position within clutch) and incubation specific (clutch size, ambient temperature, day of incubation) variables. Egg surface temperature and cooling rates were related to the position of the eggs within the nest cup, with outer eggs being initially colder and cooling quicker than central eggs. Between foraging bouts, females moved outer eggs significantly more than centrally positioned eggs. Our results demonstrate that females are capable of responding to individual egg temperature by moving eggs around the nest cup, and that the energy cost to the female may increase as incubation proceeds. In addition, our results showing that smaller clutches experience lower initial incubation temperatures and cool quicker than larger clutches warrant further attention for optimal clutch size theory and studies of energetic constraints during incubation. Finally, researchers using dummy eggs to record egg temperature have ignored important elements of contact‐incubation, namely the complexity of how eggs cool and how females respond to these changes.     

THE PHYSIOLOGICAL ECOLOGY OF REPTILIAN EGGS AND EMBRYOS. AND THE EVOLUTION OF VIVIPARITY WITHIN THE CLASS REPTILIA

1. Eggs of Crocodilia and Chelonia, like those of birds, have a pair of egg membranes separating a thick layer of albumen from the calcareous shell. In contrast, eggs of oviparous Lepidosauria have only a single shell membrane, upon which relatively small amounts of calcium carbonate are deposited; and the volume of albumen in eggs is extraordinarily small at the time of oviposition. 2. With the possible exception of certain geckos and some chelonians, eggs of oviparous reptiles seem always to absorb water from the substrate during the course of normal incubation. In so far as the rate of water absorption exceeds the rate of water loss by transpiration from exposed surfaces, the eggs swell during incubation. The term ‘cleidoic’ cannot be used to describe eggs of this type. 3. Embryos of lizards and snakes influence the water potential of extra-embryonic fluids contained within their eggs, thereby maintaining or increasing the gradient in water potential that drives water absorption. 4. Embryos of Crocodilia and Chelonia obtain a substantial portion of the calcium used in ossification of skeletal elements from the inner surfaces of the eggshell. In contrast, embryonic lizards and snakes draw upon extensive reserves of calcium present in the yolk, and obtain little (if any) calcium from the eggshell. 5. All reptilian embryos seem to produce substantial quantities of urea as a detoxification product of protein catabolism. Contrary to expectation, uricotelism may not be common among reptilian embryos, even in those few instances where development takes place within a hard, calcareous egg. 6. In eggs of Crocodilia and Chelonia, respiratory gases seem to pass by diffusion through pores in the calcareous eggshell and through spaces between the fibres of the pair of egg membranes. No pores have been observed in the shell of lepidosaurian eggs, and so gases presumably diffuse between the fibres of the single (multilayered) shell membrane. 7. Metabolism of reptilian embryos is temperature-dependent, as is true for most ectothermic organisms. For each species, there appears to be a particular temperature at which embryonic development proceeds optimally, and departures from this optimum elicit increases in developmental anomalies and/or embryonic mortality. 8. Viviparity has evolved on numerous occasions among species of the Squamata, but seemingly never among Crocodilia or Chelonia. Since the evolution of viviparity entails a progressive reduction in the eggshell, only those organisms whose embryos do not depend upon the eggshell as a source of calcium may have the evolutionary potential to become viviparous. 9. Evolutionary transitions from oviparity to viviparity could have been driven by selection related to (i) thermal benefits to embryos consequent upon retention of eggs within the body of a parent capable of behavioural thermoregulation; (ii) protection of the eggs from nest predators and/or soil microbes; and (iii) more effective exploitation of a seasonal food resource by early emerging young.     

Can Reptile Embryos Influence Their Own Rates of Heating and Cooling?

Previous investigations have assumed that embryos lack the capacity of physiological thermoregulation until they are large enough for their own metabolic heat production to influence nest temperatures. Contrary to intuition, reptile embryos may be capable of physiological thermoregulation. In our experiments, egg-sized objects (dead or infertile eggs, water-filled balloons, glass jars) cooled down more rapidly than they heated up, whereas live snake eggs heated more rapidly than they cooled. In a nest with diel thermal fluctuations, that hysteresis could increase the embryo’s effective incubation temperature. The mechanisms for controlling rates of thermal exchange are unclear, but may involve facultative adjustment of blood flow. Heart rates of snake embryos were higher during cooling than during heating, the opposite pattern to that seen in adult reptiles. Our data challenge the view of reptile eggs as thermally passive, and suggest that embryos of reptile species with large eggs can influence their own rates of heating and cooling.     

Costs and response to conspecific brood parasitism by colonial red-breasted mergansers

Costs of conspecific brood parasitism (CBP) are expected to be influenced by a species’ life history traits. Precocial birds lay large clutches, and clutches that have been enlarged by CBP can affect host fitness through a longer incubation period, displaced eggs, and lower hatching success. We examined costs and response to CBP by hosts in a population of colonial red-breasted mergansers (Mergus serrator; n?=?400 nests over 8 years) within which 29% of parasitized clutches were enlarged considerably (≥?15 eggs). Length of the incubation period did not increase with clutch size. The mean number of eggs displaced from a parasitized nest during incubation (2.8) was 2×?greater than at an unparasitized nest (1.4). Hatching success declined by 2% for each additional egg in the nest. Thus, for a nest with?≥?15 eggs, one or more fewer host eggs hatch relative to an unparasitized nest with the same number of host eggs, assuming equal probability of success for all eggs. Hosts were 40% more likely to desert nests receiving 2 or 6 experimental eggs relative to unparasitized control nests, although it is unknown whether hens deserting a nest renested elsewhere. Our study indicates that costs of CBP to hosts during nesting may be limited to those red-breasted mergansers incubating the largest clutches (≥?15 eggs), and it raises questions about the adaptive significance of deserting a parasitized clutch.     

Adaptation of Avian Eggs to Incubation Period Variability Around Allometric Regressions Is Correlated with Time

Many morphological and physiological parameters of avian eggsare known to be correlated with fresh egg weight, i.e, to exhibitallometric relationships. It has been predicted that part ofthe variability around some of these allometric relationshipsis the result of adaptations to incubation periods of differentlengths. I performed multiple regressions of three of theseparameters (the rate of water loss in the nest, conductanceof the shell to gases and the rate of O₂ consumption just priorto hatching) versus weight and incubation period to test thesepredictions. All these parameters were significantly correlatedwith weight and for eggs of the same weight inversely correlatedwith the length of the incubation period. Other investigatorshave reported allometric interrelations between weight, lifespan and other parameters. For example the average number ofheartbeats is the same in mammals of all weights. The kind ofinterrelation reported in this study appears similar to thiskind of allometric interrelation but is actually quite differentbecause it concerns organisms of the same weight. I refer tothis kind of interrelation between various parameters for eggsof the same weight as an isometric interrelation. The two kindsof interrelations are quite independent the existence of anallometric interrelation does not prove the existence of anisometric interrelation.     

Gold climates and the evolution of viviparity in reptiles: cold incubation temperatures produce poor-quality offspring in the lizard, Sceloporus virgatus

Evolutionary origins of viviparity among the squamate reptiles are strongly associated with cold climates, and cold environmental temperatures are thought to be an important selective force behind the transition from egg-laying to live-bearing. In particular, the low nest temperatures associated with cold climate habitats are thought to be detrimental to the developing embryos or hatchlings of oviparous squamates, providing a selective advantage for the retention of developing eggs in utero, where the mother can provide warmer incubation temperatures for her eggs (by actively thermoregulating) than they would experience in a nest. However, it is not entirely clear what detrimental effects cold incubation temperatures may have on eggs and hatchlings, and what role these effects may play in favouring the evolution of viviparity. Previous workers have suggested that viviparity may be favoured in cold climates because cold incubation temperatures slow cmbryogenesis and delay hatching of the eggs, or because cold nest temperatures are lethal to developing eggs and reduce hatching success. However, incubation temperature has also been shown to have other, potentially long-term, effects on hatchling phcnotypcs, suggesting that cold climates may favour viviparity because cold incubation temperatures produce offspring of poor quality or low fitness. We experimentally incubated eggs of the oviparous phrynosomatid lizard, Sceloporus virgatus, at temperatures simulating nests in a warm (low elevation) habitat, as is typical for this species, and nests in a colder (high elevation) habitat, to determine the effects of cold incubation temperatures on embryonic development and hatchling phenotypes. Incubation at cold nest temperatures slowed embryonic development and reduced hatching success, but also affected many aspects of the hatchlings' phenotypes. Overall, the directions of these plastic responses indicated that cold-incubated hatchlings did indeed exhibit poorer quality phenotypes; they were smaller at hatching (in body length) and at 20 days of age (in length and mass), grew more slowly (in length and mass), had lower survival rates, and showed greater fluctuating asymmetry than their conspecifics that were incubated at warmer temperatures. Our findings suggest that cold nest temperatures are detrimental to S. virgatus, by delaying hatching of their eggs, reducing their hatching success, and by producing poorer quality offspring. These negative effects would likely provide a selective advantage for any mechanism through which these lizards could maintain warmer incubation temperatures in cold climates, including the evolution of prolonged egg retention and viviparity.     

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.     

Characteristics, abundance and fertility of orphan eggs of the Lesser Rhea ( Pterocnemia-Rhea-pennata pennata ): implications for conservation

The Lesser Rhea (Pterocnemia-Rhea-pennata pennata) has a complex reproductive system that combines polygyny with sequential polyandry, in which males build the nest, fully incubate the eggs and care for the young. As occurs with the Greater Rhea (Rhea americana), Lesser Rhea females sporadically lay eggs outside the nest (‘orphan’ eggs), which are not incubated and thus fail to hatch. We have examined the orphan eggs of Lesser Rhea over two separate breeding seasons to determine their abundance and fertility status. During 2004 and 2005, weekly ground searches for orphan eggs were conducted in a wild population of Lesser Rhea in northwestern Patagonia, Argentina. During these searches the total number of nests, eggs in each nest and orphan eggs laid outside the nests was recorded. Orphan eggs represented approximately 7% of the total eggs laid in a breeding season. Six fresh orphan eggs were artificially incubated, four of them being fertile. Orphan eggs seemed to have two origins: some were laid near deserted nests in the early to mid-reproductive season; others were probably laid by first-time breeders and were found later in the reproductive season. Given the near-threatened status of the Lesser Rhea, harvesting and artificial incubation of orphan eggs, which otherwise would be unproductive, may contribute significantly to the conservation of this species, i.e., ‘recovered’ birds could be used for reintroduction or reinforcement of wild Lesser Rhea populations.     

Effect of intermittent incubation and clutch covering on the probability of bacterial trans‐shell infection

Microbial infection is considered a critical cause of hatching failure in birds. Although several behavioural mechanisms are believed to improve reproductive success in birds, their direct effect on the risk of bacterial trans‐shell infection (BTSI) remains to be tested. Here, we focus on the protective roles of intermittent incubation and covering of the clutch with nest lining during the laying period, when eggs are highly susceptible to BTSI. To our knowledge, this study is the first to use culture‐independent PCR‐based methods to measure quantitative and qualitative indices of BTSI. In our experiment, we exposed Mallard Anas platyrhynchos eggs that were either intermittently incubated (I‐INCUB) or un‐incubated (I‐UNINCUB), and covered (COV) or uncovered (UNCOV) with nest lining. Hatchability of I‐INCUB eggs was twice that for I‐UNINCUB eggs. The presence and degree of BTSI had no effect on hatching success of experimental eggs. The residual weights of ducklings hatched from infected and I‐INCUB eggs were lower than those from uninfected and I‐UNINCUB eggs. In addition, ducklings originating from COV eggs were heavier than those hatched from UNCOV eggs. Intermittent incubation and clutch covering had no effect on the probability or degree of BTSI. Although the effect of BTSI is considered less detrimental in temperate birds, we show that the presence of BTSI inside the egg may significantly affect hatchling phenotype. This represents a novel insight into the role of BTSI in the reproductive success of birds and implies that the use of molecular PCR‐based methods is required in future studies for a better understanding of such causality.     

Ecological,evolutionary, and conservation implications of incubation temperature‐dependent phenotypes in birds

Incubation is a vital component of reproduction and parental care in birds. Maintaining temperatures within a narrow range is necessary for embryonic development and hatching of young, and exposure to both high and low temperatures can be lethal to embryos. Although it is widely recognized that temperature is important for hatching success, little is known about how variation in incubation temperature influences the post‐hatching phenotypes of avian offspring. However, among reptiles it is well known that incubation temperature affects many phenotypic traits of offspring with implications for their future survival and reproduction. Although most birds, unlike reptiles, physically incubate their eggs, and thus behaviourally control nest temperatures, variation in temperature that influences embryonic development still occurs among nests within a population. Recent research in birds has primarily been limited to populations of megapodes and waterfowl; in each group, incubation temperature has substantial effects on hatchling phenotypic traits important for future development, survival, and reproduction. Such observations suggest that incubation temperature (and incubation behaviours of parents) is an important but underappreciated parental effect in birds and may represent a selective force instrumental in shaping avian reproductive ecology and life‐history traits. However, much more research is needed to understand how pervasive phenotypic effects of incubation temperature are among birds, the sources of variation in incubation temperature, and how effects on phenotype arise. Such insights will not only provide foundational information regarding avian evolution and ecology, but also contribute to avian conservation.     

The Gaseous Microclimate of the Avian Nest During Incubation

Interest in the gaseous microclimate of the avian nest duringincubition has greatly intensified in recent years because ofthe potentially important effects of micro climatic factorsupon embryonic respiration and hydration. Few data are availabledescribing levels of carbon dioxide and oxygen surrounding theeggs. What information is available, however suggests that inmost species levels of CO₂ and O₂ may not differ importantlyfrom that of the general atmosphere. The most likely exceptionsto this generalization are tunnel nesting birds. Data quantifyingwater vapor in the nest are more abundant. Nest humidity variessubstantially between species, but there are no apparent correlationsbetween nest humidity and breeding habitit or ambient humidity.Based upon limited empirical data and simulations using a deterministicmodel it appears that adult regulation of egg dehydration byshort-term modificttions of nest humidity may be neither necessarynor effective.     

Egg ejection cost can limit defence strategies against brood parasitism

A cost associated with the evolution of antiparasite strategies is the failure to recognize parasitic eggs, leading the host to evict its own eggs. However, there is evidence that birds recognize their own eggs through imprinting. This leads to the question of why birds accept parasitic eggs if such eggs can be identified. Here, we tested whether egg ejection per se can be costly due to increased predation risk to the remaining clutch and whether olfactory or visual cues of egg ejection increase predation. We carried out three field experiments to answer the following questions: (a) Does ejecting an egg increase nest predation risk? (b) Does the presence of olfactory cues, such as the smell of a broken egg, increase nest predation risk? And (c) Does the presence of visual cues, such as an egg shell below the nest, increase nest predation risk? We found evidence that egg ejection increases nest predation and that olfactory cues alone also increase nest predation. The presence of visual cues did not change predation rates. These data indicate that egg ejection is costly for both host and parasitic eggs that may remain in the nest. Our results suggest why host and parasite eggs are commonly found within the same nests, despite the possibility that hosts recognize and could possibly eject the parasite’s egg.     

Relationship of weight loss to ambient humidity of birds eggs during incubation

Summary Eggs of birds nesting in wet and dry habitats, have been artificially incubated at controlled humidity white weight loss of the eggs and shell water vapour conductance have been determined. Eggs of species from wet habitats loose weight at a higher rate than those from drier habitats at a given relative humidity.It is suggested that the conductance of the egg shell to water vapour is adapted to the conditions of humidity in the environment such that weight loss varies little (and less than predictable) in relation to the relative humidity at the nesting sites.The relative humidity surrounding eggs during natural incubation was found to be in the range of 30–50% in 4 different species. Humidity in the nest during natural incubation was found to be higher than what would result if ambient air was heated to incubation temperature indicating that the sitting bird conserves humidity around the eggs.     

Nocturnal incubation recess and flushing behavior by duck hens

Incubating birds must balance the needs of their developing embryos with their own physiological needs, and many birds accomplish this by taking periodic breaks from incubation. Mallard (Anas platyrhynchos) and gadwall (Mareca strepera) hens typically take incubation recesses in the early morning and late afternoon, but recesses can also take place at night. We examined nocturnal incubation recess behavior for mallard and gadwall hens nesting in Suisun Marsh, California, USA, using iButton temperature dataloggers and continuous video monitoring at nests. Fourteen percent of all detected incubation recesses (N = 13,708) were nocturnal and took place on 20% of nest‐days (N = 8,668). Video monitoring showed that hens covered their eggs with down feathers when they initiated a nocturnal recess themselves as they would a diurnal recess, but they left the eggs uncovered in 94% of the nocturnal recesses in which predators appeared at nests. Thus, determining whether or not eggs were left uncovered during a recess can provide strong indication whether the recess was initiated by the hen (eggs covered) or a predator (eggs uncovered). Because nest temperature decreased more rapidly when eggs were left uncovered versus covered, we were able to characterize eggs during nocturnal incubation recesses as covered or uncovered using nest temperature data. Overall, we predicted that 75% of nocturnal recesses were hen‐initiated recesses (eggs covered) whereas 25% of nocturnal recesses were predator‐initiated recesses (eggs uncovered). Of the predator‐initiated nocturnal recesses, 56% were accompanied by evidence of depredation at the nest during the subsequent nest monitoring visit. Hen‐initiated nocturnal recesses began later in the night (closer to morning) and were shorter than predator‐initiated nocturnal recesses. Our results indicate that nocturnal incubation recesses occur regularly (14% of all recesses) and, similar to diurnal recesses, most nocturnal recesses (75%) are initiated by the hen rather than an approaching predator.     

Nest attendance by tropical and temperate passerine birds: Same constancy,different strategy

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