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.     

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.     

Oxygen Transport in the Avian Egg at High Altitude

Oxygen transport to avian embryo tissues occurs by three steps,two of which are driven by diffusion. This results in a seriesof stepwise decrements in PO₂ between atmosphere and tissue.The PO₂ decrements for embryos of the domestic fowl incubatedat different altitudes are used here to examine potential adaptationsto hypobanc hypoxia. With exposure to moderate hypoxia embryosof the domestic fowl appear to maintain adequate tissue oxygenation.Adaptive adjustments in the shell, shell membranes and chorioallantoiscomplex were not observed. However, hemoglobin O₂ affinity wasincreased and preliminary evidence suggests a redistributionof blood flow to maintain adequate oxygenation in higher priorityareas of embryonic tissue. At severe hypoxia, embryos of thedomestic fowl show decreased O₂ consumption, embryo mass andlengthened incubition period. Thus at severe hypoxin the embryoof the domestic fowl does not appear to provide a realisticmodel. Evidence from avian embryos of species native to highaltitude suggest that they are able to maintain adequate tissueoxygenation even at severe hypoxia. Preliminary evidence suggeststhat some of the blood vascular system and tissue level adaptationspresent in the chicken embryo are also present in species nativeto high altitude. One of these, an increase in embryonic hemoglobin-O₂affinity which is physiologically mediated in the chicken embryois genetically-based in the embryo of the native high-altitudespecies.     

Avian Incubation Patterns Reflect Temporal Changes in Developing Clutches

Incubation conditions for eggs influence offspring quality and reproductive success. One way in which parents regulate brooding conditions is by balancing the thermal requirements of embryos with time spent away from the nest for self-maintenance. Age related changes in embryo thermal tolerance would thus be expected to shape parental incubation behavior. We use data from unmanipulated Black-capped Chickadee (Poecile atricapillus) nests to examine the temporal dynamics of incubation, testing the prediction that increased heat flux from eggs as embryos age influences female incubation behavior and/or physiology to minimize temperature fluctuations. We found that the rate of heat loss from eggs increased with embryo age. Females responded to increased egg cooling rates by altering incubation rhythms (more frequent, shorter on- and off- bouts), but not brood patch temperature. Consequently, as embryos aged, females were able to increase mean egg temperature and decrease variation in temperature. Our findings highlight the need to view full incubation as more than a static rhythm; rather, it is a temporally dynamic and finely adjustable parental behavior. Furthermore, from a methodological perspective, intra- and inter-specific comparisons of incubation rhythms and average egg temperatures should control for the stage of incubation.     

鸟类卵色的进化

鸟类学家提出了多种假说用于解释鸟类卵色的多态性,然而,不同学者的观点和研究结果不尽相同。本文从捕食者、警戒色、太阳辐射和巢寄生角度综述了各种假说和研究,探讨鸟类卵色的进化。     

Egg Selenium Concentrations as Predictors of Avian Toxicity

Aquatic birds are exposed to selenium through their diet by ingesting aquatic invertebrates that have accumulated selenium from water and the food chain. However, dietary composition is highly variable among species, over time, and across sites, making it difficult to provide accurate estimations of dietary exposure for particular species at specific locations. Selenium accumulates in the egg, resulting in embryo malformation, embryonic death, and decreased survival of juveniles. If the relationship between egg concentration and these reproductive parameters can be defined with sufficient certainty, then risk assessments can be performed through analysis of egg selenium concentrations. Other researchers have proposed egg toxicity thresholds that lead to conclusions of widespread selenium toxicoses in waterbirds. However, we believe these values are overly conservative and that it is unlikely that selenium is posing a significant risk to wild birds in areas where the current water quality criterion is being met. Through the use of simple statistical models (logit, probit, and Weibull functions) we are able to express mortality and teratogenicity relationships for mallards (Anas platyrhynchos) in such a manner that the risk manager can be presented with information about the probability of reduced duckling survival if mean egg selenium (MES) concentrations are known. Data analysis indicates that the two endpoints (mortality and teratogenesis) cannot be distinguished statistically. Commonly used effects thresholds (the EC₁₀ and EC₂₀) correspond to 16 and 21?mg/kg dw MES using the most sensitive endpoint, chick mortality. Both of these values are higher than the 6?mg/kg dw level proposed by Skorupa (1998) who based his estimate on field-observational data potentially confounded by other environmental stress factors. The mortality and teratogenicity endpoints presented here relate selenium exposure to risk to individuals within a population and do not provide information about the probability of selenium causing changes in population growth rates, either at the local or regional levels.     

Avian Incubation Inhibits Growth and Diversification of Bacterial Assemblages on Eggs

Microbial infection is a critical source of mortality for early life stages of oviparous vertebrates, but parental defenses against infection are less well known. Avian incubation has been hypothesized to reduce the risk of trans-shell infection by limiting microbial growth of pathogenic bacteria on eggshells, while enhancing growth of commensal or beneficial bacteria that inhibit or competitively exclude pathogens. We tested this hypothesis by comparing bacterial assemblages on naturally incubated and experimentally unincubated eggs at laying and late incubation using a universal 16S rRNA microarray containing probes for over 8000 bacterial taxa. Before treatment, bacterial assemblages on individual eggs from both treatment groups were dissimilar to one another, as measured by clustering in non-metric dimensional scaling (NMDS) ordination space. After treatment, assemblages of unincubated eggs were similar to one another, but those of incubated eggs were not. Furthermore, assemblages of unincubated eggs were characterized by high abundance of six indicator species while incubated eggs had no indicator species. Bacterial taxon richness remained static on incubated eggs, but increased significantly on unincubated eggs, especially in several families of Gram-negative bacteria. The relative abundance of individual bacterial taxa did not change on incubated eggs, but that of 82 bacterial taxa, including some known to infect the interior of eggs, increased on unincubated eggs. Thus, incubation inhibits all of the relatively few bacteria that grow on eggshells, and does not appear to promote growth of any bacteria.     

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)     

Egg Turning During Incubation has no Effect Upon the Growth of Embryos of Alligator mississippiensis

Alligator eggs are not turned during incubation, instead the embryo adheres to the top inside of the shell. Turning is alleged to shear off the embryo and kill it. Avian egg turning allegedly facilitates embryonic development by stimulating growth of the area vasculosa and minimizing the effects of unstirred yolk and albumen layers. From day 10 to day 45 of incubation, alligator eggs were experimentally turned, gently, through ± 60° in an hourly cycle. This turning regime killed only 6 out of 25 embryos. Compared with unturned controls, no significant effects were observed on the growth, production of extraembryonic fluids or utilization of albumen and yolk for those embryos that survived turning. The protein concentration of amniotic fluid at various stages of alligator development was examined in eggs incubated at 30 and 33°C. The fluid contained very little protein (max <8 mg) at any time: the protein concentration did not change consistently as development progressed. Differences in response to egg turning in birds and reptiles may be associated with the length of the incubation period, the protein content of the albumen and the mechanism of albumen utilization.     

Water and Ionic Fluxes Inside the Egg

The water/water and water/gas interfaces of the developing embryoare described. During incubation three main compartments arepresent the subgerminal amniotic and allantoic fluids. The formationcomposition and regulation of these solutions are consideredas examples of the water/water interfaces. Evaporative waterloss across the eggshell is considered as an example of a water/gasinterface. The effects of this phenomenon are considered ashaving been oversimplified in the past because they ignore phasechanges (condensing of water vapour) and the ability of theembryo to regulate its development according to the suppliesavailable.     

Experimental Shifts in Intraclutch Egg Color Variation Do Not Affect Egg Rejection in a Host of a Non-Egg-Mimetic Avian Brood Parasite

Avian brood parasites lay their eggs in the nests of other birds, and impose the costs associated with rearing parasitic young onto these hosts. Many hosts of brood parasites defend against parasitism by removing foreign eggs from the nest. In systems where parasitic eggs mimic host eggs in coloration and patterning, extensive intraclutch variation in egg appearances may impair the host’s ability to recognize and reject parasitic eggs, but experimental investigation of this effect has produced conflicting results. The cognitive mechanism by which hosts recognize parasitic eggs may vary across brood parasite hosts, and this may explain variation in experimental outcome across studies investigating egg rejection in hosts of egg-mimicking brood parasites. In contrast, for hosts of non-egg-mimetic parasites, intraclutch egg color variation is not predicted to co-vary with foreign egg rejection, irrespective of cognitive mechanism. Here we tested for effects of intraclutch egg color variation in a host of nonmimetic brood parasite by manipulating egg color in American robins (Turdus migratorius), hosts of brown-headed cowbirds (Molothrus ater). We recorded robins’ behavioral responses to simulated cowbird parasitism in nests where color variation was artificially enhanced or reduced. We also quantified egg color variation within and between unmanipulated robin clutches as perceived by robins themselves using spectrophotometric measures and avian visual modeling. In unmanipulated nests, egg color varied more between than within robin clutches. As predicted, however, manipulation of color variation did not affect rejection rates. Overall, our results best support the scenario wherein egg rejection is the outcome of selective pressure by a nonmimetic brood parasite, because robins are efficient rejecters of foreign eggs, irrespective of the color variation within their own clutch.     

蓝尾石龙子孵化期卵内物质和能量的利用

研究了安徽滁州蓝尾石龙子卵在温、湿度分别为 3 0℃、-1 2kPa的孵化条件下 ,胚胎对卵内物质和能量的利用。本研究蓝尾石龙子卵孵化期为 2 1 1d ,卵孵化时从基质中吸水导致重量增加。卵孵化过程中 ,干重、非极性脂肪和能量转化到孵出幼体的比率分别为 5 7 1 %、3 8 8%和 5 2 3 %。新生卵卵壳中5 0 %的灰分、5 9%的钙转移至孵出幼体中 ,胚胎发育所需要的无机物来自卵黄和卵壳。     

Incubation pattern and foraging effort in the female Water Pipit Anthus spinoletta

Incubation and foraging patterns of female Water Pipits Anthus spinoletta were studied in two breeding seasons in an Alpine valley of Switzerland. Decreased temperature reduced the length of periods spent off the nest (inattentive period), while decreased food availability led to a reduction in time spent incubating (attentiveness), shorter periods spent on the nest (attentive period), more frequent inattentive periods and higher foraging effort, measured as the product of frequency of inattentive periods and twice the flight distance to foraging sites. The negative relation between food availability and foraging effort resulted from more frequent foraging bouts and longer flight distances under poor food conditions. Feeding of the incubating female by the male did not affect foraging effort and attentiveness but did change the temporal pattern of inattentive periods from a few long to several short inattentive periods.     

Successes and Failures in Avian Egg and Nestling Recognition With Comments on the Utility of Optimality Reasoning

SYNOPSIS. Interactions between parasitic birds and their hostsprovide some of the best examples of recognition that has obviousand strong adaptive value. But these interactions also involvenumerous failures of recognition that result in enormous lossesin fitness. The failures are more significant than the successesbecause they are exceptions to basic evolutionary theory whichpredicts that organisms behave optimally. The egg recognitionbehavior of North American passerines separates species intotwo discrete groups. In "rejecter" species nearly all individualsare intolerant of nonmimetic parasitic eggs. In "accepters"nearly all individuals tolerate such eggs. Rejecters possessa true recognition of their own eggs, apparently learning thisin an imprinting-like process. No adaptive explanation accountsfor the lack of rejection in accepter species. Many of thesespecies are victimized heavily by parasitic cowbirds and haveeggs divergent from those of the cowbird. They readily acceptclutches containing only cowbird eggs. But accepters do showegg recognition in some contexts. They abandon clutches whosetotal volume of eggs falls below a critical value; they ejectand visually discriminate among eggs that are broken, thus showingthat they are capable of the critical behaviors needed to rejectparasitic eggs. Since most accepters occur in taxa that alsocontain rejecters, I propose that stochastic processes are largelyresponsible for their nonoptimal behavior. Even though manybirds reject parasitic eggs, all or nearly all accept nonmimeticparasitic nestlings, a situation for which there is no reasonableoptimality explanation. Although these recognition studies demonstratethe value of optimality reasoning because they are based onclearly defined predictions arising from the expectation ofadaptiveness, they also demonstrate some general problems inherentin optimization models. It is argued here that while optimalityreasoning is the best and most all-inclusive paradigm in biology,its value will be enhanced if its limitations are recognized.When warranted, conclusions of nonoptimality should be stressedand optimality itself should be tested and not employed onlyas an assumption used to formulate hypotheses.     

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.     

Avian Egg Odour Encodes Information on Embryo Sex,Fertility and Development

Avian chemical communication is a rapidly emerging field, but has been hampered by a critical lack of information on volatile chemicals that communicate ecologically relevant information (semiochemicals). A possible, but as yet unexplored, function of olfaction and chemical communication in birds is in parent-embryo and embryo-embryo communication. Communication between parents and developing embryos may act to mediate parental behaviour, while communication between embryos can control the synchronicity of hatching. Embryonic vocalisations and vibrations have been implicated as a means of communication during the later stages of development but in the early stages, before embryos are capable of independent movement and vocalisation, this is not possible. Here we show that volatiles emitted from developing eggs of Japanese quail (Coturnix japonica) convey information on egg fertility, along with the sex and developmental status of the embryo. Specifically, egg volatiles changed over the course of incubation, differed between fertile and infertile eggs, and were predictive of embryo sex as early as day 1 of incubation. Egg odours therefore have the potential to facilitate parent-embryo and embryo-embryo interactions by allowing the assessment of key measures of embryonic development long before this is possible through other modalities. It also opens up the intriguing possibility that parents may be able to glean further relevant information from egg volatiles, such as the health, viability and heritage of embryos. By determining information conveyed by egg-derived volatiles, we hope to stimulate further investigation into the ecological role of egg odours.