其实只是鸟类普遍性的，以及一些特殊的水禽的，当然也不会那么璀璨或鲜红
一个非常美的鸟站
BIO 554/754
Ornithology[size=+1]Avian Reproduction: Anatomy &the Bird Egg
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Reproductive Anatomy:
- Gonads - paired testes in males & usually a single ovary in females
- Ovary
- most birds have only left ovary but 2 ovaries are typical of many raptors
- contains from 500 to several thousand primary oocytes
- Testes & follicles increase dramatically in size as the breeding seasonapproaches.
- As day length increases, photic stimulation of the hypothalamus resultsin the secretion of Gonadotropin releasing hormone (GnRH below). When activatedby GnRH, the anterior pituitary secretes two gonadotropin hormones, follicle-stimulatinghormone (FSH) and luteinizing hormone (LH). FSH acts on sperm-producingstructures in the testes, while LH acts on the interstitial cells of the testescausing them to secrete the steroid hormone testosterone. The pituitarygland monitors the amount of testosterone in the blood, thus creating anegative feedback loop to maintain hormone levels within a set range (Akinsand Burns 2001).
- Ambient visual cues, such as daylight, activate photosensitive loci inthe brain both indirectly, through the eyes, and directly, through theskull. The hypothalamus of the bird brain contains special cells that aresensitive to extremely low light levels, intensities comparable to theamount of light that can penetrate brain tissue (Akins and Burns 2001).
[size=-2]From: Akins and Burns (2001)
The pattern of testosterone secretion in free-living populations of Song Sparrows.
Plasma levels peak in April and May as breeding got underway and thenwere maintained at a lower “breeding baseline” during the
rest of the breeding season. As prebasic molt ensued, plasma levels oftestosterone were basal and remained so throughout autumn and winter.
From: Wingfield and Hahn (1994).
Biological actions of the steroid hormone testosterone. The morphological, physiological and behavioral actions of testosterone that are essential
for male reproductive function are given on the right hand and lowersides of the figure. The “costs” of prolonged high levels oftestosterone are given on the
left hand side in italics. The patterns of plasma testosterone levelsmay be a function of secretion patterns to maintain male reproductivefunction, and “costs”
of testosterone that require that plasma levels be low. From Wingfield et al. (2000).
| Testosterone increases availability of carotenoids-- Androgens and carotenoids play a fundamental role in the expressionof secondary sex traits in animals that communicate information onindividual quality. In birds, androgens regulate song, aggression, anda variety of sexual ornaments and displays, whereas carotenoids areresponsible for the red, yellow, and orange colors of the integument.Parallel, but independent, research lines suggest that the evolutionarystability of each signaling system stems from tradeoffs with immunefunction: androgens can be immunosuppressive, and carotenoids divertedto coloration prevent their use as immunostimulants. Despite strongsimilarities in the patterns of sex, age and seasonal variation, socialfunction, and proximate control, there has been little success atintegrating potential links between the two signaling systems. Theseparallel patterns led us to hypothesize that testosterone increases thebioavailability of circulating carotenoids. To test this hypothesis, Blas et al. (2006) manipulated testosterone levels of Red-legged Partridges (Alectoris rufa)while monitoring carotenoids, color, and immune function. Testosteronetreatment increased the concentration of carotenoids in plasma andliver by >20%. Plasma carotenoids were in turn responsible forindividual differences in coloration and immune response. These resultsprovide experimental evidence for a link between testosterone levelsand immunoenhancing carotenoids that (i) reconciles conflicting evidence for the immunosuppressive nature of androgens, (ii) provides physiological grounds for a connection between two of the main signaling systems in animals, (iii) explains how these signaling systems can be evolutionary stable and honest, and (iv) may explain the high prevalence of sexual dimorphism in carotenoid-based coloration in animals. | 
Red-legged Partridge (Photo by G. Bortolotti)
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Sperm production
- occurs in seminiferous tubules of the testes (shown below)
- occurs best at slightly cooler temperatures, so spermatogenesis may occurprimarily at night when body temperatures are slightly lower
[size=-1]Light photomicrograph of a section of a testis showinga seminiferous tubule during full
[size=-1]semen production. SG indicates spermatogonia; PS, primaryspermatocyte; Ss, secondary spermatocyte;
[size=-1]MS, mature spermatocyte; and L, lumen (original magnification×800) (Samour 2002).
- sperm are stored at the terminal end of the vas deferens (seminal glomus),and this creates a swelling called the cloacal protuberance
Male birds have paired abdominal testes lying cranioventralto the first kidney lobe. Testes increase dramatically in size during thebreeding season. The vas deferens emerges medially and passes caudallyto the cloaca where it has a common opening with the ureter in the Urodeum.The terminal vas deferens is swollen as a storage organ: the seminal glomus(or seminal vesicle as in the drawing to the right). As in mammals, sperm formation is temperature sensitive,and maturation is assisted by nocturnal drops in temperature, or by thedevelopment of scrotal-like external thermoregulatory swellings holdingthe seminal glomera.
In addition, male birds tend to have relatively low extragonadalsperm reserves and sperm are ejaculated soon after production in the testes.
|  [size=-2]Source: http://wwwvet.murdoch.edu.au/Anatomy/avian/fig5.1.GIF |
Cloacal protuberance | 
[size=-1]Longitudinal section of the cloaca of a male budgerigarduring the
culmination phase of the breeding cycle. SG indicates seminalglomus;
P, proctodeum; and C, cloaca (original magnification ×12)(Samour2002). |
[size=-1]Photo source: http://www.wtamu.edu/~rmatlack/pigeon_dissection/male_reproductive.jpg
Sperm competition and testes size -- Comparative analyses suggest that a variety of ecological and behavioural factors contribute to the
tremendous variability in extrapair mating among birds. In an analysisof 1010 species of birds, Pitcher et al. (2005) examined severalecological
and behavioural factors in relation to testes size; an index of spermcompetition and the extent of extrapair mating. In univariate andmultivariate
analyses, testes size was significantly larger in species that breedcolonially than in species that breed solitarily, suggesting thathigher breeding
density is associated with greater sperm competition. After controllingfor phylogenetic effects and other ecological variables, testes sizewas also
larger in taxa that did not participate in feeding their offspring. Inanalyses of both the raw species data and phylogenetically independentcontrasts,
monogamous taxa had smaller testes than taxa with multiple socialmates, and testes size tended to increase with clutch size, whichsuggests that
sperm depletion may play a role in the evolution of testes size. Theseresults suggest that traditional ecological and behavioural variables,such as
social mating system, breeding density and male parental care canaccount for a significant portion of the variation in sperm competitionin birds.
| Testis size increases with colony size in Cliff Swallows-- By using a sample of over 800 male Cliff Swallows (Petrochelidonpyrrhonota) that died during a rare climatic event in their Nebraskastudy area in 1996, Brown and Brown (2003) investigated how testis sizewas related to body size, age, parasite load, and a bird's past colony-sizehistory. Testis volume increased with body size. After correcting for bodysize, testis volume was lowest for birds age 1 and 2 years but did notvary with age for males 3 years old or more. Birds occupying parasite-free(fumigated) colonies had significantly larger testes than did birds atnonfumigated sites. Testis volume increased significantly with the sizeof the breeding colonies a bird had used in the past. These results showwithin a species that larger testes are favored in more social environments,probably reflecting a response to increased rates of extrapair copulation(and thus sperm competition) among Cliff Swallows in large colonies. Thepresence of ectoparasites, by inflating levels of plasma corticosterone,may in turn reduce testis mass. These data provide no support for the hypothesisthat large testes, perhaps by producing more testosterone, are immunosuppressiveand thus costly for that reason. | |
Cliff Swallows
Great Tit provisioning nestlings [size=-1]
(Source: http://www.nature.com/)
Temperature and the timing of reproduction -- Manybird species reproduce earlier in years with high spring temperatures,but little is known about the causal effect of temperature. Temperaturemay have a direct effect on timing of reproduction, but the correlationmay also be indirect, for instance via food phenology. As climatechange has led to substantial shifts in timing, it is essential tounderstand this causal relationship to predict future impacts ofclimate change.
Visser et al. (2009) tested the direct effect of temperature on laying dates in Great Tits (
Parus major)using climatized aviaries in a 6-year experiment. Temperature patternsfrom two specific years in which the wild population laid either early(‘warm’ treatment) or late (‘cold’ treatment) were mimicked. Layingdates were affected by temperature directly. Because the relevanttemperature period started three weeks prior to the mean laying date,with a range of just 4°C between the warm and the cold treatments, andbecause the birds were fed ad libitum, it is likely that temperatureacted as a cue rather than lifting an energetic constraint on the onsetof egg production. Visser et al. (2009) also found a high correlationbetween the laying dates of individuals reproducing both in aviariesand in the wild, validating investigations of reproduction of wildbirds in captivity. These results demonstrate that temperature has adirect effect on timing of breeding, an important step towardsassessing the implication of climate change on seasonal timing.
Egg production
- Most birds have only one ovary and one oviduct. In early stages of embryonicdevelopment, each female bird has two ovaries; only the left one developsinto a functional organ. In some birds, such as hawks, the right ovaryand oviduct usually develop. A mature ovary looks like a cluster of grapes.and may contain up to 4,000 small ova which can develop into mature ova.
- With fertilization, the ovum(egg) becomes a developing embryo
- Theembryo passes through the oviduct; typically takes about 24 hours (forpasserines & most other birds)
- The demand for calcium to make the egg shell is very high, and so the circulatinglevels of blood calcium in birds are greatly elevated compared to mammals,typically twice as much.
[size=-1] In most birds, only the left ovary and oviductpersist. The ovary enlarges greatly during the breeding season. Activeovaries resemble bunches of tiny grapes -- the developing follicles. Theoviduct opens medially to it in a funnel-shaped ostium. Ovulation resultsin the release of an egg from a mature follicle on the surface of the ovary.The egg, with extensive food reserves in the form of concentric layersof yolk, is pickedup by the ostium and ciliary currents carry it into the magnumregion. Over about three hours the egg receives a coating of albumen.
The egg then passes into the isthmus,where the shell membranes are deposited. This takes about one hour. Theegg them moves to the uterus, or shell gland, where the calcareousshell is added and, in some birds, pigment is added in characteristic patterns.The egg then passes into the vagina and cloaca for laying. |  [size=-2]Source: www.wisc.edu/ansci_repro/lec/lec1/female_hist.html |
Variation among bird species in the relative amount of yolk in eggs andthe amount of energy available to the developing embryo (kJ-g -1, orkilojoules per gram). From top to bottom, the hatchlings are analtricial Brown Creeper, a semiprecocial Least Tern, a precocial RuddyDuck, a superprecocial Mallee Fowl (
Leipoa ocellata), and a Brown Kiwi (
Apteryx australis).Kiwis are ‘outliers.’ Female kiwis produce extremely large eggs fortheir size (with substantial amounts of yolk), but young typicallyremain in the nest for several days and so are best classified assemiprecocial (From: Sotherland and Rahn 1987).
Kiwi lays an egg
[size=-1]The left egg found inside the female oviraptorosaurian.The
texture of the shell pieces probably resembles the original
textureof the egg. Credit: Yen-nien Cheng | Eggs discovered inside dinosaur -- The discoveryof eggs inside a dinosaur has provided new clues about dinosaur reproductivebiology and more support for the hypothesis that birds evolved from dinosaurs.The pair of eggs are the first found inside a dinosaur. Sato et al. (2005)found that the dinosaur produced eggs in some ways like a crocodile andin other ways like a bird. Crocodiles have two ovaries enabling them tolay a clutch of eggs. Birds have a single ovary and lay only one egg ata time. The dinosaur's egg-producing capability lay somewhere in between,suggesting a possible link with modern birds. It had two ovaries, but produceonly one egg at a time from each ovary.
Sato et al. (2005) studied a dinosaurfrom a group of dinosaurs called oviraptorosaurians. This type of dinosaur— probably 3 - 4 meters long — is a subgroup of the theropods. The dinosaurwas excavated in China. The similar size of the eggs suggests the creature'stwo oviducts each produced a single egg at the same time. |
Dinosaur - video powered by Metacafe
Dinosaur egg
[size=-1]Femalebirds can bias the sex of their chicks.-- Whether a bird is morelikely to lay a male or female egg depends on which sex will have the greatestchance of doing well. Rutstein et al. (2004) adjusted the food intake offemale Zebra Finches [see photo of female (left) and male (right) ZebraFinches below right] & found that well-fed females were more likelyto produce daughters, while less well nourished birds were more likelyto have sons. This is exactly as predicted by the fact that female offspringneed to be better nourished than males if they are to survive and growwell. 
[size=-1] The authors noted that:“In most animals sex ratio is close to 50:50 and extremely resistant tochange. In mammals, including humans, the sex of the baby is determinedby whether the sex chromosome in the sperm is male or female. But in birds,it is the female’s egg rather than the male’s sperm that determines whatsex the chick will be. Thus the female has the potential to determine thesex of her young by whether she ovulates male or female eggs. In some way,female Zebra Finches seem to be able to exert control over whether to producea male or female egg depending on which of the two is most likely to besuccessful. Our research tells us that they do it, and we understand why.The big question is: how do they do it?”
[size=-1] In many animals, femalesneed to be well-nourished and in good condition if they are to breed, aseggs are costly to produce. Bigger eggs tend to lead to bigger young thatare more likely to survive. Such ‘sex ratio adjustment’ is well documentedin certain insects, such as bees and wasps, but is less well understoodin birds and mammals.
[size=-1] Birds are an excellentmodel to use in the study of sex ratio adjustment because, using moleculartechniques, scientists can establish the sex of each egg soon after laying.Further, all the resources given to the developing embryo are present inthe egg at laying. Thus the size and the content of the egg are measuresof the amount of resources that the female has allocated to that egg, whichaffects its subsequent survival chances.
[size=-1] The authors explained: “We manipulatedthe diet quality of Zebra Finches to look at the effects of body conditionon female investment. We found that females were able to exert a strongdegree of control over the production of male and female eggs. When femaleswere fed on a low quality diet, they laid eggs that were considerably lighterthan those laid when they were fed on a high quality diet, and they alsolaid far more male eggs on a low quality diet. This is the converse situationto that described 30 years ago for mammals, but it makes sense for ZebraFinches. Previous research has shown that under poor nutritional conditions,female Zebra Finches grow more slowly and survive less well compared tomales. Therefore, females are producing more of the sex with the highestsurvival chances under those conditions.” |
[size=-1]Two potential mechanisms for
[size=-1]determination among birds. (A) the
[size=-1]presence of the W chromosome
[size=-1]triggers femaleness or (B) the
[size=-1]presence of two Z chromosomes
[size=-1]confers maleness. | [size=-1]Avian sex determination (Ellegren 2001) -- Themolecular determinants behind sexual development in birds remain a mystery.The process is known to be different from that in mammals, with no homologto the gene that confers maleness in mammals found in birds. The failureto identify such a gene in birds is probably a reflection of the fact that,despite the occurrence of two sexes being nearly universal throughout theanimal kingdom, the genes involved seem virtually unrelated among metazoanphyla. These differences raise obstacles for comparative or candidate geneapproaches in studies of sexual development.
[size=-1] In birds, females arethe heterogametic sex, with one copy each of the Z and W sex chromosomes.Males are homogametic (ZZ). However, it is not clear whether it is thepresence of the female-specific W chromosome that triggers female development,or the dose of Z chromosome that confers maleness. An intriguingadditional possibility is that both Z and W matter! In marsupials, forexample, Y acts as a dominant testis determining chromosome, while theX chromosome determines the choice between pouch and scrotum. Maybe a systemwhere the two sex chromosomes mediate different aspects of sex differentiationis also used in birds. |
[size=-1]
Vertebrate sex determination systems. Phylogeny of major vertebrate clades showing the sex determining systems
found in members of the respective clade. ‘Multiple’ indicatesinvolvement of more than one pair of chromosomes in sex determination.
TSD: temperature-dependent sex determination (From: Ezaz 2006).
Incubation temperature and avian sex ratios -- Although common in reptiles, incubation temperature has not been considered to be a factor in determining sex ratios
inbirds. However, Goth and Booth (2005) found that incubation temperaturedoes affect sex ratios in megapodes, which are exceptional among birdsbecause they use environmental heat sources for incubation. In the
Australian Brush-turkey (Alectura lathami),a mound-building megapode, more males hatch at low incubationtemperatures and more females hatch at high temperatures, whereas theproportion is 1:1 at the average temperature found in natural mounds.Chicks from lower temperatures weigh less, which probably affectsoffspring survival, but are not smaller. Megapodes possessheteromorphic sex chromosomes like other birds, which eliminatestemperature-dependent sex determination, as described for reptiles, asthe mechanism behind the skewed sex ratios at high and lowtemperatures. Instead, Goth and Booth (2005) suggest a sex -biasedtemperature-sensitive embryo mortality because mortality was greater atthe lower and higher temperatures, and minimal at the middletemperature where the sex ratio was 1:1.
Copulation & fertilization:
- For most birds, copulation involves a 'cloacal kiss', with the male onthe female's back & twisting his tail under the female's
- copulation typically lasts just a few seconds (but there are exceptions - see [size=-1]Phony phallus puts sperm ahead in bird first below)
Bald Eagles mating
White-throated Kingfishers mating
| Phonyphallus puts sperm ahead in bird first-- "These birds wouldbe at it for 10-20 minutes," said co-author Tim Birkhead of the Red-billedBuffalo Weaver. Males use their organ to rub females and improve theirsperm's chance of success. Few male birds have a phallus; most achievefertilization via a cloacal kiss. So 19th-century reports of a mock memberin the Buffalo Weaver sent Winterbotton et al. (2001) to Namibia. Catchingthe birds in the act was tough, recounts Birkhead: "In 3 years we saw eightmatings." Pairs occasionally emerged from nests and flew to a nearby tree."I'd run after them, sweating profusely with my binoculars steaming up,"he says. The pair would start bouncing up and down - over numerous consecutivebouts. Compared to the 1-2 second tryst of most birds, their staying poweris unique. Yet, entry of the elusive organ was hard to make out. Even incaptivity "they performed beautifully," but the view was blocked, saysBirkhead. So they glued a piece of cardboard to an unlucky bird's member.This did not prevent mating, suggesting that the Buffalo Weaver organ isactually a weapon in sperm wars. By choosing a male who rubs longest orbest, females may be selecting top-quality sperm. Paternity testing revealedthat female Buffalo Weavers sire birds from multiple males, providing evidenceof sperm competition. Time spent courting must be shown to predict spermtransfer or success to really back up the idea. The 1.5-cm appendage lacksblood vessels and has a twisted furrow down its length. Males in communalnests have longer ones than those that live alone, showing that size isa factor in social success. But for males at least, the phallus is formore than foreplay. -- HelenPearson, Nature Science Update | |
- males in a few species, including most waterfowl & ostriches (see diagrambelow), have an intromittent organ; most males do not
Diagram of the left lateral view of a retracted and erectphallus of a male Emu or Rhea. The top drawing
represents the phallus withinthe pouch. A. vas deferens, B. urideum, C. proctodeum, D. pocket to containphallus,
E. erectile wall of phallus, F inverted hollow tube ofphallus, G. phallic sulcus, H. erectile tissue, and I. erect phallus
with blind hollow tube. (Source:
http://www.cassowary.com/workshop.html)
Examples of genital covariation in waterfowl.
(A) Harlequin Duck (
Histrionicus histrionicus) and (B) African Goose (
Anser cygnoides), two species with a short phallus and no forced copulations, in which females have simple vaginas. (C) Long-tailed Duck (
Clangula hyemalis), and (D) Mallard
(Anas platyrhynchos),two species with a long phallus and high levels of forced copulations,in which females have very elaborate vaginas (size bars = 2 cm). ] =Phallus, * = Testis, star = Muscular base of the male phallus, > =upper and lower limits of the vagina (From:
Brennan et al. 2007).
Eversion of a male Muscovy duck penis
Explosive eversion and functional morphology of the duck penis-- Coevolution of male and female genitalia in waterfowl has beenhypothesized to occur through sexual conflict. This hypothesis raisesquestions about the functional morphology of the waterfowl penis andthe mechanics of copulation in waterfowl. Brennan et al. (2010) usedhigh-speed video of phallus eversion and histology to describe for thefirst time the functional morphology of the avian penis. Eversion ofthe 20 cm muscovy duck penis is explosive, taking an average of 0.36sec, and achieving a maximum velocity of 1.6 m sec−1. Thecollagen matrix of the penis is very thin and not arranged in anaxial-orthogonal array, resulting in a penis that is flexible whenerect. To test the hypothesis that female genital novelties makeintromission difficult during forced copulations, Brennan et al. (2010)investigated penile eversion into glass tubes that presented differentmechanical challenges to eversion. Eversion occurred successfully in astraight tube and a counterclockwise spiral tube that matched thechirality of the waterfowl penis, but eversion was significantly lesssuccessful into glass tubes with a clockwise spiral or a 135° bend,which mimicked female vaginal geometry. These results support thehypothesis that duck vaginal complexity functions to exclude the penisduring forced copulations, and coevolved with the waterfowl penis viaantagonistic sexual conflict.
Near the junction of the vagina and shell gland of female birds aredeep glands lined with simple columnar epithelium. These are the spermstorage tubules, so called because they can store sperm for long periodsof time (10 days to 2 weeks). After an egg is laid, some of these spermmay move out of the tubules into the lumen of the tract, then migrate fartherup to fertilize another egg.
[size=-1]Avian sperm | 
[size=-1]Avian sperm storage tubules |
Photomicrographs of sperm storage tubules contained stained and unstained spermatozoa from domestic chicken
(Gallus domesticus)
hens (a, b) and turkey (
Meleagris gallopavo) hens (c, d). Arrows indicate stained spermatozoa; arrowheads designate unstained spermatozoa.
Scale bars = 25 micrometers. From: King et al. (2002).
King et al. (2002) found that spermatozoa from two differentinseminations (one with stained sperm, one with unstained sperm)generally
segregated into different storage tubules in bothchicken and turkey hens. Storage tubules contained mixed populations ofspermatozoa were
found in only 4% of chicken and 12% of turkey storage tubules examined.They concluded that the mechanism of last-male precedence does
not appear to be due to the stratification of spermatozoa within the tubules.
 | Innervation of sperm storage tubules (Freedmanet al. 2001) -- Immunohistochemical staining of a turkey uterovaginaljunction and sperm storage tubules. This micrograph shows a fluorescingneuron (green) near some sperm storage tubules (SST). The blue areas(se) are the surface epithelium lining the lumen of the uterovaginaljunction and the epithelium of the sperm storage tubules. The arrowpoints to a magenta-stained area of one SST that indicates the presenceof actin (a protein found in smooth muscle. The total image is 19micrometers across.
This association between neuronsand SSTs provides evidence that SSTs are innervated and suggests thatthe storage and release of sperm from SSTs can, perhaps, be controlled.
|
Post-insemination events(Birkhead and Brillard 2007)-- Most birds do not have a phallus and, in these species, inseminationoccurs via the so-called ‘cloacal kiss.’ Depending on taxa, sperm areejaculated into the cloaca or vagina and rely on their motility toreach the numerous sperm-storage tubules (SSTs) located at the junctionof the vagina and the uterus. As a consequence of selection duringtheir migration through the vagina, only 1–2% of inseminated spermenter the SSTs, the rest are probably ejected the next time that thefemale defecates. The SSTs contain only morphologically normal sperm,suggesting either that only normal sperm successfully traverse thevagina or that only normal sperm are ‘accepted’ by the SSTs. An unknownbut probably small proportion of sperm move directly to theinfundibulum (the site of fertilization) without entering the SSTs,although these are likely to fertilize only a single ovum.
Thatsperm in the SSTs are invariably positioned with their heads directedtowards the distal end of the tubule suggests that egress from the SSTsis passive. Sperm are lost from the SSTs more or less continuously at aconstant per capita rate. They enter the uterus and are carriedpassively to the infundibulum. Sperm accumulate or move relativelyslowly through the infundibulum so that there is usually a populationavailable to fertilize each ovum as it is ovulated. On ovulation, theovum is captured by the prehensile, funnel-shaped infundibulum and thesperm swarm over the surface of the ovum; their target is the germinaldisc, which contains the female pronucleus. At this stage, the ovum isbounded by the inner perivitelline layers (IPVL). The clustering ofsperm and holes made by sperm in the IPVL around the germinal discsuggest that sperm might use chemical signals to locate the germinaldisc.
Incontrast to most other taxa, where only a single sperm enters the ovum,polyspermy is typical in birds. Several sperm enter the germinal discregion, hydrolyzing the IPVL via the acrosome reaction of the sperm,whereby the release of enzymes from the sperm acrosome enables thesperm nucleus to enter the ovum. However, only a single spermatozoonfuses with the female pronucleus and the remaining sperm are shifted tothe periphery of the germinal disc and play no further part indevelopment. Fertilization includes the penetration of ovum by sperm aswell as the fusion of the male and female pronuclei (syngamy). Becauseembryo development begins almost immediately, many cell divisions haveoccurred by the time the ovum has become incorporated into the egg andthe egg is laid (in most species) 24 hr later.
[size=-1]Scanning electron photomicrograph of a
[size=-1]budgerigar spermatozoon. A indicates
[size=-1]acrosome; H, head; and T, tail (original
[size=-1]magnification ×20000) (Samour 2002). | 
[size=-1]Transmission electron photomicrograph of the longitudinal
section of part of the nucleus and midpiece of a Budgerigar
spermatozoon.N indicates nucleus; PC, proximal centriole;
DC, distal centriole; F, axialfilament complex; and
M, mitochondria (original magnification ×30000)(Samour 2002). |
Fertilization of the egg usually occurs in the infundibulum.
[size=-1]Light photomicrograph of a zona-free hamster ovum with
[size=-1]numerous budgerigar spermatozoa bound to its surface
[size=-1](original magnification ×215) (Samour 2002).
| [size=-1]Repelling clingy exes helps snipe save sperm --Writer Gore Vidal once said that he never passed up an opportunity to havesex or appear on television. Some male birds would disagree on at leastone count. Having mated with a female, a male Great Snipe (Gallinagomedia) will reject her further advances and even chase her away. MaleGreat Snipe form leks to eye up the talent before choosing a mate. A fewmales get the most sex. Popular birds can get more than half of the matings,perhaps 10 a day. Hence their pickiness, suggest Saether et al. (2001).As male Great Snipe take no part in caring for their offspring, it wasthought they had nothing to lose by mating as much as possible. But topmales, overburdened with potential partners, must share sperm with careand spread their favors around. Sperm budgeting is the only possible explanationfor male snipes' ungrateful behavior. Like a nightclub, Great Snipeleks see their share of aggravation. "All four kinds of mating conflictshappen" - male choice, female choice, and male and female competition -explains Saether. Males are more likely to repel clingy exes if there area lot of other females around. Females fight with one another, and malesfrom neighboring territories chase their rivals' females away. Hostilitytowards old flames might be a bid to maintain order. "If a male gets ridof an unwanted female it's one less problem to worry about," says Saether.Female snipe probably seek to mate again so that they can get enough spermto fertilize their eggs. Rejected females tend to lower their sights andsettle for less popular males. -- JohnWhitfield, Nature Science Update |  [size=-2]Photo by Saether et al. (2001)
[size=-2]www.nature.com/nsu/011018/011018-8.html |
TheAvian Egg
Birds' eggs, like the birds themselves, vary enormously in size. Thelargest egg from a living bird belongs to the ostrich. It is over
2000 times larger than the smallest egg produced by a hummingbird [size=-1](seephoto to the right; Source:
http://www.pma.edmonton.ab.ca/vexhibit/eggs/vexhome/sizeshap.htm).Ostrich eggs are about 180 mm long and 140 mm wide and weigh 1.2 kg. Hummingbirdeggs are 13 mm long and 8 mm wide and they weigh only half of a gram. Theextinct Elephant Bird from Madagascar produced an egg 7 times larger thanthat of the Ostrich! Within the egg, three extraembryonic membranes supportthe life & growth of the embryo:
- amnion
- surrounds only the embryo
- inner layer of cells secretes amniotic fluid in which the embryo floats;fluid keeps the embryo from drying out and protects it
- chorion - surrounds all embryonic structures & serves as a protectivemembrane
- allantois (or allantoic sac)
- grows larger as embryo grows, fuses with the chorion & is called thechorio-allantoic membrane
- works together with chorion to permit respiration (exchange of oxygen andcarbon dioxide) and excretion
- important in storage of nitrogenous wastes (uric acid)
Relative eggmass (corrected for adult mass) is greater in species with longerembryonic periods (days) among 64 passerine species in tropicalVenezuela, subtropical Argentina, and north temperate Arizona. Opensymbols reflect cavity-nesting species and show an interacting effectwhere their larger clutches are associated with relatively smaller eggs.
Egg size variation among tropical and temperate songbirds-- Species with “slow” life history strategies (long life, lowfecundity) are thought to produce high-quality offspring by investingin larger, but fewer, young. Larger eggs are indeed associated with fewer eggsacross taxa and can yield higher-quality offspring. Tropical passerinesappear to follow theory because they commonly exhibit slow life historystrategies and produce larger, but fewer, eggs compared with northern species. Martin (2008) found that relative eggmass (corrected for adult mass) varies extensively in the tropics andsubtropics for the same clutch size, and proposed a hypothesis toexplain egg size variation both within the tropics and between latitudes: Relative egg mass increases in species with cooler egg temperatures and longer embryonic periods to offset associated increases in energetic requirements of embryos. Eggtemperatures of birds are determined by parental incubation behaviorand are often cooler among tropical passerines because of reducedparental attentiveness of eggs. Cooler egg temperatures and longer embryonic periods explained the enigmatic variation in eggmass within and among regions, based on field studies in tropicalVenezuela (36 species), subtropical Argentina (16 species), and northtemperate Arizona (20 species). Alternative explanations were notsupported. Thus, large egg sizes may reflect compensation for increased energetic requirements of cool egg temperatures and long embryonic periods that result from reduced parental attentiveness in tropical birds.
| Egg composition and hatchling phenotype -- Parentalinvestment in eggs and, consequently, in offspring can profoundly influencethe phenotype, survival and evolutionary fitness of an organism. Avianeggs are excellent model systems to examine maternal allocation of energytranslated through egg size variation. Dzialowski1and Sotherland (2004)used the natural range in Emu (Dromaius novaehollandiae) egg size,from 400 g to >700 g, to examine the influence of maternal investment ineggs on the morphology and physiology of hatchlings. Female Emus provisionedlarger eggs with a greater absolute amount of energy, nutrients and waterin the yolk and albumen. Variation in maternal investment was reflectedin differences in hatchling size, which increased isometrically with eggsize. Egg size also influenced the physiology of developing Emu embryos,such that late-term embryonic metabolic rate was positively correlatedwith egg size and embryos developing in larger eggs consumed more yolkduring development. Large eggs produced hatchlings that were both heavier(yolk-free wet and dry mass) and structurally larger (tibiotarsus and culmenlengths) than hatchlings emerging from smaller eggs. As with many otherprecocial birds, larger hatchlings also contained more water, which wasreflected in a greater blood volume. Emu maternal investment in offspring,measured by egg size and composition, is significantly correlated withthe morphology and physiology of hatchlings and, in turn, may influencethe success of these organisms during the first days of the juvenile stage. | 
[size=-2]abc.net.au/schoolstv/animals/EMUS.htm |
[size=-1]Emu egg & embryo
[size=-1]
http://www.digimorph.org/specimens/Dromaius_novaehollandiae/egg/
- Eggs consists of 4 primary components:
- yolk
- energy-rich supply of food
- 21 - 36% lipids & 16 - 22% proteins (with the rest being water)
- the yolk is suspended in the center of the egg by twisted strands of proteinfibers called chalazae (shown below)
| Yolk contains maternal antibodies -- Antibodiesare deposited in eggs during yolk formation through the deposition of immunoglobulins,primarily IgY (also called IgG), in the yolk. In Chickens (Gallus domesticus),maternal IgY is catabolized by offspring over the first 14 days post-hatchingand, by about 5 days post-hatching, offspring begin to synthesize theirown IgY. As a result, after approximately two weeks the circulating IgYin young is principally of endogenous origin. Adult levels are attainedbetween six weeks and six months of age. However, maternal antibodies maycontinue to affect offspring phenotype even after they are catabolizedby influencing growth and developmental rates. In the absence of maternalIgY in chickens (due to surgical bursectomy of the mother during her ownembryogenesis), the number of cells in the spleen that help lymphocytes(helper T cells) attack antigens (foreign proteins on pathogens) is depressed.Also, the immune responsiveness of offspring is depressed, which couldlower the survival of offspring particularly in harsh disease environments(Grindstaff et al. 2003). | 
Antibodies 'attack' pathogens or toxins theyproduce by binding to antigens (e.g., proteinsin the membranes of bacteria) via their 'binding sites'(the black areas above). This binding can neutralize toxins and attractwhite blood cells that eliminate pathogens (by phagocytosis). |
Maternal secretion of antibodies and absorption by the young occur onlyprenatally in birds (with the exception of pigeon crop milk)
(From: Boulinier and Staszewski 2008).
 The familiar color of a chicken’s egg yolk (a) is in starkcontrast to the richly pigmented egg yolk of a lesser Black-backed Gull, Larusfuscus (b). Such high maternal investment of carotenoids into egg yolkis typical among wild bird species, suggesting that these biologicallyactive pigments serve important functions in the developing bird(From: Blount et al. 2000).
| Why egg yolk is yellow (or red) (Blount et al.2000) -- Egg yolk in birds is colored yellowish-red by carotenoids. Untilrecently, there has been no adaptive explanation of why many egg-layinganimals provision their eggs so richly with carotenoids. It now appearsthat, in developing birds, carotenoids protect vulnerable tissues againstdamage caused by free radicals. Athough embryonic tissues depend on oxidizable,unsaturated fatty acids in yolk, their abundance makes the tissues susceptibleto peroxidation caused by reactive oxidative metabolites and by free radicals,which are produced as normal by-products of metabolism. Protection againstlipid peroxidation in young birds is afforded by the actions of yolk-derivedcarotenoids and other antioxidants, like vitamin E. Antioxidants also protectpassively-acquired antibodies (IgY; see above) against break-down. Thus,maternal investment in egg composition, including carotenoids, might havea greater influence on offspring viability than has been realized. Theuse of carotenoid pigments in the sexual displays of female birds mightindicate their ability to produce high quality eggs and chicks. |
- albumen
- 90% water & 10% protein
- the embryo's water supply, but also serves as a 'shock-absorber' to helpprotect the embryo
- buffers embryo from sudden changes in temperature
- shell membranes
- attached to the shell are two membranes, the inner and outer shell membranes.They protect the egg from bacterial invasion and help prevent rapid evaporationof moisture from the egg.
[size=-1]Keratin fibers from the outer shell membrane can be seenabove, attached to the
[size=-1]calcium carbonate crystals that make up the main shellstructure.
[size=-1](Source:
http://www.rit.edu/~tld0898/SEM.html)
- shell
- protects the embryo
- contains thousands of pores (see diagram below) that permit gas exchange
- generally white in cavity-nesters & colored and patterned in open nesters(see Ecology of egg colors below)
- color is added to the eggshell from pigments secreted by cells in the wallof the uterus
[size=-1]Thousands of tiny pores like the one pictured above,cover the shell, providing a passage for gas exchange.
[size=-1](Source:
http://www.rit.edu/~tld0898/SEM.html)
| WeakerBirds Use Steroids to Boost Offspring -- Verboven et al. (2003)reported that female gulls in poor condition were more likely to give theirchicks a hormone boost to improve their chances of survival. Verboven andher colleagues experimentally enhanced maternal condition by supplementaryfeeding LesserBlack-backed Gulls (Larus fuscus) during egg formation and comparedthe concentrations of steroids (including testosterone) in their eggs withthose in eggs laid by control females. Egg androgens could affect offspringperformance directly through chick development and/or indirectly throughchanges in the competitive ability of a chick relative to its siblings.Contrary to expectation, females with experimentally enhanced body conditionlaid eggs with lower levels of androgens. This suggests that less healthyfemales pass on more steroids than healthy ones in a bid to enhance theperformance of their young. Verboven noted that “We originally thoughtthat gulls in good condition would put more steroids in their eggs. Butwe discovered that healthy birds don’t tend to give their eggs the extraboost.” She compared the situation to struggling athletes who take performance-enhancingdrugs. She added: “A poor sports person maybe wants to use steroids toconceal poor performance but if you are good you don’t need to use them.” | 
[size=-1]LesserBlack-backed Gulls
[size=-1]©Arthur Grosset |
Enhanced social behaviors (frequency h-1) and dominance in 10-month-old gulls from eggs injected with androgens
(black bars) compared to birds from eggs injected with oil (open bars). O, oblique display; F, forward display; C, charge;
P, aggressive peck; D, displacement. Sex did not affect behavior.
| [size=-1] Avian mothers create different phenotypes by hormone deposition in their eggs-- In birds, mothers deposit substantial amounts of androgens in theireggs, and experimental evidence indicates that these maternal androgensinfluence the chick's early development. Despite the well-knownorganizing role of sex steroids on brain and behavior, studies on avianmaternal egg hormones almost exclusively focus on the chick phase.Eising et al. (2005) found that, in Black-headed Gulls, maternalandrogens in the egg enhance the development of the nuptial plumage andthe frequency of aggressive and sexual displays (see Figure above)almost 1 year after hatching. The long-lasting effects may be mediatedby an upregulation of androgen receptors later in life. Alternatively,the early hormone exposure may have influenced thehypothalamus-pituitary-gonad axis, resulting in higher androgenproduction later in life. The long-lasting effects of egg androgens arealmost certainly beneficial for Darwinian fitness. Successful territoryestablishment and defense by means of aggressive interactions areessential for reproductive success in this colonial breeder. Inaddition, the displays are important for mate selection. Clearly, inbirds, maternal hormone deposition in eggs may profoundly influenceindividual differentiation of fitness related traits. Since thesehormones suppress early immune function of the chick and reducelong-term survival, mothers may be faced with a trade-off betweenproducing offspring with lower survival prospects but higherreproductive success per year, or with higher chances of survival andlower annual reproductive output. By producing eggs that differ inlevels of maternal hormones, mothers seem to produce a variety ofphenotypes, perhaps an adaptive strategy in unpredictable environmentalconditions. Since natural selection acts upon such phenotypicvariation, shaping a population's demography, the role of maternalandrogens in this selective process may be much greater thananticipated until now. |
The Ecology of Egg Colors [size=-1](see
Birds:A Virtual Exhibition - The Provincial Museum of Alberta)
Egg colors and markings have strong adaptive values. Originally, birds'eggs were probably all white, as reptile eggs are. Eggs that are laid onthe ground or in open nests in trees, rather than in cavities, often exhibitcryptic coloration. The eggs blend in with their surroundings and are muchless visible to potential predators (e.g., a
Killdeernest).
Sometimes eggs that are laid in open nests are white at first. Theythen become stained by the mud and rotting vegetation in the nest. Grebeslay white eggs that become stained and cryptically colored over time.
[size=-1]Red-necked Grebe nest
[size=-2]Source:
http://www.wetlands.org/programs/RussiaCD/eng/3/32/321/red-05.htm
In some species, such as the Common Murre, where different females layeggs with very different markings, the uniqueness may have a purpose. Distinctivepatterns, as in the eggs shown below, help females identify their own eggin a
colony where thousandsof eggs may dot a cliff face.
[size=-2]Source:
http://www.absc.usgs.gov/research/seabird_foragefish/seabirds/flash_cards/common_murre.html
Eggs of kingfishers and other cavity nesting birds, such as woodpeckersand some owls, are often white. The brightness of the eggs may help theparents to more easily locate them in the cavity. Shown here is the eggof a Barn Owl.
[size=-2]Source:
http://www.amonline.net.au/birds/gallery/eggs.htm
[size=-3]http://www.skullsunlimited.com/bird-eggs.htm
Evolution of egg color and patterning in birds --Avian eggs differ so much in their color and patterning from species tospecies that any attempt to account for this diversity might initiallyseem doomed to failure. Kilner (2006) reviewed the literature that,when combined with the results of some comparative analyses, suggeststhat just a few selective agents can explain much of the variation inegg appearance. Ancestrally, bird eggs were probably white andimmaculate. Ancient diversification in nest location, and hence in theclutch's vulnerability to attack by predators, can explain basicdifferences between bird families in egg appearance. The ancestralwhite egg has been retained by species whose nests are safe from attackby predators, while those that have moved to a more vulnerable nestsite are now more likely to lay brown eggs, covered in speckles, justas Wallace hypothesized more than a century ago. Even blue eggs mightbe cryptic in a subset of nests built in vegetation. It is possiblethat some species have subsequently turned these ancient adaptations tonew functions, for example to signal female quality, to protect eggsfrom damaging solar radiation, or to add structural strength to shellswhen calcium is in short supply. The threat of predation, together withthe use of varying nest sites, appears to have increased the diversityof egg coloring seen among species within families, and among clutcheswithin species. Brood parasites and their hosts have probablysecondarily influenced the diversity of egg appearance. Each drives theevolution of the other's egg color and patterning, as hosts attempt toavoid exploitation by rejecting odd-looking eggs from their nests, andparasites attempt to outwit their hosts by laying eggs that will escapedetection. This co-evolutionary arms race has increased variation inegg appearance both within and between species, in parasites and inhosts, sometimes resulting in the evolution of egg color polymorphisms.It has also reduced variation in egg appearance within host clutches,although the benefit thus gained by hosts is not clear.
Many small songbirds have eggs with just a ‘ring’ ofsmall spots around the broad end that does little to make the eggscryptic. Evidence now suggests that such spots are located where theeggshell is a bit thinner (likely due to a calcium deficiency in thediet of female birds), with the pigment serving to strengthen the shell(Gosler et al. 2005). The spots consist of protoporphyrin pigment thatbirds synthesize during production of the heme component of hemoglobin(Burley and Vadhera 1989) and integration of this pigment into theeggshell provides additional strength. When a female bird hasinsufficient calcium to deposit in a shell, protoporphyrin moleculesthat have a semi-crystalline structure similar to that of eggshells areapparently deposited instead instead of calcium. As a result, the spotsoccur precisely where the shell is a bit thinner.
Several species of birds have blue eggs, and DavidLack (1958) suggested that, in habitats where light levels are low,blue eggs might be cryptic. If true, that could help explain the blueeggs of some open-cup nesting birds that occur in forest habitats suchas Wood Thrushes. However, Lack’s hypothesis cannot explain why somebirds that nest in cavities, like European Starlings and EasternBluebirds, also have blue eggs. One hypothesis is that the blue-greencolor of eggshells represents a signal of female quality to their mates( Moreno and Osorno 2003). The pigment responsible for the blue-greencolor is biliverdin, a substance produced when the hemoglobin ofdamaged red blood cells is catabolized and also known to have strongantioxidant properties. Antioxidants are important because they canconvert free radicals, molecules that can damage DNA, proteins, andother macromolecules, into less reactive substances. Deposition of thispigment in eggshells by laying females may, therefore, signal theircapacity to produce antioxidants and control free radicals. Male birdspaired to females of such quality that they are able to depositantioxidants in eggshells rather than retaining them may then expendgreater effort in caring for their superior offspring (Kilner 2006). Insupport of this hypothesis, the provisioning rates of male PiedFlycatchers (Ficedula hypoleuca) and the intensity of the bluecoloration of eggs were found to be positively correlated (Moreno etal. 2004). Also, female Eastern Bluebirds in better body condition werefound to lay more colorful eggs, supporting the hypothesis thatbiliverdin pigmentation in eggshells reflects female condition(Siefferman et al. 2006).
Egg-laying:
"Initially the female stood motionless in the nest cup. Thefirst sign of approaching egg- laying was usually intensifiedbreathing, occasionally with rhythmic opening and closing of the billthat pointed either horizontally forwards or more or less upwards. Thehead was drawn in and the body feathers were somewhat fluffed out; theCoal Tit in addition raised its crown feathers. The tail was kepthorizontal or elevated up to about 45 degrees”. Then the tip of thetail started nodding movements synchronously with rhythmic depressionsof the rump.These movements which apparently were caused by throes ofparturition when the egg traveled down the oviduct, were almostinvisible to begin with but gathered in strength and ended with asudden elevation of the rump that marked the moment of egg-laying. Thenthe bird “froze” in a motionless posture, termed “recovery phase.” Thislast rise of the rump clearly indicated that the egg had just beenlaid.
Duration of egg-laying varies a great deal even withinspecies. The opening and closing of the bill and rhythmic movements ofthe back and tip of the tail occurs repeatedly for up to 4 minutes inthe Prairie Warbler, presumably corresponding to the duration of egg-laying. For 3 eggs of the Goldcrest, only 8-9 seconds elapsed betweenthe first visible sign of pressure and the moment of egg- laying. Intits, this period varied from about 10 to 77 seconds, mostlv 20-30seconds. The Cuckoo (
Cuculus canorth) which is a broodparasite, is known to lay the egg remarkably swift, usually within 10seconds with a lower limit of only 3-4 seconds. Presumably this shortduration is an adaptation to its parasitic behavior."--- From: Haftorn(1996).
[size=-2]Source:
http://www.afn.org/~poultry/egghen.htm
Female birds turns part of the cloaca and the last segment of the oviductinside out ("like a glove"). The vent is then everted and the egg emergesfar outside at the end of the bulge. As a result, the egg does not contactthe walls of the cloaca and get contaminated by feces. In addition, theintestine and inner part of the cloaca are kept shut by the emerging egg,and their contents cannot leave when the hen strains to deliver the egg.Therefore, eggs are always clean when laid (van der Molen 2002).
