Bird Sex and Eggs - Chapter 12

Chapter 12: Bird Sex and Eggs

Avian Female Anatomy

• Interesting similarities and fascinating differences exist between avian and mammalian reproductive systems.

• Key components of the avian female reproductive system:

  • Ovary: Produces the yolk and undergoes oogenesis. Avian females typically have only one functional ovary (left). The right ovary usually regresses during development.

  • Oviduct: A long, coiled tube where fertilization and egg development occur. It is divided into five distinct regions:

    • Infundibulum (1): Captures the ovulated oocyte. This is where fertilization typically occurs if sperm are present. Sperm can be stored in specialized tubules within the infundibulum.

    • Magnum (2): Adds albumen layers (the egg white). This process takes 3-4 hours, with albumen being approximately 90% water and rich in proteins like ovalbumin. The albumen provides hydration and nutrients to the developing embryo.

    • Isthmus (3): Deposits the shell membranes around the albumen layer. These membranes provide a protective barrier against bacterial invasion and aid in moisture retention.

    • Uterus (shell gland) (4): Where the shell, made of calcium carbonate, is added. This process takes 19-20 hours. The shell provides structural support and protection. Pigments can also be added to the shell in this region.

    • Vagina (5): Connects the uterus to the cloaca. It plays a role in sperm transport and egg expulsion.

  • Cloaca: A common opening for the digestive, urinary, and reproductive tracts.

• Shell membrane: Inner and outer layers that protect the egg contents.

• Cleaving blastodisc: The early-stage embryo on the surface of the yolk.

• Shell: Primarily composed of calcium carbonate, providing a hard, protective outer layer.

• Right Oviduct: Typically regressed and non-functional in most avian species.

• Intestine: Part of the digestive system, located near the reproductive organs.

• Chalazae: Two twisted, cord-like structures that suspend the yolk in the center of the egg.

• Vitelline membrane (yolk sac): A membrane surrounding the yolk that provides nutrients to the developing embryo.

• A hen can produce around 50,000 cells, and the shell takes approximately 24 hours to form. The entire process of egg formation, from ovulation to laying, takes about 25-26 hours in chickens.

Chicken Cloaca and Follicles

• The cloaca of a chicken includes the vagina opening and intestine opening. It is a versatile opening for excretion and reproduction.

• Follicles: Structures in the ovary that contain developing oocytes.

  • Mature Follicle: Contains a fully developed oocyte ready for ovulation.

  • Discharged Follicle: The follicle after the oocyte has been released (ovulation).

  • Atretic Follicle: A follicle that has stopped developing and is degenerating.

• Pedicel: A stalk-like structure that attaches the follicle to the ovary.

• Stigma: A specialized region on the follicle where it ruptures to release the oocyte during ovulation. It is relatively avascular to minimize bleeding during ovulation.

Seasonal Changes in Gonad Size

• The ovary and oviduct (typically only on the left side of the bird) change in size with age and season.

  • Winter: Smaller size, reduced activity.

  • Spring: Larger size, increased activity due to hormonal stimulation.

• Testes also change dramatically in size with season and age. This is influenced by photoperiod and hormonal signals.

Hormonal Control of Reproduction

• Specific behaviors in reproduction, molt, and migration are directed by hormones.

• Hormone Production: Hypothalamus & Pituitary Gland

• Process:

  • As day length increases, photic stimulation of the hypothalamus results in the secretion of Gonadotropin-Releasing Hormone (GnRH). Photoperiod (day length) is a primary environmental cue that initiates the breeding season.

  • Activated by GnRH, the anterior pituitary secretes follicle-stimulating hormone (FSH) and luteinizing hormone (LH).

  • FSH acts on sperm-producing structures in the testes (seminiferous tubules), promoting spermatogenesis.

  • LH acts on the interstitial cells of the testes (Leydig cells), causing them to secrete testosterone.

• Testosterone Levels:

  • Plasma levels peak in April & May as breeding commences. Testosterone influences male sexual behavior, secondary sexual characteristics, and aggression.

  • Maintained at a lower “breeding baseline” during the rest of the breeding season.

  • As prebasic molt ensues, plasma levels of testosterone are basal and remain so through autumn & winter. Molting is energetically expensive, and high testosterone levels would interfere with feather growth.

• Wingfield and Hahn (1994) studied the pattern of testosterone secretion in free-living populations of Song Sparrows (2005) J Evol Biol 18:557-567. This research demonstrated how hormone levels correlate with breeding behavior and environmental conditions.

• Testes size was significantly larger in species that breed colonially than those that breed solitarily, suggesting that higher breeding density is associated with greater sperm competition involving 1010 bird species. Sperm competition is a selective pressure that favors larger testes and higher sperm production.

Testes Size and Mating Systems

• Monogamous taxa had smaller testes than taxa with multiple mates. This is because monogamous males face less sperm competition.

• Testes size tended to increase with clutch size, suggesting that sperm depletion may play a role in the evolution of testes size. Larger clutches require more sperm to fertilize all eggs.

• The size of the cloacal protuberance, a site of sperm storage, varies across species. This structure is more developed in species where males store sperm for extended periods.

• Very few birds have any kind of an intromittent organ (i.e., penis), and for most birds, copulation is little more than a brief “cloacal kiss.” This method is efficient for species without a penis.

Avian Copulatory Organs

• The female is generally in control of who sires her eggs. Females can eject sperm from unwanted males, select sperm for storage, and influence fertilization.

• A few birds have an erectile, penis-like cloacal phallus (special modification of the ventral wall of the cloaca). This is more common in species with high levels of sperm competition.

• Only three percent of avian species belonging to two main clades have retained the ancestral copulatory organ: the Paleognathes (e.g., ostriches, rheas, and tinamous) and the Galloanseridae (e.g., cracids and ducks). These groups rely on direct sperm transfer.

• Drawing of phallus within the pouch:

  • A. vas deferens: The duct that conveys sperm from the testes to the cloaca.

  • B. urideum: Part of the cloaca that receives the ureters.

  • C. proctodeum: The posterior part of the cloaca.

  • D. pocket to contain phallus: A storage space for the phallus when not in use.

  • E. erectile wall of phallus: Tissue that fills with fluid to cause erection.

  • F. inverted hollow tube of phallus: The internal structure of the phallus.

  • G. phallic sulcus: A groove along which sperm travels.

  • H. erectile tissue: Spongy tissue that engorges with blood for erection.

  • I. erect phallus with blind hollow tube: The phallus in its erect state, showing the internal structure.

Sperm Diversity

• Only three percent of avian species belonging to two main clades have retained the ancestral copulatory organ: the Paleognathes (e.g., ostriches, rheas, and tinamous) and the Galloanseridae (e.g., cracids and ducks).

• A few birds have an erectile, penis-like cloacal phallus (special modification of the ventral wall of the cloaca).

• Brennan et al. (2010) Proc. R. Soc. B 277:1309-1314 researched explosive eversion of the muscovy cloacal phallus in air reaching 5 cm. This study highlighted the rapid and forceful extension of the phallus during copulation.

• There is a lot of diversity in the size and shape of avian sperm. Sperm morphology can influence swimming speed and fertilization success.

• Examples:

  • Collared trogon

  • Great black-backed gull

  • Eider

  • Blue ground dove

  • Chicken

  • Yellow-rumped warbler

• Sperm Components:

  • a = acrosome: Contains enzymes that help the sperm penetrate the egg.

  • af = axial filament: The core structure of the sperm tail, providing motility.

  • mp = midpiece: Contains mitochondria that provide energy for sperm movement.

  • n = nucleus: Contains the genetic material (DNA).

  • tm = tail membrane: The outer covering of the sperm tail.

Sperm Storage in Female Birds

• In chickens, average concentrations of sperm are 3.5 million per cubic millimeter of semen.

• A single ejaculate passes from 1.7 billion to 2.5 billion sperm, with records ranging from 7 billion to 8.2 billion. High sperm numbers increase the chances of fertilization.

• Near the junction of the vagina & shell gland of female birds are deep glands lined with simple columnar epithelium called sperm storage tubules. These tubules are essential for prolonged fertility.

• These tubules store sperm for long periods (10 days - 5 weeks). This allows females to fertilize eggs over an extended period from a single mating.

• After an egg is laid, some of these sperm may move out of the tubules into the lumen of the tract, then migrate to fertilize another egg. This process ensures fertilization of subsequent eggs.

• Female fertile period = 5 days before first egg. The female needs to store sperm before the first egg is ready to be fertilized.

• Sperm storage up to 10 days for the female zebra finch. Different species have varying sperm storage capabilities.

• Sperm storage time is related negatively to hatching success. Prolonged storage can reduce sperm viability and fertilization rates.

Egg Size Variation

• The colors of many birds' eggs make adaptive sense. Camouflage helps protect eggs from predators.

• Eggs vary tremendously in size. Kiwis lay the largest eggs for their size.

Eggshell Pigmentation

• Many songbirds have eggs with just a “ring" of small spots around the broad end that does little to make the eggs cryptic. These markings may serve other functions such as structural reinforcement or species recognition.

• Evidence suggests that such spots are located where the eggshell is a bit thinner (likely due to a calcium deficiency in the diet). Calcium is crucial for shell formation.

• The pigment seems to strengthen the shell. Pigments like protoporphyrin can increase shell hardness.

• The spots consist of protoporphyrin pigment, synthesized during production of hemoglobin (Burley and Vadhera 1989). This pigment is derived from the breakdown of red blood cells.

• Gosler et al. 2005 researched why birds’ eggs are speckled in Ecology Letters 8: 1105-1113. This study explored the adaptive significance of eggshell speckling in relation to nest camouflage and egg strength.

Egg Size and Clutch Strategies

• Within a species, egg size differs. For example, younger birds of a species tend to lay smaller eggs than older birds. Older birds are more experienced and can allocate more resources to egg production.

• Even eggs laid by a particular bird vary in size, both within & between clutches. This can depend on the female’s condition and resource availability.

• Food availability is a key factor. Adequate nutrition is essential for producing large, nutrient-rich eggs.

• If you were advising a bird, would you recommend that it lay four large eggs or five smaller eggs? This depends on the environmental conditions and the bird's ability to provide for its offspring.

• Imagine situations where one strategy is better than the other. For example, in resource-rich environments, larger eggs with more nutrients may be beneficial. In harsh environments, laying more eggs may increase the chances of some offspring surviving.

Egg Shape and Contamination

• Research suggests another possible factor influencing egg shape, at least in the Common Guillemot (Murre) (Uria aalge) studied by Birkhead et al. (2017).

• Considerably more “contamination” (i.e., feces) on Common Guillemot eggs compared to Razorbill, and most contamination is on the pointed end of the egg, where it is in contact with the substrate.

Pyriform Egg Shape

• Pyriform shape thus keeps the blunt end of the egg, which has the highest porosity, relatively free of contamination. This shape reduces the risk of pathogens entering the egg.

• This, in turn, may facilitate both gas exchange during incubation and the hatching process; the chick emerges from the blunt end of the egg (Birkhead et al., 2017). Proper gas exchange is crucial for embryo development, and a clean, porous surface facilitates this process.

Egg Laying Intervals and Formation

• The intervals between successive eggs are variable across and within species.

  • Gyrfalcon: 2.5 days

  • Tricolored heron: 1.7 days

  • Small shorebirds, domestic chickens, woodpeckers, rollers, and most passerines lay eggs about 24 hours apart.

  • Other birds, such as ostriches, rheas, herons, storks, cranes, bustards, gulls, doves, owls, hummingbirds, swifts, kingfishers, as well as some hawks and cuckoos, generally lay eggs every other day.

  • Adélie Penguin: 3-6 days

• Sperm swim directly to the infundibulum or sperm storage tubules. Sperm can survive in these tubules for days or weeks.

• EGG: Oocyte + yolk layers = ovum (carbohydrates & proteins)

• Released => infundibulum fertilization window (15-30 min). This is the critical time frame for fertilization to occur.

• Magnum => albumen layers added (90% water; 3-4 hr). The albumen provides a protective cushion and nutrients.

• Uterus => shell gland, calcium carbonate (19-20 hr). The shell protects the developing embryo from physical damage and dehydration.

Egg Composition and Components

• Eggs consist of:

  • Yolk = energy-rich supply of food (21-36% lipids & 16-22% proteins; the rest is water). Lipids provide energy, and proteins are essential for tissue development.

  • Albumin = 90% water & 10% protein. It provides hydration and some essential amino acids.

  • Amnion, chorion & allantois: These are extraembryonic membranes that support the developing embryo. The amnion protects, the chorion aids in gas exchange, and the allantois stores waste.

• Maternal carotenoid investment. Carotenoids in the yolk enhance the chick’s immune system and antioxidant defenses.

Yolk Formation

• How long does it take swallows to form a yolk?

• We can count the nocturnal light bands to get a notion of how long it takes. These bands reflect variations in hormone levels and nutrient deposition.

• 4-5 days in passerines, 6-8 days in larger birds such as ducks and pigeons, and as long as 16 days in some penguins. The duration varies with body size and energy requirements.

• For swallows, food availability while the egg is being “yolked up” has a significant (though noisy!) effect on the size of the yolk. Adequate food intake directly contributes to the yolk’s nutritional content.

Yolk Amount and Clutch Size

• Variation among bird species in the relative amount of yolk in eggs and the amount of energy available to the developing embryo.

• Brown Creeper (altricial): Altricial chicks hatch in a helpless state and require extensive parental care.

• Least Tern (semiprecocial): Semiprecocial chicks are partially developed at hatching and can move around but still need parental care.

• Ruddy Duck (precocial): Precocial chicks are well-developed at hatching and can feed themselves.

• Mallee Fowl (superprecocial): Superprecocial chicks are fully independent at hatching.

• Brown Kiwi (semiprecocial; outlier): The kiwi has an exceptionally large yolk relative to its egg size.

• CLUTCH SIZE

  • Fixed in some species:

    • Some shorebirds: CS = 4 eggs

    • Some seabirds = 1 egg

    • Penguins = 2 eggs

  • Usually variable:

    • Red-winged Blackbirds: 2-5 eggs

    • Tree Swallows: 4-7 eggs

Factors Influencing Clutch Size

• Factors of variation in clutch size:

  • (1) Female Age: First-year females lay smaller clutches. They are less experienced and have fewer resources.

  • (2) Season: Decrease in clutch size through the breeding season. Environmental conditions may deteriorate as the season progresses.

  • (3) Latitude: Gradient of increasing clutch size going from the equator in both directions.

    • Tropics => stable environment => Pops @ K (carrying capacity). Populations are stable, and reproductive rates are lower.

    • Temperate => more variable environments => pops “r-selected” => higher clutch sizes. Populations are more opportunistic with higher reproductive rates.

    • Temperature => warmer in tropics during lay period => induce embryos to start developing even when eggs are not actively incubated. Early development can be risky in less controlled conditions.

  • (4) Nest type: In general, cavity nesters lay larger clutches than open-cup nesting birds, possibly due to lower predation rates generally experienced by cavity nesters. Cavities offer better protection from predators and the elements.

  • (5) Fat reserves at the initiation of nest: A gradient of increasing clutch size with increasing fat reserves. Females with more energy reserves can invest more in reproduction.

• Most females finish