Chapter 16: Egg Laying Chapter overview: Chapter 16 reviews the reproductive anatomy of the bird and the physiology of egg laying, including: structure and production of the egg anatomy of male and female bird factors affecting egg laying

Parts of the Egg: Shell - produced in the uterus Shell membranes - produced in the isthmus Albumen (white) - produced in the magnum Yolk (ovum) - produced in the ovary Germinal disk - chromosome container on the yolk

Egg Shell: Two main layers Composed of calcium carbonate Porous Translucent when laid; becomes opaque when dry Covered by a thin film called cuticle which tends to seal the pores

Shell Membranes: Two membranes are located just inside the shell and surrounding the albumen Membranes are bonded together except at the blunt end, or “air cell” end With time and evaporation the air cell forms by the membranes pulling apart

The Egg White: The egg white (albumen) is high in protein and water and has four principal layers: Outer thin white Thick white Inner thin white Thick white surrounding yolk; twisted into a rope-like structure called the chalaza on opposite sides of the yolk

The Egg Yolk: The egg yolk (ovum) is suspended in the center of the egg by the chalaza and is: High in fat and water Arranged in alternating light and dark layers Contained in a thin yolk sac called a vitelline membrane The location of the chromosomes in a germ cell in a tiny area (“spot”) on its surface

Weight of Selected Bird Eggs, g:

Anatomy of the Avian Male: Testes - paired; located in the abdominal cavity near the kidneys; sperm of the avian survive at internal body temperature Vas deferens - paired; convey sperm outside the body during mating Cloaca - terminus of the digestive and urinary tract, also receives the vas deferens Vent - external opening of the cloaca

Anatomy of the Avian Female: Ovary - single; in chickens the right ovary regresses in early life Stigma line - nonvascular suture line on the developing follicle that ruptures to release the ovum Infundibulum - funnel-like structure that receives the ovum and guides it to the remainder of the oviduct; site of fertilization

Anatomy of the Avian Female: Oviduct - single; anatomically divided into several distinct areas: Magnum: adds albumen Isthmus: adds shell membranes Uterus: adds shell; rotation creates chalaza Vagina: prepares egg for laying (oviposition) Cloaca - terminus of the digestive tract; egg passes through by inversion of the vagina

Hormones and Egg Laying: Follicle stimulating hormone (FSH) - stimulates growth of ovarian follicles; source: anterior pituitary Luteinizing hormone (LH) - causes ovulation; source: anterior pituitary Parathyroid hormone - maintains blood calcium balance for egg shell formation; source: parathyroid glands

Hormones and Egg Laying: Thyroid hormone - influences molting and seasonal changes in egg laying; source: thyroid gland Oxytocin - causes oviposition and laying; source: posterior pituitary Prolactin - causes manifestation of maternal instincts; source: anterior pituitary

Factors Affecting Egg Laying: Age at sexual maturity - commercial egg laying is optimized by managing hens to begin production at about 20 weeks of age Light pattern - optimize production with 14-18 hours of (artificial) day length Removal of eggs from the nest - removal increases rate of egg laying

Intensity of Egg Laying (Chickens): Interval between ovulations is 24-26 hours “Clutch”- an uninterrupted series of laying days of 10-14 days is desired; inheritable Laying begins at 18 to 20 weeks of age Peak production (90 percent laying per day) is reached at about 30 weeks of age Laying period is typically 12 to 14 months

Molting and Recycling: Molting - shedding and replacing of feathers A rest period to rebuild hens that are to be recycled Reduce light, feed, and water Requires a period of 6 to 8 weeks to rebuild Recycling - allowing hens a second laying cycle Shorter production period, but larger eggs

Factors Affecting Egg Size: Genetics - egg weight is highly heritable Age of bird - older birds lay larger eggs, therefore recycling can be an advantage Size of bird - larger hens usually lay larger eggs Environment - heat stress reduces egg size Nutrition - balanced diets maximize production and shell strength