Falco-Peregrinus Broedproces Mystery of the Egg

The bird’s egg is in its complexity still a mystery. And at the same time one of the greatest wonders of nature. In about 32 days develops in the inside of the pereginus egg from the egg cell of the female and the sperm cell of the male a fluffy alive and kicking chick. With a mix of strong genetic qualities of both.

SW and Buckeye with their eggs.
Photo: Scott Wright

The little one in the egg has sufficient food, water and oxygen with him to grow and is protected by membranes and an eggshell. A wonderful incubator formed by nature. Only the body-heat of the two parents during the incubation is still needed in order to make this miracle happen. The miracle of coming into being of a new precious life. It is the mystery of the egg.

Growth and development of the follicle.

The miracle begins with the formation of a follicle in the ovaries from the female. Furthermore, the falconiforms together with the brown kiwi bird are the only birds with two active and functioning ovaries. In all other birds functions only one ovary.

Ovary and oviduct.

At the instigation of follicle-stimulating hormone (FSH) grows a germ cell from a diploid eggcell mothercell, around which a vesicle, the follicle, is formed. The follicle cells exude a liquid, the follicle liquid. The fluid flows together and creates a cavity within the follicle: the follicle cavity.

The growth of the egg follicles can be divided into 4 stages:

1. The organisation of pre-primary still white follicles and the first growth of primary follicles.

2. Pre-hierarchy stage. Here in this stage begins a slow growth of primary follicles. This can take weeks to months.

3. The selection of one follicle for the first place in the preovulation hierarchy. The female influences whether her first egg will produce a female or male chick. That choice is made in the meiosis.

4. The rapid growth of preovulation follicles, which takes place in the last days before ovulation.

The continued availability and development of the follicles in the ovary in the breeding season is in particular to a large extent dependant on gonadotropins in the blood, paracrine (the factors that the cell produces influencing the neighbour cells) and autocrine (the factors influencing the same cell that produces them), factors made by the ovaries itself and humoral neurochemical and neuro factors of the nervous system.

Growth ovocyt in the follicle.

The future egg cell is located in the follicle and is surrounded by a layer of (muco)polysaccharides, the zona pellucida. In the ovaries granulosa cells form the perivitelline membrane that enwraps the primary ovocytes. They develop to a primordial follicle with a size of about 8 mm. The start of the primary follicle growth coincides with the forming of the theca layer from stem cells. The theca layer is separated from the granulosa layer by the basal lamina. Primary follicles are 0.8-1 mm in diameter. During the time that the follicle grows to the hierarchical link follicle stage (1-8 mm) also begins the collection in the cytoplasm of the lipo-protein rich yolk parts and the differentiation of the theca into internal and external layers.

There can be only one follicle released or ovulated. So there is always a succession of maturity of follicles. That is referred to as the hierarchy of follicles

Now there grows in the follicle the theca layer and a vascular-nervous system, through a channel that is known as the pedicle, through which each ovocyte is connected to the wall of the ovary. As a result, the rapidly growing ovocyte in the follicle receives blood and, therefore, nutrition. Through this channel with the blood, the yolk components are created in the liver under the influence of estrogen.

The follicle now becomes within the pre-ovulatory hierarchy the preovulatory follicle. This grows within a few days from 9 to 40 mm. Preovulationary granulosa cells of the preovulational follicles make it now possible to receive large amounts of vitellogenin and lipoprotein in the entire cell, except in the area of the germ. The next stage is the ovulation of the largest and most mature preovulatory follicle.

Vitellogenin is a protein and a precursor of several yolk proteins.

Histological compartment of developing follicles in various stages of maturation.

The yolk is made in shells in the cytoplasm of the ovocyte in the follicle. When the yolk is mature and the ovocyte completely ready, the follicle breaks finally along a line where there are few or no blood vessels. This line is called the stigma. When the yolk with ovum is released from the ovary it is protected and kept intact by the vitelline membrane: the ovulation is a fact.

There are several hormones secreted by the follicle that affect the entire oviduct, including progesterone. Progesterone suppresses ovulation. Estrogen ensures, among other things, that the calcium level in the blood is very high. During the laying of eggs it can reach values of 30 mg/dl in the blood. The female will eat calcium rich food at that time and therefore also eat the bones of her prey. Under the influence of estrogen approximately 10 days before egg production begins bone tissue is accumulated in the normally hollow spaces of the skeleton. For this the femur, humerus and tibia are used. This is a temporary supply that can be used for the forming of the eggshell

Yolk protein and antibodies.

Formation of the yolk proteins (the precursors vitellogenin (VTG = phosphoglycoprotein) and very low density lipoprotein (VLDL = mainly transport material for triglycerides, phospholipids and cholesterol)) occurs in the liver under the influence of gonadotropine and steroid hormones. After transport through the bloodstream they penetrate the ovarian follicles through the basal membrane and the granulosa cell layer to the plasma membrane of the developing ovocyte in the follicle.

Peregrine egg nutritional value.

Composition per egg

Weight 45

Water g 32
Energy kjoules/ kcalories 282/68

proteïn g 5.6

Carbohydrate g trace

Fat g 5.1

Inc saturated f.a. g 1.7

Monounsaturated f.a g 2.1

Polyunsaturated f.a. g 0.9

Dietary fibre g none

Sodium mg 70

Potassium mg 65

Calcium mg 27

Phosphorus mg 100

Magnesium mg 6.2

Iron mg 1.0

Zinc mg 0.7

Copper mg 0.04

Iodine mg 27

Chlorine mg 83

Sulphur mg 93

Selenium mg 6

Vitamin A mg 98

Vitamin D mg 0.9

Vitamin E mg 0.57

Vitamin C mg none

Thiamin (B₁) mg 0.05

Riboflavin (B₂) mg 0.24

Niacin mg 0.05

Vitamin B₆ mg 0.06

Folate mg 26

Vitamin B₁₂ mg 1.3

Biotin mg 10

Pantothenic acid m 0.91

In the cytoplasm of the ovocyte a conversion takes place to a variety of lipids and proteins (phosvitin, lipovitellin, phospholipids, cholesterol and triglycerides). During most of the growth of the yolk, it consists of about 50% protein and 50% fat, but the final composition of the yolk is in dry material 32.6% fat, 16.6% protein, so in the final stage of growth (= rapid growth stage) proportionally more fat must be stored. 90% of the yolk material is formed within 7-10 days for the laying of the egg. Probably the making of the yolk is complete 24 hours before ovulation.

Necessary antibodies for the defence of the chick come from the body of the mother that gives IgG immunoglobulins to the yolk. In the first 14 days after hatching the mother’s IgG is dismantled and replaced by the chick’s own created IgG, whose first traces are present after 5 days. After about 2 weeks the circulating IgG is entirely made by the chick itself. The necessary level is reached between six weeks and 6 months.

Therefore, the yellow yolk.

The yolk gets its yellow color from carotenoids. Originally the yolk was white in the first stage of follicular development. Carotenoids protect the vulnerable embryo tissue from damage caused by free radicals. Embryonic tissue is dependent on unsaturated fatty acids which react with oxygen, but their abundance makes the tissues sensitive to peroxidation caused by waste products and free radicals

Many copulations in the nest: Mariah and Kaver of Rochester.

These are normal by-products of metabolism. Protection is given by the carotenoids and other antioxidants such as vitamin E. Antioxidants protect also the given IgG globulins against degradation. The beautiful deep dark yellow-orange colored legs, cere and borders around the eyes of the peregrine female in the pairing time is a sign that she is capable of producing eggs of high quality. And thus healthy and strong chicks!

Fertilisation.

After copulation the mobility of each sperm cell determines if he can reach one of the many sperm storage tubes in the vagina wall. Only 1-2% will ultimately succeed in this, so the first large selection has already occurred. The rest is excreted in the next stool by the female. A small number of sperm cells go directly to the infundibulum where the final fertilization takes place. There they wait for the ovulation.

In the meantime the gender cells must still be formed. The meiosis has already started to do so. The progress of meiosis happens in several successive stages: the germ cells in the ovaries have had the prophase of the first meiotic division take place already before the birth of the female. During courtship the other phases of the first meiotic division take place. In ovulation the second occurs, it stops meiosis division in metaphase. Fertilization brings the completion of the second meiotic division.

Formation of the egg.

The oviduct is a large round tube where the ovocyte, in which the new life is originated, makes her journey to be laid as egg. During this journey the yolk gets a mantle of protein, membranes and finally the eggshell. The oviduct has 5 specific components: Infundibulum, Magnum, Isthmus, Shell gland, and Vagina.

After fertilisation the yolk with embryo receives in the Infundibulum a layer of albumin (protein) and the chalazae that the yolk will fix when the membranes get made. After approximately one hour, the embryo-yolk is pushed to the Magnum, where a broad protein mantle is built around the yolk. This serves as shock absorber and food for the developing embryo. In this part of the oviduct the egg gets a bit of its final shape. This takes approximately 3 hours.

To get to the Isthmus the forming egg must go through the translucent narrowing that is the border between the two parts of the oviduct. Here there are no glands. The Isthmus is a short part of the oviduct. It is strongly folded to make the passage of the ever-increasing egg possible. The inside of the folds in the wall of the isthmus are coated with a strong muscular layer that can move the egg. During the 2 hours the egg is here, the inner and outer membranes are formed that are laid around the yolk and protein. When an egg is broken these are the white films inside the egg shell. Also here is the starting point for the creation of the egg shell.

The components of the egg.

In the Uterus finally the valuable shell is formed around the egg. The shell is made up of calcium carbonate. The chalazae that were formed in the Infundibulum are now definitively attached to the membranes laying against the inside of the shell. The chalazae are the 2 strands that keep the yolk centered. They also serve as an axle where around the yolk can rotate and so ensure that the plate area always is at the top. The egg stays here approximately 20-26 hours. During that time is also added to the egg water and salts.

The aqueous-saline solution makes the protein volume double. The shell gland is the latest in the series of oviduct that must be decent-sized to pass the egg. It is therefore highly folded with a large lumen. The pigments are added in the final hours of the shell formation.

In the last part of the journey through the fallopian tube the egg arrives in the Vagina where a thin coating is made on the shell made to protect it against the bacteria and dust which could penetrate the pores of the egg shell. The egg moves through the oviduct with its small end first. But it will be laid the other way around with the wide end first. In the Vagina the egg is horizontally rotated just before the laying starts. It is still soft and gets her shape from the shape of the pelvis of the mother. The egg with the embryo on board starts the last part of its journey through the mother's body and arrives in the cloaca and then is pressed out the body of the mother. This goes hand in hand with a kind of contractions.

Three beautiful falcons eggs.

The female becomes impatient, her breathing speeds up. She sits on the nest and stands up often. She stands then in the hole with her spine curved and clearly she is pressing. Then again she lies down, stands up, turns around in the hole and is standing again in the press attitude. The male is in the vicinity, but not with her in the nest. When the egg is laid she lays down on it tired. Very exhausted. Her eyes are closing down. After a minute or 10 the male arrives. He wails from the distance and she answers. That is for him the sign that he can come in. The two are echupping against each other, with the new-born egg in the hole between them. For minutes and intense. He smells the egg and touches the egg with his tongue. And leaves. She echups a little and takes a nap. She doesn’t leave the nest when all the eggs are laid.

The peregrine falcon egg is elliptical to short elliptical. The size is 50-54 mm by 41-43 mm. The egg is smooth without gloss. Creme to brown reddish covered with a few scattered spots and speckles of varying warm brown to dark red tones. When the egg leaves the body, it has, of course, the body temperature of the mother. The contents fills the entire egg shell. However, the shell is hardening and drying. While it cools down, the content shrinks, loses volume and the density changes slightly. This creates pressure that sucks air into the egg and an air space is formed between the 2 shell membranes along the long side of the egg. There the shell is the most porous and the air can more easily come in. This is where the chick gets her/his oxygen during the hatching.

In the meantime 26-32 hours have passed. The embryo has since its conception in the Infundibulum undergone quite a development. Cell division started directly after fertilisation even though the rest of the egg has yet to be formed. The cell divisions will continue as long as the egg is warmer than 19 degrees. The first cell division is complete when the egg arrives in the Isthmus. The following divisions happen approximately every 20 minutes. At the time that the egg is laid there are some hundreds of cells. After it is laid the egg cools down and stops the embryonic development till all the eggs are laid. The brooding usually starts after the laying of the egg that is the next to last one to be laid

A typical bird chromosome.

Gender of the chick.

The gender of the chick is with birds determined not by the father but by the mother. That is the opposite from humans. Human beings have 23 pairs of chromosomes. 22 pairs provide the inherited properties. These are nuclear DNA autosomalic chromosomes. From those 22 pairs, each is double/diploid available, so each person has a total of 44 autosomal chromosomes and they are NOT a determining factor for the gender. The chromosomes in the 23rd pair determine the gender: human females have XX, and males have XY. The karyotype features of the human being exist therefore from 22 autosomal chromosome pairs and 1 pair of autosomal gender chromosomes.

With birds and thus the peregrine it is totally and completely different. The chromosomes of birds are divided into macrochromosomes and microchromosomes. A microchromosome is approximately 10 times smaller than a macrochromosome. In contrast to the macrochromosome, the microchromosome is lacking the highest chromosome order. More than half of the avian genes are located on the microchromosomes. The karyotype of the peregrine consists of 2n = 80 of which a quarter are macrochromosomes.

With birds the gender is also defined by a heteromorphologic chromosome pair, the gender chromosomes. As noted above, for people these chromosomes are called chromosomes X and Y. For birds we use Z = X and W = Y. A male has ZZ and is therefore homogametic and the female has ZW and is heterogametic. (It is assumed that the W chromosome contains no genes and therefore is empty. That is why ZW is also written as Z0.) The gender of the chick is determined by the egg and not by the sperm.

Mariah heavy with eggs in her abdomen.

For the peregrine the determination of the gender of the embryo therefore lies with the female and by that she has the potential and the possibility of determine the gender of the ovocyte which is ovulating. Research shows that the female indeed can determine whether the egg will be female or male. But how? That can happen in 2 ways.

Intervention in the meiosis.

The types of cell division are mitosis and the meiosis. The normal cell division of body cells is called mitosis. This is a process in which a cell splits into two genetically identical parts. Each part becomes a full-function cell. These new cells can once again split so that from 1 cell two identical cells originate. Then from these two are produced four cells. From those four again eight cells and so on.

The gender cells are formed by a cell division called meiosis. Meiosis takes place in principle like mitosis, but the chromosomes will not be doubled. This will produce gender cells, each with half the number of chromosomes of the original cell. Gender cells thus contain 1 chromosome of each pair. This is necessary because in fertilization the gender cells combine. The sperm and the egg fuse together. During this merger the chromosomes come together. After fertilization a fertilised eggcell has the chromosomes of the eggcell and spermcell. In total, the same number of chromosomes as the elders. From this cell can by mitosis a new animal develop.

Copulation will begin, in her body is a ripe ovocyt to ovulate.

It appears that the female can intervene in the earliest stage of this meiosis. In stage 1 of the meiosis and that is very remarkable. Bird mothers can thus manipulate the gender of their chicks. The gender is determined before the first meiotic division, when 1 sex chromosome stays behind in the ovocyte. In the meiosis 1 phase the steroid production is limited to progesterone. It appears that the higher the progesterone levels, the greater the chance of a female ovocyte. The chances can be high as 75%.

Intervention in the ovocyte hierarchy.

Less drastic but equally effective is the control over the ovocyte hierarchy in the maturing of the follicles in the ovary. There are many eggs maturing at the same time in the ovary. A male follicle has different hormones than a female. The body of the female can distinguish between a male and a female follicle. At one point a hierarchy is determined in the maturing process. The choice of that is highly dependent on outside conditions such as the availability of food, population, environmental conditions. In addition the female has control over what hormones and the amount of them that she puts in an egg. And therefore on the formation of the character of the chick. Above all force and the will to fight is then something that is important.

In the ovary is therefore determined which follicle will ovulate first.

Eggs of the Hemweg 2006.
Photo: Ruud Altenburg

The influence of prolactin on developing follicles is such that if the female has already begun brooding as the last egg is made this could lead to a smaller last egg. And also is known that when the reserves of the female are getting lower that can cause a small last egg and the chick has then little chance of survival. Although the mother always tries to put little extras in the last egg to prevent that.

Research Results.

And what shows the investigations? First it is clear that the earlier the eggs are laid in the season, the greater the chances of success. And that the last chick in a nest always has it harder and the probability that it will survive is small. Irrespective of gender. The last chick always gets less food and the consequences of that are for a female chick much more severe than for a male chick. In particular in the long term. It appears that if the last arrived chick is male he recovers quickly after fledging and easily can find a territory for himself. A last arrived female chick has much less success with that. Many of these females roam long as floaters.

In the nest a female chick must grow more than a male chick. For the first and second chick it does not matter whether they are male or female, they will have the same amount of food and are growing equally. Unless there is very little food. Then in that case the female chick is much worse off with a shortage of food than the little male. In times of plenty of food the female chooses for female offspring. Females who like to have large nests are often choosing for a complete female nest. An example is Mariah of Rochester.

Beautiful large SW of Cleveland.

It is known that the mother puts in all the eggs after the first additional nourishment to prevent the oldest chick from displacing the rest. It doesn’t matter if the first egg is female or male. Given the large chance of mortality among juveniles the investment in the eggs is very big to let them fly out well. Therefore, additional nourishment is a kind of insurance for breeding success..

The manipulation of the gender of her chicks is dependent on many factors: the laying time, preference for large broods, food supply. And the long-term options for the last chick. It seems like the peregrine tunes her offspring carefully on the chances of survival of either a male or female first chick and the amount of eggs she is going to lay.

And all this in the perspective of the survival of the species. And of course embryos with a large amount of food on board are most likely to survive after hatching and fly out successfully and have a good quality of life thereafter. The better the start in the egg and the weeks in the nest (and that always means much and rich food), the more likely they are to survive.

The mystery of the egg