Birds are the only vertebrates that have 100%  Oviparous reproduction ( 100% egg laying). In some reptiles and all Mammals the young ones develop within the body of the adult mother, while in Birds the maximum development of the young happens in a hard-shell container that has all the elements necessary for the development of embryo like Oxygen, Nutrients etc.

Why do birds lay eggs? :

Birds lay eggs to increase the chances of survival of young ones and the parents

• The primary movement of a bird is by flying. This is true for fending themselves from predators, looking for food or any other purpose. To fly the bird has to fight gravity. And to fight gravity it has to be very light.  So in order to be light at all times the birds do not carry the young ones inside their bodies.
• The flightless/ Ground dwelling birds also lay eggs because they were once capable of flying and lost the ability as they stopped using it. Also the fact that with eggs out of their body, they are agile and can outrun their predators.
• Laying of eggs also provides security to the young ones as these eggs are either laid at place that are difficult for predators to reach (ex.  Baya weaver, Broadbill) or camouflaged in colours and patterns of its surrounding ( ex. Ostrich, jacana ). The young ones have a great chance of surviving the attack from predators even before they are born.
• Once the egg is laid, the development of young one is not dependent on its mother. So even if the mother dies , the young ones can be incubated by father or others in the family. This further increases the survival chances of the young ones
• Egg laying also leads to a interesting breeding system called POLYANDRY, where the female can have clutches by mating with different males ( ex Bronze winged Jacana) ( for more on polyandry pls. visit www.ogaclicks.com/wordpress/mating-in-birds-part-i-different-type/)

 

  How are eggs formed ? :

The female reproductive cell is call OVUM. This is joined to a YOLK parcel in a tube like organ called OVIDUCT.  The YOLK contains nutritional package that is required for the chick to develop within the egg. Then the male reproductive cell called the SPERM fertilizes with the OVUM .The fertilised OVUM along with the YOLK are then covered with a translucent substance that is rich in water and protein. This is called ALBUMEN, this is provides the embryo with aqueous medium required for it to grow and also act as a buffer and a shock absorber protecting the embryo

The embryo is connected to Yolk and Albumen by a “Umbilical chord” equivalent called CHALAZA. This whole package is further surrounded by two tough membranes called  CHORION and ALLANTOIS. This  whole parcel then travels to a part of Oviduct where it is covered with a Shell of Calcium Carbonate . The actual shape of egg is formed here and the egg is either Smooth or pitted . The egg at this stage is either white, off-white or blue.

 

Picture credit https://upload.wikimedia.org/wikipedia/commons/f/fb/Anatomy_of_an_egg_labeled.jpg

 

Till here the process is same for all birds. From here on depending on the family , surrounding and nesting habits the eggs are provided extra features

The eggs then passes through a section of Oviduct where the glands coat it with yellowish pigments that buff the shell and darkens it resulting in Beige, Cream or Off-Brown  colours.

The egg is then passed through a section of Oviduct that paints it in patterns and other colours to provide it the camouflage effect. This depends from Species to species basis their nesting and surroundings

The eggs are then passed through oviduct into the Urogenital opening called CLOACA

The egg laying process is a very delicate and strenuous one for the bird. It can take anywhere between a  day to a few days depending on the size of the bird and its eggs.   But one thing is common, each egg has to complete its sequence from Fertilization to formation to ejection from CLOACA before the next egg is formed. This means that the egg formation  cannot run parallel to each other  and has to be sequential and that too only after the completion of the previous egg.

This is because of many reasons. The egg formation like flying is a high energy consuming process and the bird with its high metabolic rate need rest  to get recouped and be ready for it again.

The bird is very vulnerable during this stage and hence will not want to expose itself to the predators for long . So once the egg is out it can defend itself , something that it will not be able to do if there were multiple eggs in the OVIDUCT in various stages.

By producing one egg at a time the bird is able to give the egg maximum size and attention to its features and thereby a greater chance of survival

Also as the birds can store the Sperm for many days within their body , they can use it for fertilization as and when required and this make egg laying easy

( To know more about bird copulation pls visit http://ogaclicks.com/wordpress/mating-in-birds-part-iii-copulation/)

 

The whole process of Egg formation in a diagram

 

 

Picture credit http://people.eku.edu/ritchisong/Avian_FemaleReprodTract6.jpg

 

How are egg coloured?

Eggs get coloured from various methods  . Some are coloured in Oviducts by special glands so that they can merge with the surroundings and get a camouflage . Espl in the ground birds. They also have patterns drawn on them so that they can look like stones, animal droppings or any other rubble.

Some eggs like that of the water birds get stained by the wet water plants that the parents cover them with when they leave the nest. The wet water plants stain the eggs.

Eggs of Cormorants, Egrets, Herons and Storks get coloured by the faeces and the regurgitated food droppings on them by the adults and buy the firs born siblings. They stain the egg  with these wastes.

Lammergeirs or Bearded Vulture get their bellies coloured red/orange  due to roosting  in crevices in cliffs. These are Iron rich soil that stick to their feather barbs. When these feathers are wet and as the bird sits on the eggs , they get stained orange.

The birds that lay eggs in dark Holes ( ex Owls) have bright white eggs so that the eggs can be visible to them in the darkness and they do not step on them by mistake   .

 

Shape of Eggs :

The Birds have to consider a lot of factors to adapt to an egg shape. There are three main shapes of eggs.

Round : Perfectly spherical

 

Picture Credit http://www.takayaiwamoto.com/Egg_Draw/preston_eggshape.jpg

Oval : Narrower at one end

 

Picture Credit http://www.takayaiwamoto.com/Egg_Draw/preston_eggshape.jpg

Pyriform : Triangular ( Like a top) with the narrow end tapering to a “rounded” point

 

Picture Credit http://www.takayaiwamoto.com/Egg_Draw/preston_eggshape.jpg

Internally the egg has to pass through the narrow oviduct and hence an oval shape is a better choice. But the spherical shape allows the most optimal use of space, strength and integrity to structure and weight saving for a given volume. But the most critical factor is the shape of nest and its surroundings.

Birds with open nest , nest on cliffs  , Nesting on floating plants and high points will have a pyriform shaped egg as the egg doesn’t tip over the edges of the nest. The egg if knocked by wind or some disturbance will spin like a top and topple with he narrower end towards the inside of the nest. This shape also allows the birds to put all eggs ( Max of 4) together and incubate them easily. Birds Like Skimmers, Jacana, Gulls, Avocets, terns etc. have this shaped eggs

Birds with Cup, Saucer  or Bowl like nests where the shape of nest prevents the eggs from rolling over will have Round or Oval eggs.

 

Clutch Size :

This is defined as the number of eggs laid by a single female bird in an uninterrupted sequence in a single nesting or breeding attempt

Smaller clutch size  means in case the clutch /chick is killed the breeding effort is wasted. Also the smaller clutch size is literally putting “ all eggs in one basket”. But the larger clutch size has its own disadvantages  too. The birds that produce large clutch sizes and breed early do not live long. This is the disadvantage of  hyper energetic breeding. So an optimal clutch size is decided by nature for each species . The clutch size is linked to the life history of the species and is influenced by various factors like

• Size of the Bird :  The larger birds like Raptors and cranes take care of the young for a longer period of time and the survival rate of the young is very high. They also start breeding after a few years of Juvenile status and their mortality rates are low. So they have lesser eggs in their clutch.. maximum of 2.  The smaller birds like Tits, Larks, Flycatchers etc. spend very little time with their young and the young ones are forced into self dependency very early .The mortality rates are very high hence there will be larger clutches and multiple breeding attempts.

 

• Breeding location : Clutch size also depends on Quantity, Quality and density of food resource in a location. Within a species there can be variation in clutch size due to location.In areas where the temperature is suitable, Food is plenty (Western Ghats, North East, Jungles ) the clutch size will be small ,because the competition in the area will be high for nesting, partner selection and resource gathering. Also as the resources are found around the year  there will be multiple breeding attempts.  In areas where the food is dependent on Seasons ( like Himalayan region, Desert region etc) and the suitable temperature exists only for a limited period of time the clutch size will be larger. This is to ensure the maximum use of the favourable season and produce as many new birds as possible. Birds that nest in tree hole that ARE NOT EXCAVATED BY THEM ( Myna, Parakeet, Rollers etc.) they tend to have a larger clutch as they are not sure of getting the nest again. So they lay more eggs and ensure that they increase the population when they have a chance of having a good nest.

 

• Predation : The smaller birds are more prone to predation and hence need to produce more chicks than the larger birds. Their clutch size is bigger than the large birds. But within the smaller birds there is variation in terms of clutch size. As a larger clutch would mean attracting predators due to movement and noise and thereby endangering the whole brood , the birds tend to have smaller clutches.  Also in smaller clutches the chicks will get more proportionately fed by the parents and hence will grow up faster and leave the nest.

 

• Egg laying date: The birds that are in prime of their health and breeding abilities will breed early in the season. They will lay large clutch size. This is because of hormones in the body and also availability of food. Also once the young ones hatch and grow uo they will still be in season and they can start fending for themselves as the availability of food is good. The best of the genes is passed in these birds. The birds that breed late are of inferior qualities and their clutch size is also small. This is because internally their hormones are getting over and also externally the season is ending. This will lead to shortage of food for the young ones and they could starve to death. So the clutch sizes are small to ensure that the young ones survive. Generally the late breeders have inferior genetic material and they keep breeding late and smaller clutches and will slowly fade away so that the birds with superior genetic material survive

 

•  Size and age of the female :  The younger birds have smaller clutches as their oviducts are smaller and they can produce very little embryonic material. Their young ones also have high mortality due to inexperience of the mother.

 

Status of young : There are two types of Chicks  of birds

• Altricial : Highly dependent of parent for food and grooming. Takes a lot of time to leave the nest and will stick with parents even after leaving nest for a fair amount of time
• Precocial : Very little dependence on parents . Grows up quickly and they leave nest early Learn survival skills very fast and moves away from parents
• So the birds that have the Altricial brood will have smaller clutch. This is because the strain on the parents in considerable and also a larger brood over a longer time will attract unwanted attention. The Precocial  will have larger clutch and will also be able to breed  multiple times in the season.

 

• Bird Family : There are variations within different species and groups amongst birds for the clutch size but it is also seen that within a family the clutch size is of same standard. The clutch size is dependent on Survival conditions , Feeding regimes and diet.
• Doves and Pigeons will always lay two eggs. This is because of the special way they feed their young=g initially. They feed the young with a milk like secretion from the crop called crop milk. The quantity is usually enough for only two young ones, hence the clutch size.

 

• Most waders will have 4 eggs in a clutch. The reason is shape of the egg. The egg is of Pyriform shape so 4 is the ideal way to pack the eggs in the open nest ( with tapering ends towards inside , looking like a sliced Pizza ). This prevents the eggs from rolling out . This also makes it the smallest area for the bird to cover for incubation. Hence the clutch size is limited to 4

 

 

 

 

The egg laying process completes the second stage in reproduction cycle / Nesting cycle  of a bird

 

Source :  Wikipedia, Beat About the bush -Birds by Trevor Carnby