Birds have an internal development, meaning their embryos develop inside the egg rather than directly in the physical environment. This contrasts with amphibians and many marine animals which often have an external fertilization and development.
When does a bird’s development begin?
A bird’s development begins when an egg is fertilized internally by the male. Unlike some animals that fertilize externally, bird reproduction relies on internal fertilization. This occurs during mating when the male transfers sperm into the female’s reproductive tract where it can then fertilize the ovum.
What happens after fertilization?
After fertilization, cell division begins rapidly. However, full development does not occur until the fertilized egg is laid. Once laid by the female, the embryo continues developing inside the hard eggshell, protected from the external environment.
The eggshell not only protects the growing embryo but also contains the nutrients, water, and oxygen needed for development. The shell’s pores allow gas exchange to occur.
What are the main stages of development?
The main stages of development inside the egg are:
- Fertilization – sperm fertilizes the ovum
- Cell division – rapid cell multiplication and growth
- Gastrulation – cells differentiate into specialized tissues and organs
- Organogenesis – organs form, like eyes, lungs, intestine
- Muscle and skeletal development – bones, muscles, and feathers form
- Hatching – the chick breaks out of the eggshell using its egg tooth
How long does development take?
The total incubation time varies by species. For chickens, the incubation period ranges from 21-25 days. Larger birds tend to have longer developmental times. For example, peafowl incubation ranges from 28-32 days while swan incubation can last 35-45 days. The table below shows approximate incubation times for various bird species:
Species | Incubation Time |
---|---|
Chickens | 21-25 days |
Ducks | 28 days |
Geese | 28-35 days |
Pigeons | 17 days |
Peafowl | 28-32 days |
Ostriches | 35-45 days |
What happens after hatching?
Once emerged from the egg, the hatchling enters a new phase as a baby bird. Having fully developed inside the egg, the chick has all it needs to survive external to the egg. The brain, organs, skin, feathers, legs, wings, eyes, and beak are all fully formed.
Initially after hatching the chick is wet, tired, and vulnerable. Once dry and rested, their energy returns and they are ready to exit the nest. The parents continue caring for the hatchlings until they learn to fly and hunt on their own.
How does the embryo get oxygen?
The embryo gets the oxygen needed for development through the egg’s porous eggshell. Oxygen is able to diffuse through the shell’s microscopic pores along with carbon dioxide diffusing out. The shell provides the protective barrier while still allowing gas exchange.
The embryo also has special membranes that line its respiratory and cardiovascular systems bringing oxygen to tissues. The chorioallantoic membrane forms to transport oxygen from the shell to the embryo.
What provides nutrition?
The yolk provides almost all of the nutrients needed for the embryo to fully develop. The yolk contains crucial proteins, fats, vitamins and minerals. As the cells divide, the yolk nutrients are consumed.
The yolk spheres inside the egg decrease in size throughout incubation as the embryo uses up the available food source. A portion of the yolk may also be absorbed into the hatchling’s abdomen to provide nutrients for a few days after hatching before the chick can forage.
Why do birds develop internally?
There are a few key advantages to internal development in birds:
- Protection – The hard eggshell provides protection from predators and harsh environments during the vulnerable developmental period.
- Support – The embryo has all the nutrients and moisture necessary in a self-contained support system.
- Retention – Being internal allows the embryo to go through development isolated from the external world.
- Adaptability – Birds can nest and breed in more extreme locations since the egg protects the embryo.
Overall, the excellent insulation, protective casing and built-in food supply offers the best chance of survival for bird embryos. External development in many species results in high mortality.
How does the embryo orient itself?
Inside the egg, the developing embryo orientates itself so its head is towards the blunt end. Its head develops adjacently to the air cell which provides oxygen. The embryo moves into this position using sensory cues and muscle contractions.
Having the head by the air cell ensures the embryo has sufficient oxygen supply during the incubation process. It also orients the embryo to be in ideal position for hatching since it will hatch from the blunt end after pipping through the air cell.
Why does orientation matter?
If an embryo develops with incorrect orientation it can impair its development and ability to hatch properly. Incorrect positioning could put the head far from the oxygen source or orient the body so pushing through the shell during hatching is difficult.
How is orientation controlled?
The embryo appears to use a combination of factors to control its orientation inside the egg:
- Gravity receptors – sense which direction is up
- Touch stimulation – guide position based on contact with shell
- Chemo-attraction – move towards high oxygen at air cell
- Light stimulation – orient towards light shining through shell
- Muscle contractions – physically shift position
With these mechanisms, the chick actively moves itself into the optimal head-upward facing position during development.
What happens if the egg is rotated?
If the egg is rotated, meaning turned upside down at some point during incubation, the embryo will reposition itself. As long as the egg is turned early enough, the embryo can fully reorient itself so its head develops adjacent to the air cell again.
However, if the egg is rotated late in development, there may not be enough time for the chick to fully reposition before hatching. Late changes in orientation can sometimes lead to physical abnormalities or death since critical systems like breathing and hatching are affected.
Ideally, the egg should not be drastically turned after day 7 of incubation. Minor shifting is acceptable, but flipping the egg upside down late can be harmful.
What causes embryo mortality?
There are several causes of embryo mortality, meaning embryo death during incubation:
- Genetic defects – Abnormalities preventing normal development
- Incorrect incubation temperatures – Extreme highs or lows kill the embryo
- Insufficient gas exchange – Oxygen deprivation suffocates the embryo
- Bacterial infections – Can spread from the parent bird into the egg
- Physical damage – Cracks or punctures kill the embryo
- Incorrect egg positioning – Limits oxygen and nutrients
- Nutritional deficiencies – Lack of vitamins/minerals inhibits growth
Careful egg handling, proper incubation equipment, and using fertile eggs helps minimize embryo death rates. Even then, some percentage of mortality is expected.
Are there any disadvantages to internal development?
There are a few potential disadvantages or compromises associated with internal development in birds:
- The egg must contain all the resources needed – makes egg production more metabolically taxing on the female
- Limited gas exchange through the shell – can sometimes be a bottleneck
- Requirements for parental egg incubation – time and energy intensive for parents
- Prevention of water loss is difficult – dehydration can occur
- Difficult to monitor embryo health non-invasively – problems not spotted until hatching
However, overall the advantages far outweigh the disadvantages which is why birds universally rely on internal development.
Conclusion
In summary, birds exhibit internal development with eggs providing the protective, nourishing environment for embryos to grow. The fertilized egg contains the specialized membranes, nutrients, and structure needed for a chick to fully develop its organs, tissues, skeleton, feathers, and other systems until ready to hatch. This internal incubation period allows birds to develop safely despite their fragile structure as embryos. Without the isolation of the egg, birds could not survive external exposure during these formative developmental stages.