Candling: Shining Light on the Development of the Avian Egg

The breeding season at the Keauhou Bird Conservation Center usually begins in the middle of March and continues until August. This is the most rewarding time of the year for our staff and interns, who work tirelessly during the rest of the year to ensure that the endangered Hawaiian birds under our care are in prime condition for breeding. Careful consideration is put into aviary setups, diets, and pairings, all to give the birds the best opportunity possible to successfully reproduce. Caring for newly hatched chicks is a demanding and delicate job. If you'd like to read more about chicks and their care you can read a previous blog I wrote, Hand-rearing Hawaiian Honeycreepers.

For now, let's take a step back. Before there was a chick, there first was an egg. Eggs are perfectly contained systems that allow an embryo to safely develop and then, when the time is right, hatch! Until the chick hatches though, how do we know what is happening inside of the egg? To study the developing embryo we rely on a technique known as candling. This involves shining a very bright light into an egg so that the progression of development can be observed and recorded. Before machines designed for candling eggs were invented poultry farmers relied on candles to do this job, which is where the technique got its name.

Speaking of poultry, chickens have roughly the same incubation period of 21 days as the ʻAlalā, or Hawaiian crow, a corvid endemic to Hawaiʻi Island that has been “Extinct in the Wild” since 2002. The incubation period of the chicken provides a nice reference to draw comparisons from regarding embryo development.

When an egg is brought into our incubation room one of the first steps we take, after weighing the egg, is to candle it. This allows us to carefully examine the shell for any irregularities or damage. Candling illuminates tiny cracks or punctures in the egg that can otherwise be difficult to see. It is essential that any damage to the shell is repaired, or the egg will lose weight in the form of water evaporating too quickly. This will result in a weak and dehydrated chick that may be unable to hatch. Any compromises to the integrity of the shell are also avenues for bacteria and other foreign contaminants to enter the egg, which could result in the loss of the developing embryo or a chick that hatches with an infection. Repairs to the eggshell are done with non-toxic glue.

During this time we also look at the air cell of the egg. The air cell is formed at the large, rounded end of the egg immediately after it is laid. Over the course of incubation the air cell will grow larger as water evaporates from the egg and escapes through thousands of microscopic pores in the shell. These pores also facilitate the diffusion of oxygen and CO2 in and out of the egg. Eventually the air cell will be where the chick takes its first breath when it begins to hatch. If an egg receives parental incubation before it can be candled, but it is unknown exactly when incubation began, the approximate age of the embryo can be determined by evaluating the size of the air cell.

If sperm from a male bird is not present in an egg, the egg is called infertile. The yolk will have a tiny white dot on its surface called the germinal vesicle. This is the nucleus of the egg. If sperm has made contact with the egg then it is fertile, and this layer of cells is referred to as the blastoderm. Instead of a small, white dot, the blastoderm will look more like a bullseye. This stage in early-embryonic development cannot be seen by candling the egg, but is visible with the naked eye if the egg is broken open, although we don’t break open a developing Hawaiian bird egg! After a fertilized egg has been incubated for roughly 24-48 hours the yolk will appear much larger in size. We refer to this as being “spread”. The blastoderm on the yolk will enlarge and protrude outwards. This is the area where cell division is occurring and the embryo is beginning to form. This enlargement on the yolk, commonly called a “bump,” can occasionally be visible when an egg is candled.

However it can often be difficult to see, especially with small eggs or eggs with a pigmented shell. Fertility in chicken eggs is usually evident at day 3 or 4. However, ʻAlalā eggs are light blue in color with lots of reddish-brown blotching, making it more difficult to determine. It is after six 24-hour incubation periods that we are generally able to determine if an ʻAlalā egg is fertile. At this point in development the embryo will be visible upon candling. It will have a tiny, beating heart and small circulatory network of veins surrounding it. This is commonly referred to as a “spider” because of its similarity in appearance to the arachnid. Movement from the embryo is generally always seen during candling and recorded. The eyes will have pigment by now and can also be seen. The presence of eye pigment is an indicator that the embryo has progressed from early-embryonic development to mid-embryonic development.

From mid-embryonic development to late-embryonic development, eggs are candled by incubation staff to monitor a few particular developmental milestones. One of these is when the circulatory network of veins completely surrounds the air cell of the egg. Air cell circulation is usually complete after 8 days of incubation for ʻalalā. The development of the chorioallantoic membrance, also called CAM, is also monitored.

The CAM is the fusion of the chorion and the allantoic membranes. It is a large, vascular network that functions to deliver oxygen to the embryo and remove wastes, like CO2 and uric acid. It also is responsible for transporting calcium to the embryo for bone formation. The CAM for bird embryos is the equivalent of the placenta in mammal embryos. The CAM will visibly fill the egg, from the air cell all the way to the pointed end, or apex, of the egg. The CAM is generally completely formed for ʻalalā after 13 days of incubation. By late-embryonic development the embryo will begin to occupy most of the available space within the egg. The egg will appear darker as incubation nears the end because light is unable to pass through it. The chick’s first downy feathers are occasionally even visible. On approximately day 20 the air cell of the egg will become uneven and extend further down one side of the egg than the other. This is known as drawdown and is the first indication that the hatching process has started.

Check back again for an upcoming blog all about hatching!