When you are hatching chicken eggs, it’s difficult, if not impossible to see how things are going inside the egg from the outside. Candling is the process of looking inside the egg without disturbing the developing embryo. By using a bright light it is possible to look into the egg and make sure it is developing properly.
Candling Eggs Before Incubation
Candling a chicken egg before it goes into the incubator won’t tell you much, but should be done anyway so that you have a baseline for comparison. You can’t tell fertilized chicken eggs from unfertilized eggs by ordinary candling, but you can see what an undeveloped egg looks like so that the changes are more evident when you candle the egg later. You’ll also be able to see the air space at the large end of the egg. You’ll also be able to tell if there are any cracks in the egg too small to see normally. These micro-cracks will reduce the likelihood of the egg hatching since they make it easier for bacteria to enter the egg and may interfere with the egg retaining the proper moisture level.
Which End is Up?
Candling the egg to find the airspace before it is placed in the incubator is also important if you have an egg that is rounder than usual so that it is difficult to tell the large end from the small end. The large end will have the air space in it and should be placed higher than the other end of the egg. Egg placement and treatment while in the incubator is critical to proper hatching. For directions on hatching eggs go here.
Maintaining Temperature Stability
In the first few days after the egg goes into the incubator and begins to develop in earnest, things are generally too small to see clearly unless you have a really good candling set-up. Even if you could see what was going on in there, developing chicken eggs are temperature sensitive, so it would be a bad idea to pull them out of the incubator and upset the temperature balance on a daily basis. When you do candle the chicken eggs, try to get everything done as quickly as possible so that they are only out of the incubator for a few minutes. Bear in mind that opening the incubator will release all the warm air inside and it may take a while to get back up to temperature. Resist the temptation to adjust the thermostat for at least a couple of hours after opening the incubator for candling. It will likely return to the correct temperature on its own.
Egg Candling Device
A basic candling device can be made with any bright light shining through a hole of about one inch in diameter. In a dark room, the egg is placed over the hole to block all the light from escaping. The result of this is that the light penetrates the egg shell and lights up the egg from the inside. Brighter lights work better, but bear in mind that powerful light bulbs give off quite a bit of heat and may even set fire to cardboard or other combustibles. Compact Fluorescent Bulbs (CFL) should NOT be used for this because of the risk of breaking and releasing mercury. Candling will give better results if the room is darkened.
Day Seven Candling
On day seven, after the eggs have set, (been brought up to incubator temperature) there should be enough development to determine how the embryo is developing. It is helpful to have the eggs numbered so that candling results for each egg can be recorded and tracked against the eventual results. The first thing you might see when candling the egg is nothing. If the shell is particularly dark or thick, it may be difficult to see anything much inside the egg. Even in these eggs, however, the air space at the large end (top) of the egg should be visible. The air space will get a little bigger over time. It is interesting for kids to mark the outline of the air space by candling before the egg goes into the incubator and to compare that line with the air space size on day seven. Eggs too opaque for good candling should be treated as if everything is going well and left in the incubator.
Eggs with No Visible Development
If the shell is not too opaque for good viewing, you should be able to determine that the egg is in one of three states. The first possibility is that no development has occurred. Upon candling, the inside of the egg will look just like it did before the eggs were placed in the incubator. They’ll be relatively clear with no distinct features. This can happen for a variety of reasons such as the egg wasn’t fertilized, it wasn’t stored properly before going into the incubator, it was shaken roughly, or a variety of other reasons. Mark these eggs and check them again at day 14. If there is no change by then, you can remove them and discard them.
Signs of Properly Developing Eggs
If an egg is developing properly, on day seven you may see a clear demarcation of light and dark within the egg. Candling devices of lesser brightness may show the top half of the egg as darker while the bottom half remains clear. A visible network of fine blood vessels reaching toward the air space is a very good sign that everything is going perfectly. If you have a good candling device, you may be able to see the embryo itself. The eye spots will be the darkest and are the easiest part of the embryo to see during candling. If you’re lucky, you may see the embryo moving. Record the results for these eggs as well.
Eggs with Arrested Development
The third possibility is that the eggs started to develop and then stopped for some reason. This is a little harder to determine for the novice. The telltale sign here is a well-defined ring, called a blood ring, that runs around the outside of the shell, generally in the upper half where the developing embryo would have been located. This line may look like someone drew a circle on the inside of the shell with a fine point marker. Chicken eggs may stop developing for a variety of reasons ranging from poor temperature or humidity control in the incubator to bad genetics. Perfectly healthy adult chickens may pass on recessive genes that may be lethal in homozygous embryos. Unless you are sure that the embryo has stopped developing (has died), then mark the egg as suspect, but leave it in the incubator and treat it as you’d treat the others.
Candle Questionable Eggs a Second Time
If all the eggs appear to be doing fine on day seven, then you need not open the incubator again (except for turning the eggs daily) and for maintaining the water level for proper humidity until day eighteen when all turning should stop and the humidity should be raised according to the directions that came with your incubator. If you had eggs that showed signs of no development or arrested development when you candled them on day seven, then you can candle the suspect eggs a second time on day fourteen. At this time, any egg that is still largely clear and appears unchanged from the way it was before it went into the incubator can be removed and discarded. Eggs that still show a clear blood ring AND no change since day seven, can also be removed and discarded. When in doubt, leave the egg in the incubator. Even eggs that looked fine when candling may not hatch; it is very rare to get a 100% hatch rate. If they don’t hatch on day twenty-one, give them another three days, sometimes they’re a little slow, especially if the average incubation temperature was a little low.