Raptor Propagation: What goes on in those eggs, anyway?

As captive breeders, we carefully monitor the progress of our eggs to insure that the embryo is developing properly and make adjustments in the care of the egg when need be. The eggs are weighed and candled on a regular basis.

By the time they hatch, avian eggs lose approximately 18% of their fresh egg weight, or the weight of the egg upon laying. Weight loss is a good measure of the health of a chick and is charted and graphed along the way.

Ideally, eggs are incubated by the parents. When this is not possible, the eggs must be fostered by another bird or incubated (see picture of incubator below) and carefully managed. To manage weight loss, we weigh the egg every three days and adjust the humidity inside the incubator to help speed or slow the weight loss. If an egg loses weight too quickly or slowly, it may lead to problems for the embryo.

Candling allows us to observe the contents of an egg without breaking the shell. Initially, we candle eggs to determine fertility as well as the extent of incubation. Depending on the egg’s pigmentation, fertility can usually be determined in 5-10 days after laying, with darkly pigmented eggs taking the longest. The eggs must be handled gently and candled for no more than 10 seconds to avoid over-heating the embryo.

An egg candler (above) is a small machine that puts out a narrow beam of light. When candling eggs, the room must be completely dark. An experienced candler makes several observations of the egg:

First, the shape and condition of the air sac is noted. A change in the air sac is one indication of possible fertility.

Next, we look for a shadow, or “half-shading,” at the top half of the egg. Half-shading is caused by a network of capillaries and veins that occur as an embryo develops. The half-shading should be reddish in color.

At 10 days, the air sac should be noticeably larger and half-shading very pronounced. At this point a small embryo or “eye spot” is visible, floating in the egg.

By 15 days, the network of blood vessels is highly visible and the embryo is moble, often responding to bright light with a swimming motion.

Towards the end of the incubation period (approximately 30-31 days) the air sac expands down one side of the egg in a process called “draw down.” Shortly after draw down, the chick breaks through the membrane with its head in the air sac and begins pipping at the shell.

In this picture, you can compare the relative sizes of a Harris Hawk egg (right) and a Bald Eagle egg (left).


For further reading, I recommend James D. Weaver’s Falcon Propagation: A Manual on Captive Breeding, which was a valuable source for this post.