A metal head egg is a biomechanical vessel from which metal heads are born, appearing in Jak II and Jak 3. There are five different types of metal head eggs that appeared in the series, most commonly in pod shape and dark gray and/or green in color. It is unclear if the difference between these eggs reflect its stage in growth or the different species of metal head, though some appear to be more common than others.
In most cases, however, metal head eggs are indestructible through common means, such as the Morph Gun (only the drill platform eggs are destructible with only the Peace Maker). Only heavy weaponry such as the gunpod (shown in "Destroy eggs at drill platform"), Sig's Peace Maker, or the weapons on buggies (shown in "Destroy eggs in nest") were capable of destroying a metal head egg. Pure force is also sufficient, as was shown in the mission "Destroy eggs in strip mine", when a cluster of eggs were crushed by a steel crate.
The most common egg type is an ovoid pod with a metallic gray color, and a green, ovular, "window"-like device in the front. This type of egg is commonly grouped together with others and plastered on surfaces such as a wall or other large, stable objects. These eggs in particular have been known to drain eco from the surface on which they lie. These egg pods constantly rattle, or vibrate, sometimes emitting a noise. Other similar egg pods grow on large stalks and can be found in places rich in dark eco or with a landscape typical to metal head infestations such as Metal Head city.
Another type of metal head egg is the metal-pede egg, which is a cone-shaped egg pod, completely green in color, with an acidic defense system that will fire hazardous projectiles in random directions. These are connected to a network of tubes, and when the eggs are destroyed, these tubes emit a green gas that is harmful to humans. These eggs were only seen during the mission "Destroy eggs in nest" when Sig and Jak were sent by Damas to the Metal Head cave to eliminate them.
In Jak II, during the final boss fight with Metal Kor, it is shown that the Metal Head leader is capable of producing stinger eggs. These in particular are hollow, glass-like (yet flexible) translucent egg pods with small stingers inside of them. If the egg is destroyed prematurely, the stinger will not emerge, otherwise, a stinger will appear in a matter of seconds from being deployed. These eggs appear to grow on the backside of Metal Kor, and when they are mature, he will shoot them from his back onto the ground.
A rarer egg type seen in the Metal Head tower near the end of Jak 3 is a translucent tank-like construct connected to various tubing and clustered with many other eggs. The larger eggs in these clusters contain a premature metal head. The exact species of metal head remains undetermined, though it closely resembles the body and head of a mantis.