Generally speaking, any food produced by a non-kosher animal is non-kosher. Thus, the egg of a non-kosher bird is not kosher but the egg of a kosher bird, such as a chicken, is kosher. If one comes across an egg and does not know which bird laid it, how does one tell a non-kosher egg from a kosher egg?
The Talmud gives us two ways to tell if an egg is of a non-kosher bird. If the egg is totally round like a ball rather than round at one end and tapered at the other end, a non-kosher bird laid the egg. So, too, if the yolk of the egg surrounds the white of the egg rather than the other way around, or if the egg has no white but is filled with yolk. If, however, the egg is round at one end and tapered at the other, the white of the egg surrounds the yolk and it looks like the egg of a chicken or of another identifiable kosher bird, the egg is kosher and may be eaten without any further investigation as to its pedigree.
In accordance with the general principle articulated above, the egg of a neveilah chicken that died before it was properly slaughtered, as well as the egg of a treif chicken that suffered from one of the treif-rendering defects previously discussed, is prohibited for consumption.
If a non-kosher egg, such as the egg of an eagle, was cooked in the same pot as kosher eggs, the kosher eggs remain kosher and may be eaten provided the non-kosher egg was cooked in its shell. The reason for this is that a kosher item cooked with a non-kosher item only becomes non-kosher if it absorbs the taste of the non-kosher item cooked together with it.
The shell of the non-kosher egg acts as a buffer through which the taste cannot be transferred to the kosher eggs. If, however, the non-kosher egg is taken out of its shell or its shell is cracked and it is cooked with the kosher eggs, the kosher eggs become non-kosher and cannot be eaten even if the kosher eggs remain in their shells unless the ratio of the kosher eggs to the non-kosher egg is at least 61 kosher eggs to the 1 non-kosher egg.
What is the status of an egg with bloodstains or blood specks?
The basic principle here is that the Torah only prohibits the consumption of blood of meat but not the blood of eggs. Nevertheless, there is a concern that a blood spot in an egg may indicate the presence of the beginning of the formation of the embryo of a chick. Although a hatched chick would be permissible for consumption if properly slaughtered, the embryo of a chick inside an egg of a bird, even a kosher bird, is forbidden for consumption. Such a blood spot that raises the concern of the existence of the beginnings of an embryo in the egg is referred to in the halacha as dam rikum.
The concern of dam rikum, however, is only present in the case of an egg laid by a chicken that mated with a rooster but it is not present in the case of an egg laid by a chicken that was not fertilized by a rooster. A fertilized egg may hatch into a chick after the hen has sat upon it for a period of three days whereas an egg that was not fertilized by a rooster, referred to in the halacha as beitza hamuzeret, will never hatch into a chick never mind how long the hen sits on it. Accordingly, any blood found in an unfertilized egg cannot be dam rikum and under Torah law may be eaten.