Crop Milk
Doves and pigeons along with some species of parrots are the only birds that produce crop milk. 
Studies have shown it is manufactured in the crop and the process beings while the parents are brooding the eggs. Soon after sitting the eggs the cell division of the crop increases about 600 percent. After about week the process increases at a higher rate. 

When analyzed the milk was found to contain About 70 percent water. The other 30 percent contained, 38 percent fat, 58 percent was protein and 98 percent of this protein, is true protein.  
Also found was small amounts of ash, calcium, phosphorus sodium, potassium,
lecithin, and all of the known B vitamins. 
One teaspoon also contained as much vitamin A as drop of cod-liver oil.

By the way both the parents Hen and Cock produce it.