Coltsfoot is called eggs without adequate calcification of the shell. The etymology derives from the word "falgalala" expression that was used in the Iberian Peninsula between the eighth and fifteenth centuries to describe an internal fabric of an egg.

This alteration may occur at the beginning or end of the laying period, with no apparent pathological cause, but when the birds have reached 15% of lying, presentation of eggs in coltsfoot is linked to changes in food-related deficient intake of calcium, phosphorus and magnesium or vitamin D3 and calcium loss with hyperventilation in hot season.

Other less common causes include injuries, secondary infectious bronchitis virus or adenovirus type III (egg drop syndrome) and even less common are genetic causes. In the case of eggs in coltsfoot by viruses they must be presented with calcified eggs but deformed shells.