First, there are defects in the structure of the eggshell and the eggshell membrane. The fibers of the eggshell membrane are loose, the thickness is insufficient, the gap between the eggshell papillary layer increases, and the density of the palisade layer is low, which leads to easy leakage of moisture. 

The other issue is the quality of the egg white. The concentration of the egg white is low, the strong protein has weak water retention ability, and there is excessive free water, which easily penetrates into the papillary layer of the eggshell, the pores, etc.

In summary, the formation of dark spot eggs is caused by abnormal structures of the eggshell and eggshell membrane, as well as the leakage of water, which ultimately leads to this phenomenon.

The three main causes of dark spot eggs: 

1. Physiological and disease factors. 

Older laying hens (generally those over 50 weeks old) experience a decline in physiological functions, resulting in a decrease in eggshell quality; Inflammation of some organs such as the fallopian tubes can also damage the function of the eggshell glands, thereby causing dark-spotted eggs.

2. Factors related to the breed  

The incidence of dark spot eggs in brown-shelled egg-laying chickens is higher.

3. Environmental and Management Factors  

Sudden strong light, noise or fright can cause stress in the chicken flock, leading to abnormal contractions of the oviduct and disordered calcium secretion. As a result, spots will form. In addition, if eggs are stored in a high-temperature and high-humidity environment (with a temperature above 25℃ and a humidity above 70%), it will also accelerate the migration of moisture. If the storage time is too long (more than 30 days) or the humidity is too high (> 75%), at this point, the carbon dioxide inside the egg is lost, the pH of the egg white rises, and the water permeability increases.

4. Nutritional Factors  

Imbalance in the calcium and phosphorus ratio in the feed, deficiency of vitamin D3 and trace elements such as manganese, zinc, and copper, affect the calcification of eggshells and the synthesis of shell membranes. Excessive calcium forms calcium spots, while insufficient calcium results in thin and brittle eggshells with uneven pigment deposition. Insufficient available protein or amino acids in the feed also leads to a weak shell membrane structure.