Unthrifty feather is a term used to describe a poor quality or defective feather that is not suitable for use.

Unthrifty feathers in birds can occur for a variety of reasons, including:

1. Natural molting: Birds naturally shed old feathers and grow new ones in a process called molting. During this time, new feathers may be shorter, paler, or otherwise different from the mature feathers they will eventually replace.

2. Poor nutrition: A diet lacking in essential nutrients can result in feathers that are weak, frayed, or discolored.

3. Infections: Bacterial or fungal infections can cause feathers to become damaged or fall out.

4. Physical trauma: Injuries from fights with other birds, predator attacks, or other sources of physical trauma can result in unthrifty feathers.

5. Environmental stress: Exposure to high heat or humidity, lack of sunlight, or other environmental stressors can result in unthrifty feathers.

6. Disease: Certain diseases, such as avian pox or psittacine beak and feather disease, can result in abnormal feather growth or feather loss.

Overall, unthrifty feathers in birds can be a sign of an underlying health issue and may indicate that a bird is not in optimal condition. It is important to consult with a veterinarian if you notice any changes in your bird's feathers or behavior.