There are several things that you can do to help a bird gain weight, but the very first thing that should be done if you suspect weight loss in your bird is to schedule an appointment with your avian vet to get a full checkup done as soon as possible. A number of serious health problems can be the cause of weight loss in birds, and in order to preserve your pet's health, any noticeable drop in weight should be thoroughly investigated by a licensed veterinarian to rule out any illness or disease.
Identify the Reason
Odds are that your veterinarian will be able to identify a reason for your bird's weight loss and recommend a course of action for you to get your pet back to a healthy weight. If this happens, of course, you should follow your veterinarian's advice to the letter. However, if no problem or illness is identified in your feathered friend, it may just need a change in diet in order to get back to a healthy and ideal body condition. Introducing larger amounts of some of your bird's favorite foods may be all that's necessary to stimulate your pet's appetite.
Weight Gain Diet
One food that has been proven to pack a heavy punch of both calories and fat is peanuts—and luckily, most birds absolutely love them. Upping a number of peanuts in your bird's diet will increase your pet's fat intake, helping it to hang on to a few more calories and gain weight as a result. Just be careful not to overdo it. Birds are notoriously picky eaters, and many have been noted to pick through their bowls and eat only their favorite items, even when they're served a mix of great variety. This can cause nutritional deficiencies and is one reason why pelleted diets are widely recommended, in addition to fruits and vegetables.
Another thing that you can do to help your bird put on some weight is to offer some extra sunflower seeds either as part of its dinner or as treats throughout the day. Also high in fat, these tasty little seeds are a favorite of many types of birds. You should have no trouble coercing your bird into gobbling them up.
Millet spray is another favorite among pet birds and is also a higher calorie treat that could help your pet gain weight. Caution should be used to avoid feeding too much millet, however. While it can help your bird pack on weight, it is not as nutritious as some other options may be and contains what you could refer to as "empty" calories. To help make this treat more nutritious, you might try incorporating it into a recipe such as a birdie bread. The added ingredients will help you make sure that your bird is getting a wider variety of important vitamins and minerals.
Invest in a well-made bird scale and keep a log of your bird's weight every day to every week. Any time you notice a fluctuation from your bird's normal weight amounting to an ounce or more in either direction, your best bet is to check with your avian vet to make sure that your bird's diet is on the right track. Failure to do so could have devastating effects on your feathered friend.
Miscellaneous Diseases of Pet Birds. Merck Veterinary Manual.
Fruits and Vegetables in Birds' Diets. VCA Hospitals.
Cockatiels. VCA Hospitals.