Birds choose food with highest nutrition. Such “nutritional wisdom” allows animals to make a diet choice that satisfies their physiological needs. For example, carotenoids are immuno-stimulants and also help with feather pigmentation. Therefore, colorful species will seek out food with high levels of carotenoids.
Mealworms left to feed just on the bran in their container don’t contain as many carotenoids as my mealworms which are fed a variety of fresh fruits and vegetables every 2 weeks. Although there is no specific information on the how the birds can tell which mealworms are cartenoid-rich, it’s thought that the preference may be based on smell or taste.
You can help your birds out too by feeding your mealworms right before they are presented to the birds. Some good foods to feed mealworms are bananas, apples, carrots, sweet potatoes, dark leafy greens, broccoli or cantaloupe.
Source: Specific Appetite for Carotenoids in a Colorful Bird - http://www.plosone.org/article/info:doi%2F10.1371%2Fjournal.pone.0010716
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