Finches are song birds living primarily in the Northern Hemisphere, although some subspecies live below the Equator. Finches are classified as passerine, or perching, birds of the Fringillidae family and were a central part of Darwin's theory of evolution. Darwin noticed how the shapes of finch beaks on the Galapagos Islands changed to adapt to food sources. Finches eat a range of food, but have evolved to consume whatever food source is naturally available.
Seeds make up the vast amount of a finch's diet. In the wild these seeds are either taken from the ground or directly from the plants. In captivity bird seed can be purchased at any pet store. In captivity millet seeds are a good food source, but need to be supplemented with other foods. During the winter more oily seeds will be required to help the bird gain body fat. Sprouting seeds are also a great food source as the seed provides the base nutrition and the sprout extra vitamins.
Berries and Fruit
Seeds are not a food source on their own, as they do not contain enough vitamins to sustain a finch. In the wild, as with the seeds, finches will eat berries off plants and trees and have evolved to be able to take advantage of food sources. In captivity it is more of a case of finding out what a particular finch likes. Apples, oranges and bananas are a good place to start, but it is a case of trial and error.
Vegetables and Plants
Anther good source of vitamins for finches is vegetables and plant leaves. Cucumbers, broccoli and grass are all popular foods for finches. It is important to make sure the vegetables are organic, as pesticides could be harmful. Sprouting plants are especially good for finches, as the young shoots are particularly nutritious. Young and spouting grasses, chickweeds and thistles can all be used. Sprouting plants give finches seeds as well.
There is a lack of protein in a vegetable-only diet, and like other birds finches will also eat small bugs found on the ground. Maggots and mealworms can be bought from any good pet food store and can be fed periodically to the birds. It is important not to feed them insects from the back garden, as they may be carrying parasites, whereas shop-sold live foods are treated for potential disease.