Is Grit Really Necessary?Simply stated, no, grit is not necessary in the diet of finches, and in fact can cause health problems.
In order to understand the reason why, one must first understand what grit is and what purpose it serves in the body of the bird. Grit is defined as a dense, insoluble, granular, mineral material.5 Normally formed from granite or quartz, grit does not dissolve in the body, but rather becomes trapped in the ventriculus of bird which consumes it.5 Because oyster shell and related substances dissolve in the avian body, they cannot serve as true grit products.2
Grit's purpose is to aid in the digestion of whole, intact seeds. These grains contain a fibrous coating (often called the shell or hull) which is so inert that digestive enzymes are relatively ineffective against it.5 In the bird's ventriculus, grit helps to grind whole seeds and provide a substrate that digestive enzymes can act upon.5 Only birds which consume whole, intact seeds require grit.5 Examples of such birds include: doves, pigeons, free-ranging gallinaceous species, and Struthioniformes, species which naturally eat whole grains as a portion of their diet.5
Normal, healthy passerine birds do not require grit in their diet because they remove the fibrous hull of the seed and only ingest the kernel.5 The kernel is easily acted upon by the digestive enzymes,5 leaving grit without a purpose in the bodies of these avian species. Therefore, grit is not essential7 to finches. Furthermore, grit is not even beneficial to these passerine species, as a study done with canaries (which are closely related to finches) shows. The canaries were divided into two groups, one which was given access to soluble "grit" and the other which was not given access to any form of grit. No significant differences were found between the two groups regarding food intake. Additionally, this study found that insoluble grit (true grit) had absolutely no effect whatsoever on digestibility values of feed.
Not only does grit lack any real value in the diet of passerine species, but it also may lead to health problems. Some avian species will consume too much grit and become impacted7 or suffer from other digestive disturbances.2 These crop or gastrointestinal impactions most often develop in birds which suffer from health problems or depraved appetites and ingest large quantities of grit.5 With no perceived benefit, and the real potential for risk, ad libitum feeding of grit should be avoided.5