Nutritional deficiencies can occur for various reasons. From parent stock, feed consumption (improper mixes) to malabsorption issues within a particular bird. Sometimes you can resolve these with vitamin supplements or changing the current diet. Typically vitamin supplements are a good start to see if the bird shows improvement (straightening of the toes) and responds to treatment. If they do, then you need to identify the source of the problem to ensure the bird's nutritional needs are being met.