In this question, there are no options given to choose from. So i would answer this question based on my knowledge. The most likely cause of the observed variation is adaptation and evolution. The birds of that island adapted to the food source and then they evolved as per the requirement of the place and food source.
        
             
        
        
        
Are there choices to choose from, because that is a very general question?
        
             
        
        
        
Answer:
genes
Explanation:
literally just answered this question i guessed and got it right 
 
        
             
        
        
        
There are four main types of tissue: muscle, epithelial, connective and nervous. Each is made of specialized cells that are grouped together according to structure and function. Muscle is found throughout the body and even includes organs such as the heart. Our outer layer of skin is epithelial tissue.