Energy is the ability to do work. The form of energy that living things need for these processes is chemical energy, and it comes from food. Autotrophs make their own food. Heterotrophs obtain food by eating other organisms. Organisms mainly use the molecules glucose and ATP for energy.
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I think that birds have different bill lengths because of their habitats and their diet. Also natural selection and evolution has also played a factor due to the fact that birds have not always looked the way that they do now, for example the Archaeopteryx a  bird that lived around the time of the dinosaurs had a shorter and more powerful bill to grind insects and small mammals
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This means having no charge
        
             
        
        
        
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Autotrophs are the organisms which have ability make their own food using various sources of energy. Examples are can green plants and algae such as <em>Spirogyra, Chlorella, Volvox</em> make their food using sunlight carbon dioxide and water.  they are photoautotrophs
Some bacteria are also make their food using chemical energy through oxidation. They are chemoautotrophs.