How to send an email to a node author after selecting a specific value in a selected list field? Actually this is my content and I have one select list field in it. If I select a "New litter" and click on save, I have to generate an email to the author. If however I select any other value for that field (and click on save), then there is no need to generate an email. 

How can I do this?
You can use hook_node_presave() and in this function check if the select value is "new litter" and generate email.

function YOUR_MODULE_NAME_node_presave($node){
    if($node-&gt;type == 'YOUR_CONTENT_TYPE'){
        //get your field value
        $field = field_get_items('node', $node, 'YOUR_FIELD_MACHINE_NAME')[0]['value'];
        //check if value equals "new litter" key
        if($field == 'NEW_LITTER_KEY'){
            generate_email(); 
        }
    }
}


change uppercase strings with your module and machine names