In the first days of spring, birds undergo a spectacular metamorphosis. After a long winter of migration and peaceful coexistence, they suddenly begin to sing with all their might, varying each series of notes as if it were an audiophonic novel. They cannot bear the presence of other birds and begin to threaten and attack them if they cross a border, which might be invisible to human eyes but seems perfectly tangible to birds. Is this display of bird aggression just a pretenc