Establish territory - usually done by males but sometimes also by

females.  The territory must be both announced to potential mates and  
defended from competing birds of the same gender.  In some cases the  
territory is also defended against birds of another species.  This  
may be part of the secret behind mimics such as thrashers and  
mockingbirds, they mimic species they don't want for close neighbors  
(perhaps jays, ect.) to discourage the unwanted species.