The Territorial Birds 
SINE
Birds are Aura’s department, but when I'm outside looking for rocks, I’ve seen them sing to warn off intruders. I guess that’s easier than taking on every one that crosses your path. Aura came up with some reading matter on the subject if you’re interested.
Eliot Howard, Territory in Bird Life
Predecessors Altum (1868) and Moffat (1903) also made similar observations, though their ideas were not accepted at the time
Ernst Mayr, Bernard Altum and the territory theory
C. B. Moffat, Spring rivalry of birds