The black cat is a local one
THe white cat is the stray
Birds nest is near by
White cat tried to get to the nest
And that is why the birds are so concerned with the white cat. It is the only source of danger to the nest as far as the birds are concerned.
The fight between the two cats is because of the stray invading the local cats space.
The two birds are not concerned at all with the fight between the two cats. THey only want to get rid of the stray and move him away from the nest.