It comes from a brown-headed cowbird. The house finches, goldfinches, sparrows and lovebirds all tolerate this visitor. He seems innocuous enough... but I am not sure I am pleased.
Cowbirds are brood parasitic birds. They lay their eggs in other birds' nest. Their young are fed and raised by the 'host' parents. If the host 'knocks' the cowbird egg out of the nest, the adult cowbird may/often return to destroy the host's own eggs.
It "seems that Brown-headed Cowbirds periodically check on their eggs and young after they have deposited them. Removal of the parasitic egg may trigger a retaliatory reaction termed "mafia behavior"." (Hoover, Jeffrey P. &. Robinson, Scott K. (2007). "Retaliatory mafia behavior by a parasitic cowbird favors host acceptance of parasitic eggs". PNAS 104 (11): 4479–4483).
So it seems odd that the other birds at our feeders and in our garden sit along side the cowbirds. They eat side by side, they hop around on the grass together. They don't fight one another, nor do they scream or dive-bomb each other.
(Photo above shows a cowbird and a lovebird sharing a tree)
Cowbirds might be like some relatives... drop the kids and run. And I wonder whose behavior came first - the bird's or the relative's?
You can bet I will be watching bird behaviors more closely now...