When a chalk-browed mockingbird is annoyed with the presence of intruder, it lets him know by pouncing on and yelling at him, sometimes with overt attacks. No matter the size of the intruder, the mockingbird confronts him all the same. We have seen the mockingbird face raptors and other unfeathered wild fauna in the reserve, like the black-and-white tegu lizard and the urutu pit viper. It is not always successful. Sometimes it does drive the intruder away, other times it gives up and leaves, but this does not discourage the mockingbird from trying again on another occasion. A mockingbird may act alone or may be assisted by other mockingbirds to stalk the intruder.
A few birds of the reserve display this aggressive behaviour. No doubt the chalk-browed mockingbird is the most contentious of all the passerines in the reserve.