Brood parasitism sets up an arms race. The parasite gains if the host raises its chick; the host gains if it can detect and remove the egg. Most small songbirds fail at detection, which is why cowbird chicks are commonly raised by warblers and vireos that ought to know better.
Catbirds are among the reliable rejecters. Experimental studies placing real or model cowbird eggs in catbird nests recorded high ejection rates, with the host grasping or puncturing the egg and carrying it out. Because a cowbird egg differs from a catbird's plain blue-green egg in both colour and spotting, the discrimination task is relatively easy, and catbird rejection is close to complete in many samples.
The wider pattern is worth stating carefully. Rejection ability is not simply a matter of intelligence or exposure. Hosts with a long shared history and with easily distinguished eggs tend to reject; hosts newly encountering the parasite, or whose own eggs resemble the parasite's, often do not.