When I get a nice, up close look at a Loggerhead Shrike such as this one I simply enjoy the view--and am grateful that I am not a lizard, rodent, bug...you get the picture. But when I see said Shrike from a distance, I find it rather easy to be mislead into believing that I am seeing a Northern Mockingbird. Color and pattern are similar--grey with a white wing patch; perch location is similar--tops of trees; and topography is similar--open fields. (Shrikes do appear in urban settings, but far less frequently than Northern Mockingbirds, so I am usually pretty careful before claiming to have seen one in the city).
With experience--and magnification--they really aren't all that similar. Their Gestalt is in fact rather different. While the Northern Mockingbird (and yes, there are southern Mockingbirds--but none are called Southern Mockingbird) is long and slender, the Loggerhead Shrike is long and rather rectangular. In addition, Shrikes in general have large, fat heads while Northern Mockingbirds have rather flatter, smaller ones (all this in comparison to their respective body sizes). The bill shapes, too, continue this distinction as the Shrikes have rather fat, hooked bills while the Northern Mockingbirds have slimmer, thrush-like ones. Lastly, their calls differ: the Loggerhead Shrike has a buzzy one while the Northern Mockingbird sounds more like a 'chack' 'chack'. Most helpful, however, is their song: while the Shrike has a repetitive, buzzy note, the Northern Mockingbird indulges in a wide variety of songs--from mimicking car alarms, to alarm clocks, to mourning doves...you can pretty much count on the fact that if you hear a 'weird' song, you're listening to a Northern Mockingbird. Now a real challenge would be an urban Shrike being imitated by a nearby Northern Mockingbird....I'll post that if it ever happens!