Complex sentence: Sibley and Ahlquist, though, put penguins in the same super "family" as divers (loons), tubenoses, and frigatebirds; that is, penguins are closer to those birds than herons are to storks.
Split into simpler sentences: Sibley and Ahlquist, though, put penguins in the same superfamily as divers (loons), tubenoses, and frigatebirds. According to their view, penguins are closer to those birds than herons are to storks.