Because it's an available, memorable domain name with literary and classical connotations and a mildly amusing personal link.
Tasso was a poet of the Italian Renaissance and his name is associated with an oak tree (click for more). By coincidence, we live on Tasso Street, just beside an immense oak tree, so "Tasso's Oak" is a fitting symbol for our self-publishing ventures. But apostrophes aren't allowed in domain names, and "tassosoak.com" reads as "tasso soak" -- not good. Hence the hyphen.
The bird is an Acorn Woodpecker.
tassos-oak.com is hosted at flighthost.com.