A Long Island World War II vet has been sentenced to 30 days for shooting a squirrel in his yard.

William Thomas. 81, of East Northport was convicted of cruelty to animals in February after a neighbor complained that he was killing animals with a BB gun in the back yard of his Confederate-flag-festooned home.

Yesterday was his first full day behind bars, and Thomas, who suffers from dementia and various medical ailments, was hospitalized for observation, his ex-wife told The Post.

“He’s 81 years old,” said the ex, Elizabeth Thomas. “I think they should have looked at his condition more closely before they sent him to jail.”