Nessy - my neighbor used an existing pattern on the internet and improvised the turtleneck by extending the rows of stitches upward, decreasing them as she got to the top. I had her make it about eight inches long so I could fold it over because we all know how chickens telescope their necks.
By the way, the hen pictured appears to be very cute and fashionable in her sweater, but she hated it and pulled all the stitches out trying to get the thing off. I had my neighbor modify it to a simple neck gator that I slipped over her head, but she unraveled that, too.
She is now sweaterless and even balder than before. She refuses to wear a saddle of any sort, and is so preoccupied trying to get out of it, she even forgets to eat. I gave up.