"the good bird"
Presuming you can do all the others from a long press on a mobile device, copy and paste the ȳ :-( This is a less than ideal answer. But Duolingo should only mark you as having a typo, not as wrong, if you write y instead of ȳ.
On the desktop version, the letters including the ȳ appear on tiny buttons under the text entry area.
You might also want to look at these two discussions: