Maybe not the most reliable source of information, but I counted the species on this page:
List of birds of Niue - Wikipedia
The eBird data was a bit spotty, but I only got 15 there. I figured that if there were more bird species, more would have been reported.
https://ebird.org/barchart?byr=1900&eyr=2018&bmo=1&emo=12&r=NU
*It is worth noting that the Wikipedia article states that 29 bird species may be found in Niue. The eBird checklist states 34. But keep in mind this includes accidentals and vagrants.
*Also note that isn't the only duck on Niue. After looking over Niue's Mallard reports on eBird, I found this statement: