I saw a little group of ducks on a small reservoir near Hapuna. Most are ring-necked ducks, the bird on the right in the top photo being a male, and the two birds on the left below, being females. The odd one out is the bird on the left in the top photo and on the right below. That’s a female lesser scaup.
According to my bird book, a small number of lesser scaups migrate to Hawaii every year, but ring-necked ducks are considered uncommon visitors.
Thanks to birdforum.net for help with the identification of the female lesser scaup.