No, this is not strange. There have been many documented cases of various species which normally would not pair up as friends having done so when a mate is lost or unavailable. We have even heard of Australian Black Swans becoming pals with tractors, boats, etc.

Geese, ducks and other birds have been known to pair up with other waterfowl species as well as dogs and cats. So, in this case, at least the two species have someone to pal around. The Regal Swan