Phil brings up a great question.

One photo rarely tells the story when shape and structure are important.
Relative neck length can vary a lot, and sometimes you have to watch a bird for a
while to get a good feel.

This Cackler is dramatically smaller, to my eye. Neck proportions seem
similar to the Canadas, alas, but the bill seems proportionately a tad smaller and
the forehead steeper, giving the Cackler a boxy headed appearance in comparison
to the more Canvasback-like head profile of the Canadas.

Cheers
Steven Mlodinow
Everett WA