Barrow's Goldeneye

You may be right on the scaup. I went with Lesser by default because of head shape and because Greater tend to stick to the coast (though I reckon in this case that may not apply, with it being a possibly captive bird).
Not quite sure what you mean regarding the iridescence. Both Greater and Lesser show iridescence in proper lighting, though Greater's tends to be more golden-bronze while Lesser's tends to be purplish (not horribly reliable, though, as both could show either color, depending).
Very white flanks and bill nail size do seem to suggest Greater, though.

Liam, I think pic 2 is a Greater Scaup due to the iridescence which is seen in the Greater Scaup and not the Lesser.

I identified the one from the first pic in the suggestion above. The second pic is a Greater Scaup (Aythya marila), the third looks like a Heron but not sure and the fourth I have not found yet.

The same applies to these photos as to your other ducks. They need to be split up and loaded separately so that all the photos in a particular spotting are of just a single species.

your forth photo here is a female Shelduck

Hi Kris, once again, I request each of these photos gets it's own spotting.
Here's their identification:
1. Barrow's Goldeneye
2. Lesser Scaup
3. Demoiselle Crane
4. Orinoco Goose

Hi, Kriss! For easier identification, can you please separate the into four different spottings?