Feeding birds - with a puzzle in the middle!!!

We keep eight tamed cockatiels in a huge aviary in the garden. Last winter was the test if they can survive there (our vet said that in case they were not exposed to wind and rain they would survive even in very low temperature - they just developed their feather accordingly as the weather turned cold). We have covered the aviary with polycarbonate panels, and this way they can fly and play the same way as during summer. (Our first idea was to build a smaller insulated winter shelter for them within the aviary but we could not convince them to get in, so finally we decided to cover the entire aviary.)

They really enjoy being there and we also like the fact that despite having them in a cage where you can only watch them, we can enter the aviary and be with them. We are lucky that we have four males and four females and three of these form a pair - the fourth male and female seem to dislike each other - LOL. One of my sons have the most patience to teach them, so they seem to like him the most. Whenever he enters the aviary with some food they all fly on his hands.

Each of them has nicknames and we tried to buy them in a way that they are all different (there are two that are very similar in colour but we can tell them apart because the feather on their bellies is different - one of them "wears" a mini waistcoat 😂).

For example, the couple above on the first picture are Snowball (female) and Hairy (male). This boy is the bravest, he even welcomes foreigners entering the aviary by flying on their head or shoulder. And he is the most curious of all. When we brought newcomers to the aviary, he seemed to greet them with a welcome speech. He stood in the middle, walking back and forth and having a long talk (as if he was teaching the rules - LOL).

On the picture below you can (almost) see all eight of them. In the top row from left to right:
Pepi (which is something like a wig in Hungarian - she had no feather on her head when we bought her),
Rica (which is something like 'Noisy' - you can guess why... )
Rio (based on the animated movie, she forms a couple with Rica)
Next, one of the girls (the only one without a pair), Owl is hiding behind the others.
Alter-ego (he has gotten this name, because he is very similar to Hairy, he forms a couple with Pepi)
Greenhorn (he was one of the last ones to arrive and he hasn't got a pair yet)
And the couple in the front I have already introduced... 😁

This is how Owl looks like (this was taken during summer), she is really pretty having a very peculiar combination of white-spotted grey feather:

On this snapshot,​ one of them is missing... Can you figure out who? 😁
The first person to send in the correct answer will receive​ a full reward from me!

On the picture below you can see Pepi and Greenhorn eating, while Alter-ego is just sitting next to them. In the background there are Snowball, Rica and Hairy.