Should I get another mallard?

Last year, I received a rescued Easter duckling who is now a year old. She was my first duck, and she seemed lonely with only chickens, so this year, I got 3 ducklings to be her eventual companions. Ming, my first duck is a pekin. 2 of the ducklings, who are now 8 weeks old, are female pekins (one is crested), and the third one is a male snowy mallard. The 3 ducklings all seemed perfectly happy together up until the last week when Speedy, the mallard seemed somewhat sad. He'll fall behind the girls when walking, he won't jump into their tub with them, and he gets bossed around by Ming. Is it because he's so much smaller than they are? Is it because he's the only male? Should I get another mallard to be his friend? If so, when can I put this new duckling (who would be 8 weeks younger) with them? Would the duckling be OK with them since I would only get one more and not two, so I'd basically have the new duckling with me all the time like I did with Ming when I received her (she was about 6-7 weeks old when I got her last year). A single, tiny duckling is new territory for me. He just seems really depressed, his whole personality has changed, still sweet, but sad.

Do you suppose he could be ill? Do you have a vet?

I agree it is possible that he is depressed about something. It is the time of year for hormones to start kicking in, too.

I don't think he's ill. He's still eating, drinking, and pruning normally. When they were all smaller, things were fine. It's when the difference in breeds became MUCH more noticeable and when they all started to befriend Ming when he changed. He wanted to befriend her more than anyone at the start. Now, he's just kind of there. Are 3 pekins and 1 snowy mallard an alright combination? No one is physically attacking anyone, it's all verbal and mental.

Yes, since this used to be all farmland, several vets nearby are avian vets. I have a main one and a backup who check my birds regularly (if someone so much as sneezes, I take them in). No one is currently sick, thank goodness.

Ducks' relationships and emotions are complex, in my opinion. Some of it is hormones and some personality. Just my feeling.

Anyway, yes, he could just be feeling like the odd duck out. Just as well your smallest is the drake. I get worried when there are large drakes and small ducks.

I knew he was a drake when I got him. One thing I can do successfully is sex 1-2 day old ducklings. I wanted others of Ming's breed so she'd feel at home with them (hopefully), but I also wanted a male since several female ducks would get really loud, but I didn't want a pekin drake for fear he'd hurt my chickens, especially if he went after some of my silkies. A mallard seemed perfect. When I got the three of them, he was the only well-looking mallard duckling left (the others had bare spots on their necks or their eyes looked cloudy, etc.) He's the sweetest little guy, just seems down. I hope it's something he'll get over and will be back to his plucky self soon.