Why Does my Gander do This?

This is very interesting to me and I would like everyone's insights.

I have had a white Chinese gander and a brown Chinese gander for a couple years now. They hang out with the few ducks. I have always wanted a pair of Sebastopol, but the price of good birds has kept me away. I finally found an affordable older pair, not the best quality, but all I want is to enjoy them in my yard anyway as pets. I purchased them and the egg that was in their cage. I put the egg right in the incubator.

As soon as I got them home, I let them in a small area, they both had a drink, and started wandering around. No problems with the other geese. The girl found a nice spot under a tree and I thought, great, maybe she will give another egg. No, in fact the next day she was dead. No signs of trauma, she must have been ill or too much stress or whatever, but the gander was upset and called to her for awhile.

Anyway, he continued on with the other geese, I have seen no scuffles or anything, they all roost together in the barn at night. And then the egg in the incubator hatched! YAY! I think it's a girl. She was inside for a couple weeks but now she is older and the weather is better so I put her out the other day in a small pen on the lawn with the couple ducks that hatched in the incubator with her.

Now the Sebastopol gander found her on the lawn and won't leave the cage's side. He will chase off anyone and anything that comes near it. The other geese came over and checked it out but didn't seem interested. When I put the babies in at night, he calls and calls all night until he is shut into the barn. When he is let out in the morning, he goes to where the pen is, and since I don't bring the babies out til late morning, he again calls and calls.

So what is the theory behind that? It's fascinating to me. Does he just know it's another goose and wants to buddy up? Why don't the other two seem to care?