Wound?

[SrPato]

Hello again! Sorry for my English but I use a translator.
For a few days I have observed a brown liquid (?) in my duck in the area of its tail, just glued to the wings. I wanted to go to clean that area but she won't let me and I don't know what it could be. I was able to pour physiological serum to clean it and it seemed to me to look like a lump. Could anyone identify what could happen and how to act?
She could not be attacked by another animal since she lives at home and there are no predators.
Thanks for your attention

Video of what she has:

 

[Isaac 0]

That is her oil gland, and in the video, she was exhibiting some preening type behavior. I bet she rubbed her head on the gland before and after the video, correct?

 

[SrPato]

That is her oil gland, and in the video, she was exhibiting some preening type behavior. I bet she rubbed her head on the gland before and after the video, correct?

Yes! But before that I also noticed that I had it. It is normal? It looks like a wound that oozes and stained the cotton a little

 

[SrPato]

That is her oil gland, and in the video, she was exhibiting some preening type behavior. I bet she rubbed her head on the gland before and after the video, correct?

Yes! But before that I also noticed that I had it. It is normal? It looks like a wound that oozes and stained the cotton a little

 

[Isaac 0]

Yes! But before that I also noticed that I had it. It is normal? It looks like a wound that oozes and stained the cotton a little

The yellow, around the oil gland, is normal.

 

[SrPato]

The yellow, around the oil gland, is normal.

Uff Thank you so much! I thought it would be a bad thing and i was already worried, i had never been seen and since when i wanted to look at the wound she said "honk" so I thought It hurt her