Cause of abcess in her head? *Pictures*

[chkn]

Is it possible it's a sinus infection. Try some antibiotics anyway and see how it responds.

 

[Animalian]

that doesn't sound good that it's become painful

You could try the warm compress just to try and make her more comfortable (help the contents soften if its an abcess, I also use warm compress's when I have sinus infections), she might flinch away at first then realise it feels better when she lets you touch it and you're trying to help.
If it is an abcess it might rupture soon if it continues to swell. Just be ready to clean it up and then treat it like you would a normal wound. If you have any poultry antibiotics on hand I'd give them to her, since it may have ruptured on the inside the antibiotics will be able to get in there and do their job!
If you don't have any you could try apple cider vinegar or even better grapefruit seed extract (anti-biotic, anti-viral, anti-fungal properties) in her water.

She may be able to fight it off herself and the lump will just go down.

IF she was a PARROT I'd say she has beak and feather disease! Some individuals fight the infection off and never show symptoms which would explain why the others are still ok. It causes the beak and nails to overgrow and the feathers to fall out, its not fatal if the bird is cared for (beak/nails filed so they can still eat/function etc). BUT it compromises the birds immunity so they get other infections (aka an abcess) more often and can have a hard time fighting them off. It's quite common for them to die from the secondary infections.
Maybe there's a poultry version of this disease?

 

[Moochie]

Quote:
I tried the compress today. The swelling looks no different from yesterday.
And it may be a sinus infection! I've noticed today when I picked her up and had her in my lap, she was breathing through her mouth and it looks like her (cheeks? the little pockets underneath her eyes) puff in and out. She doesn't have that bad smell coming from her though and she has normal poop.

 

[Speck & Gerard]

I'd just start her on anti biotics and leave her be, it'll most likely go away fast, if you keep messing with it, it'll stress her out and make it worse. i know how you feel wanting to help her, but sometimes things just need to heal on their own with a little anti biotics

 

[Moochie]

Where do I get those?

 

[Speck & Gerard]

you can get them at any feed store, or can order them online, if you get it from the feed store you can ask someone that works there which kind would be your best bet.