Vermillion Flycatcher

That clinches it. Thanks for sharing this, I learned something from it. :)

Great observations on this bird Liam, I agree with the immature male identification. I've posted one more photo where you can see the tail and note the white on the outer tail feathers.

Also, in the first and second pictures, I see evidence of Avipoxvirus, an Avian sickness.

One way to tell for sure is the tail. Vermilion Flycatchers have white outer tail feathers, Say's Phoebe don't. Looking again, I think you may be right with Vermilion, I see some flecks of red on the crown, which would suggest a juvenile male. Usually I expect female/juvenile Vermilion Flycatchers to show some streaks on the breast, but I reckon when its puffed up like this it's not as visible.

Yes Liam I do believe this bird was possibly sick. She was photographed in an urban park but no other birds in the area appeared sick so I'm not sure what was going on. I'm confident this is a Vermillion Flycatcher and I think it is a female though could be an immature male?

Was s/he sick?

Lovely!