To be honest Jack, I am puzzled with this one - some parts of both images look like they were shot through glass so I wondering if there may have been some movement which took away some of the contrast to leave this effect. The effect looks like one I used to see on my 100-400 from time to time, always at the 400mm end when using it wide open.
Perhaps it is vignetting?
What processing did you perform?
Oh and by the way, nice pose, especially on the first shot, which could have been improved a smidge by getting lower but the vegetation in the foreground may than have got in the way. Nicely exposed too

John Stuart-Clarke, ukBPN Moderatormy website - a completely new gallery of
SCOTTISH WILDLIFE images is now
LIVE... Capercaillie, Crested Tit, and the obligatory Mallard shot ;0)
Comment & critique welcome but please don't edit my images