Well done everybody that put themselves out there, there were some very good guesses!
1Ds MkIII, 300mm f2.8IS MkI, 400iso, f2.8, 1/30 (handheld).
The interesting bit? It is a 75% crop, or slightly over 5MP from a 21MP capture, then re-sampled to 240ppi at 16"x24" (well done Sporgon, I expect no less from you).
Now a couple have made comments on the quality, I can assure you as a high end printer this image prints perfectly to 16"x24", the issue with detail, as I said at the outset, is not lack of resolution as evidenced by the cobweb, it is because of the very low contrast in the scene, the fine feather details are there when they go against the green background, just not when they are on top of each other.
So a few things I found interesting.
A 75% crop gives me an equivalent crop factor of 2, so I'd get a M4/3 sensor with a 300mm lens, or an equivalent fov of a 600mm lens and a dof of f5.6 and an iso of 800, very close to almost all the genuine guesses.
IS is amazing, even older IS. 1/30 sec with a 600mm fov is pretty darn good in my book.
You can play with noise reduction until you are happy, especially with an image like this, the more time you spend processing and masking the better the result. I only did very basic global adjustments to the image, if I spent an hour on it I could remove all the noise and sharpen up the detail. To be honest it was a throwaway crop my wife wanted to see, the original is a landscape orientation image, I'd never crop as hard. But it got me thinking what I could actually do with it and I surprised myself.
If you want genuine unbiased and friendly advice on actual image quality here trust the opinion of Sporgon and Eldar, they both know what they are looking at and obviously look at large numbers of images, and to our great benefit can use that experience to make honest assessments.
Here is the full frame of the original image.
Hope everybody found something of interest.