Tony, I've just had a read through this thread again, and while it didn't make my head hurt like it did nine years ago, it still leaves me with some unanswered questions.

It seems to be the general consensus that at the same time, same place, same good light, same lens, same subject, same camera settings, the crop sensor is going to capture more detail due to it's higher pixel density.

I'm assuming we are talking 'good light' here, and that is not only subjective, but variable, depending on the subject.

For example, a Blue Wren in open sunlight is going to be a challenge for any camera and settings, trying to cope with that highly reflective plumage, while striving to get some detail in the dark and light feathers. BTDT ! An exercise in frustration.

And when that same wren hops into some heavy shade, or a cloud covers the sun, or both, does the pendulum then swing toward the extra light gathering ability of the full frame's bigger pixels, or will today's sensors, with their better control of noise at higher ISO settings still enable the crop sensor to win out.

When I look back at my bird shots over the last nine years, I'm hard pressed to find many stand-outs taken in direct sunlight, and most keepers seem to have been taken under the canopy, or when cloudy, or both.

My jury is still out on the best type of birding camera for me, but my gut feeling is that for my usage a 35+MP FF is the way to go. Shame I can't afford a $10K lens to get the best out of it.