Sorry Tom, and thanks for your clarification.
Honestly, I was not replying to you, but clearly my statement would be taken the same way by anyone that had made the same recommendation.
My feeling regarding the suggestion being silly, was based on a few factors, 1st the OPs question being asked.
"Lens for Full frame and shooting birds"
Posted in the EF lens forum.
I think we all (myself included) can get off base when we reply to that sort of question by completely flipping the paradigm, and suggesting that rather than find what was being looked for, suggest a totally different approach. There is merit in this sort of reply. I don't deny that, but at times, it feels a little heavy handed. A little too gear centric as well. EG: The same day I replied to this thread, I had just replied to a thread where someone was trying to find a lighter weight set up do to medical condition / pain, and was being told as a solution to buy a grip and that weight to the load..
I believe that it may be that this is where my "silliness" comment truly came from.
Pixel density is a big advantage for birding. No question. Especially when cost of lenses is what it is. I would not deny it's importance. I do however contend that it is not the only thing one need to look at, and that it is not a magic bullet. (I don't imply that you said it was)
Part of the different view point may be due to differing experiences.
Some numbers to consider:
The 5D3 will put 14MP on the same "1.3x crop" as my 1D MkIII did with only 10MP and my 1DII did with only 8.5MP.
That to me, and to many 1D shooters, which were the majority of birders for some years, is a huge step up in pixel density.
The 7D puts 18MP on the same crop as the 5D3 puts only 9MP.
Tom is absolutely correct, thsi is DOUBLE the pixels on target at the same field of view.
So half the pixels compared to the 7D,.. but still more cropped to 1.6 than my long soldiering 1D MkII had for it's full field of view.
So yes, the 5D3 needs a longer lens to do a similar job re pixels on target compared with the 7D,.
But, I also know that 9MP is enough to print good solid 17" x 24" prints and sell them in an art gallery. I've done it with less (cropped 8.5MP images)
Anyway, I am sorry my choice of words ruffled feathers.