[sassan]

X or Y brand are choice you make depending on several factors including your passion toward a brand, availability, cost, peer pressure (Yes you heard it right) and many other logical or non logical reasons.

I got into Canon FTb in 1972 when I was a young barely teenager. Went for Nikon F3 because I thought it makes a better me (Photographer) though it was my egos that got most satisfaction out of the change (Soon to find how .... the pros must be when could see what FM2 or FA would offer at much lower cost). With the first digital camera getting out in market I knew what I always wanted so many years of non DSLRs Olympus, Nikon coolpixs, Fuji, then Canons and after getting serious about DSLR, it has always been Canon that made me happy. I have many other camera bodies directly or in vicinity (Child and other family members) and still stick firmly with Canon. I know very well what I will buy next and that is 5D MK II once it is released hopefully early next year.

If you go with canon (Considering you said primary interest is Bird Shooting) I suggest 40D as body, Sigma 50-500 for birds (Though it falls a bit short of Canon L 100-400 that gives you much crisper and color richer image but Sigma is one wonderful lens for the money and the very unique range it offers, not being beaten by anyone in photo industry yet).

But for real life the two lens that by virtue of experience I know are most needed ones (Never off my gear on any shooting situation) are:
- 70-200 L IS F/2.8 and
- 10-22mm None-L None-IS (You really don't need IS for shorter focal lenght lenses and no L is available in this category yet.

Once feel comfortable from the damage if you go in this route, you many want to address low light situations so a
-50mm F/1.8 at about $100 or if you want to spend more for bigger lens, a -80mm F/1.8 both excellent for sport shooting from corner of field if you can get there, then a
- 24-70mm L F/2.8 that is one of the sharpest canons lenses ever made.
Now if you have the budget to get into world of white red collar elegant prime super teles then you have many choices. My suggestion is
- 600mm L IS F/4

You can easily see that either you need to break the bank or you have some special talent with very influential sponsors.

Bottom line:
Body: 40D or 5D (Doesn't take EFs lenses)
Lens: 70-200mm L IS F/2.8
10-22mm EFs
50mm F/1.8
Then build up your gadget as the need may be. Forget about any other third party lenses unless you like to be in buy/sell status just for the sake of it.