Thanx a lot Marc for ur response. If wanna buy SLR with a zoom lens.. What are ur recommendation? How much minimum size should i buy 600mm. Also if i crop a pic taken by 600 mm.. ..to 1200mm will the quality be same? Which SLR camera do u recommend?
Budget and your weight limit are the controlling factors. I should also mention that just jumping in to a top-end system will not get you great photos. It is a skill and you need to practice and understand the equipment. Birds in flight are hard. Personally, if I were just staring out, I'd look for something on the lower price end and build up the skills and make sure you enjoy it before putting down some serious cash. I want to emphasize that gear alone will not give you great pictures, you need technique and skills.
For just starting out, a Nikon d5600 + 70-300 DX VR lens would be a good combo. Or you could try a 100-400 (Tamron or Sigma). If you are comfortable buying used, you can also try the d5500 or d5300 for about the same pictures (the 70-300 DX VR has slight user-interface limitations with d5500 and d5300). I'm sure there's the equivalent Canon, but I don't know that off the top of my head.
For entry-level, most any more-or-less current DSLR and a 300mm or 400mm lens (e.g. 100-400 f/6.3) would work. If that is enough zoom for you, you will get quicker handling, zippier autofocus, better pixel quality, and better high ISO performance than a bridge camera. For example, a Nikon d5600 or Canon Rebel T7. You can crop pretty well on one of those cameras and they give you a built-in 1.5x or 1.6x advantage due to smaller sensor size, so a 400mm lens is like a 600mm and you could crop it down to maybe a 1000mm equivalent.
In Sony-land, the a6500 with Sigma 100-400 or Sony 200-600. Sony is not cheap.
For mid-level, the Nikon d7200 or d7500 with a Tamron 150-600G2 or Sigma 150-600 C or Nikon 200-500 f/5.6 is a very nice kit. Or you could do a Canon 7d mark 2 with the same lenses (well, not the Nikon). These would give you a 900mm that could be cropped down to maybe a 1200mm equivalent, maybe even 1500mm. This option is generally the best balance of cost / weight / performance.
Towards the top-end is the Nikon d500 plus 150-600 or 500mm f/5.6e or the Canon 90d (not really as good as the d500 for wildlife from reviews I've seen). There's also some high-end 100-400 lenses (like the Canon L series) or the Nikon 80-400. This gets into the top-end of crop sensors for dslr wildlife cameras. Same focal lengths as above, but the d500 shoots faster, focuses better, and performs overall better. More money, more weight, more performance.
You can also look at full frame (which means you lose effective focal length), such as the Sony A7III, A7rIII, Nikon Z6, Nikon d750/d780, Canon 5dIV, Canon R. For Sony, you'd look at the Sony 200-600mm lens. For nikon or canon, likely the lenses I mentioned already.
At the top-end, you start looking at lenses in the $6000 - $13,000 range and they weight 6 lb - 8 lb just for the lens.
Once you start getting into the "mid-level", weight will be maybe 5 - 6 lb for camera + lens. You need a good support sling or monopod or tripod with gimbal head. Yet more cost and weight and bulk.
Marc