I have bought a Sigma 150-600mm F/5-6.3 DG OS HSM | C lens for my Canon 70D. Is it okay for bird photography? Also kindly suggest a filter for the same.
Never add a filter unless the benefit outweighs the disadvantage. The major disadvantage is, a filter add two polished surfaces that prompt a light loss due to reflection. A coated filter reduces the light loss from reflections to about 2%. The most commonly mounted filter is a UV. This filter absorbs ultraviolet light and this is helpful because distant landscapes and aerial shots are likely views through atmospheric haze. The UV filter penetrates, to some degree this haze. The UV filter is likely redundant on a digital camera because the image sensor protective cover is also a UV filter. Most folks mount a UV to protect their cherished optics from scratches.
My advice is to buy a polarizing filter. Hands down, this is the most valuable filter for the digital photographer. The polarizing screen filter darkens blue sky causing clouds to standout boldly. A polarizing filter also cuts haze like a UV. A polarizing filter mitigates reflections from non-conductors like glass, plastics, and painted surfaces. Also a polarizing filter allows some penetration so objects near the water’s surface can be imaged. These come in two flavors, buy a circular polarizer.
I have bought a Sigma 150-600mm F/5-6.3 DG OS HSM | C lens for my Canon 70D. Is it okay for bird photography?
My experience with that lens is that it should be fine for bird photography, particularly on a crop sensor camera like the 70D.
I don't have a bird photo handy, but perhaps a squirrel will be a useful stand-in...
I took this photo with a Sigma 150-600mm f5-6.3 DG OS HSM | C on a Canon 6D (full frame), with the lens set at 600mm, f/6.7, shutter speed of 1/750s, ISO set to 2500. I think I was shooting handheld, but it's possible that I used a monopod.
I was at least 25 feet from the squirrel, and I'd guess it was really more like 35 feet. Even so, you can see a lot of fine detail in the following crop, including individual whiskers and a reflection in the eye.
For bird photography you typically want reach and speed. That lens will give you reach (150-600mm). Is it fast enough (f/5.0-6.3)? With that it's a matter of budget. If you can afford to spend three times as much, then do so and get a faster lens. If not, use this, as it's decent for its price range.
If you want a filter just for lens protection, avoid it. It doesn't protect very well and it adds unnecessary flare and distortion. If you're going to use a filter for some photographic purpose, the baseline is just to try and minimise the flare and distortion a filter adds by ensuring it's multi-coated and a good quality one. Beyond that, what you should get depends on the purpose.
Is it okay for bird photography?
How do you define "OK?" Maybe it will meet or exceed your expectations of an "OK" lens, maybe it will not.
Also kindly suggest a filter for the same.
Leave the filter off and use the supplied hood. In addition to providing better impact protection than a filter, it will also increase optical performance by preventing flare from off-axis light. Filters, on the other hand, can increase flare and reflections while also decreasing optical performance by placing two additional flat surfaces between your subject and your camera.
I'd recommend against filters for this lens. I got the premium 95mm Hasselblad UV filter for it, and it reduced resolution by 20%. The same Hasselblad filter mounted on my Nikkor 24-70 mm f/2.8 E lens showed no drop in resolution. Here's a link to an article I did on that subject: https://www.photoartfromscience.com/single-post/2017/05/11/Do-Long-Lenses-Not-Like-Filters