I have a 70-200mm F/4L that I would consider affordable. You could also get a range extender if the 200mm isn't enough.
If cost and distance is important / issue, I have had great luck with the Tamron 150-600 F/5-6.3. A good used Gen 1 version is relatively inexpensive.
I believe the 100-400 L Canon runs about the same of the Tamron if you get the Gen 1 version (1300 bucks). Gen 2 is about 2k Seems that the 100-400 is pretty popular for wildlife and birding. The Sigma 100-400 seems to have good reviews and is only about $800. You list 70-200 as a lens you own, do you have image stabilization? If you do, I'd say get the tele-converter to increase the reach. Otherwise Id say maybe consider the Sigma if your looking to stay under a grand, as you will want the option of stabilization.
I have the EF 75-300mm and can say it is not worth the trouble, at 300mm (minimum for birds) it is not sharp, even not with tripod and Liveview (no mirror movement) I think I will go for a Pentacon 300mm f4.0 manual lens. I have the 200mm and it is sharp as h... at f5.6 It is more of a challenge to quickly focus though...
I use the 100-400 MkII for any wildlife/aviation stuff. It is crisp, image stab is great. I know it is a bit expensive but it is worth every penny
I shoot with the Canon 100-400 L Mk2 lens and find it fantastic , If you then couple it with a 1.4X converter you have a 140-560mm F8 lens , On the 7 D Mk2 it will still auto focus quickly enough for most situations and retains its sharpness, a great combination for Wildlife \ Birding photography. You cant go wrong with L Glass.....
I own the basic EF-S 55-250mm STM..stopped down to 6.3, it performs well in certain light conditions. The stabilize definitely helps a lot. Just last week i was given an old manual focus Koboron 75-300mm. Never imagined i would have so much fun with it !! Has a tad much of purple fringing for my liking but when stopped down 2 stops or more, it appears well controlled. Here's a few shots from the 2 mentioned lenses taken in the last 3-4 weeks. Straight off camera, only cropped or scaled down where relevant.
I have personally found the sigma 150-500mm to be a really useful lens. I have an early version of the lens but it performs well and is (relatively) cheap
There is some old M42 lenses, like 3M-5A (600 mm/F6.3), MTO-500 (500 mm/F8). They are cheap (~200 USD) and good. You may use them with converters to Canon mount.
I agree with the first gen Tamron 150-600. I've seen good used versions for under $700. The best prime for the price is Canon's 400 f5.6. David
I use a 120-600mm F5.6-32 vivitar with a fd mount and a cheap fd to eos adapter, a fd mount 2 times extender with the camera set on Manual or av mode and they work fine. My other go to lens is a Tamron 500mm mirror lens. You have to be precise with the focus and you can have any aperture size you want as long as it F8. If you are patient and can work around these little annoyances you can get excellent photos for a low out-lay of money. And good tripod also helps a lot.
The 70-300 IS USM 4-5.6 is also a good option, lightweight, image stabilize and around 200 buck. Tamron 70-300 VC USD which also have image stabilize is slightly cheaper is also a good choice.
if the birds aren't too far away, i agree with denn on the 55-250 STM. i find that stopped down a bit, it delivers good shots. that being said, if the birds are far away, i'd suggest a minimum of 400mm, more if possible.
In my view there are two types of situations. Birds standing, and birds flying. For birds standing, a longer lens is neccesary, maybe 400mm or more. A stabilized lens is easyer to use, but some old prime lenses are also very good. But not all. For birds in flight, a faster lens is better, like a 70-200mm/2.8. When I was navigating in the Pentax waters, I have used both Tamron and Sigma 70-200mm/2.8 with good results for flying birds. I have tried the same with a Sigma 150-500mm, and it was very hard to keep the birds in the lens field, If they fly low, or fast.
200mm works for BIF if they're close enough. often for me, they aren't, and at 200mm, the bird is a tiny spec too small to crop with any detail at all. since i mostly shoot BIF in daylight, a faster lens isn't as useful for me as a longer lens. but i guess it all depends on where and what kind of birds in flight you shoot. i do better for far away birds not in flight with a nikon P610 or - even better - a nikon P900. no raw and small sensor, but you can't beat 2000 or 1440mm for getting far away subjects.
I have a passion for swallows. But I also had troubles keeping other birds in the field when they fly low and close, with the longer lens. But for other situations, the longer the better. 2000mm? I didn't try a bridge camera for a very long time, maybe 8 years, but what I have seen on youtube about those Nikon Pxxx and others like that is very interesting. Very high magnification, that you can get with a DSLR mounted on an astronomical telescope. But resolution with those bridges camera is still, ...how to say, so and so. Close, but not spot on. And I wonder what would be the results in a cloudy day, on a bird hidden in the branches.
oh, there's no doubt that those cameras leave a lot to be desired. i did better with the nikons on cloudy days than i did with the canon SX50, but i mostly use my P610 on sunny days. that being said, some of the shots you can get with 1440mm or 2000mm you can't get otherwise except with a VERY heavy lens and a full frame camera and then cropped. here are a few shots i did with a rented P900 https://www.flickr.com/photos/50019820@N03/albums/72157651709364692/ and here's the flickr account for a dpreview buddy who should be the spokesman for the P900 - he rocks! https://www.flickr.com/photos/markbrigham/
Impressive. Kudos. It could be a good choice for a reserve camera for me. And this reminds me another problem in shoting birds. They almost always moves. A little or more, they move. And this brings the need for a smaller exposure time. One think to be kept in regard for choosing a lens.
not just a lens - a camera's viewfinder. bigger or brighter viewfinders or viewfinders with an angle that works for you make it easier to catch pics of birds in flight. i also find it harder to shoot birds in flight if the viewfinder, whether EVF of OVF, isn't stabilized. it's why i didn't pull the trigger on a pentax - i shoot birds in flight one shot at a time. most people shoot them with burst mode, which makes the finder's stability not too important.