I'm looking for a decent option to shoot wildlife and birds with. Currently I'm asking way too much from my 70-210 beercan. I find I'm almost continuously using it at 210mm where it obviously doesn't perform best and even then I need more reach. I need to stay under £1000 if at all possible as I'm on a budget (aren't we all?) but I'm not averse to second hand if I can get it. Currently I'm considering hiring and trying...
70-400mm 4-5.6 G SSM although I'm worried it may prove too heavy for being in the field all day. I'd rather avoid relying on a monopod if possible to keep myself fully mobile.
As has been mentioned Tamron has the new 150-600 tele coming out at a very attractive price within your budget. Look around and there are several threads on it which can lead to seeing samples of the images it can do. This lens is according to Tamron the direct replacement for the very old 200-500 who's basic design is several decades old. I do have a 200-500 and it can get very good images, but is lagging behind lenses like the Sony 70-400G which is what I use now and certainly is unlikely to keep up with the 150-600 which is what I have on pre order. I do also use the Sony 70-300G which works pretty well with things like butterflies, dragonflies and such like. And I also use the 100mm Minolta D macro lens a lot for wildflowers. Along with some shorter lenses like the Tamron 18-270 for walkabout shooting. Outdoor forays into natural areas can turn up a very wide range of subjects requiring several good lenses to cover them all.
Going out trying to use a long lens with poor support is really somewhat a waste of money. The budget for a long lens should always include good support. Monopod is only part way there, same with cheap tripods designed to support only shorter lenses. You can be mobile with a tripod. And it's what makes a quality long lens worth the money. But you need to go with tripods designed for long lens use. If going for the 150-600 then you probably need a high quality gimbal head as well as a heavy duty tripod under that. The two tripods I have that I'll be using with the 150-600 will be the Gitzo CF GT3541XLS or the GT5541LS and on them the Custom Brackets Gimbal head. When I got these I got them for way less than the list prices off ebay, and never regret having them. They are the levels that Gitzo recommends for long lenses. Yes, combined with the cost of the 150-600 you will exceed your budget, so hunting bargains and saving until you can afford them will be necessary.
For long lens support I also at times use a Kirk window mount on my Ranger, or a Shoulder mount, or just beanbags and found support out there. Each choice will have a different keeper rate, but all of those will beat handheld. Yes, I do handheld, even though a lot of the time it's a exercise in optimism even at 400mm let alone 600mm.
70-300mm 4.5-5.6 G SSM although I've read it can be a little soft at 300 which is where I'm likely to use it.
Thirdly I've debated about getting a teleconverter but I have little to no knowledge in how much impact they'd have on IQ or light as well as being less researched in what I could team it with for good results.
I'd really welcome some advice from those who have used or know these lenses as I'm currently going round in circles reading reviews that praise and slate them in equal measure!
Teleconverters are kind like a lottery. You really don't know if a combo other than the manufacturer's expensive matched TC and lens will give you good images without trying the combo.
The idea of using a TC rather than a long tele, or cropping a lot on a shorter tele is generally going to add a lot of empty magnification which is not the way to get top images. If you are after the best images those come from using a quality long tele that gets you there without cropping or TC for most of your shots. Note that does not mean you have to spring for the way high priced single focal length primes in long tele. There are quality images coming out of zooms too.
While fieldcraft and luck can get you close to some wildlife there's lots more that are not going to be at all helpful getting close to them. I'm a field biologist and they don't give me any respect either though I've spent a lifetime learning fieldcraft.
I've used many long tele's, it's a major portion of my shooting along with macro. -- And it is true you can never have enough focal length. I do use 1.4x TC at times in combinations I know work reasonably well. Also long tele is where I've done the most moving from lens to lens as better ones show up. And I've been doing that since the MF film days, better stuff does appear from time to time even for folks like me who have no sales to support my habit. I expect the 150-600 won't be the last, but I hope it will keep me going for many years. Just like the 70-400G and the Tamron 200-500 did before. I expect that the 70-400G and 70-300G will still keep seeing a lot of use, the 150-600 really covers a different range.
If you think any of those are too heavy I also have and occasionally use a couple of excellent Tamron primes 300mm f2.8 and 400mm f4. And either of those exceeds 5 lbs. I can handhold their weight for only 10-15 minutes at a time but do get nice shots even with that limitation. They don't see a lot of use mostly because they are single focal length primes, not because of their weight.