1K or 1000 in what currency? You didn't say and your profile doesn't tell us where in the world you are, but looking at your Flickr it appears it's not the U.S. Still, I'm going to use U.S. currency for some suggestions... you might need to convert.
- The Sigma 120-400 OS sells new for just under $1000 U.S.
- The Siggy 150-500 OS sells new for just over $1000, but you might be able to find it on sale or used for close or under $1000. (In the U.S. it is currently on sale new for $1020, until Feb. 28.)
- Canon 400/5.6 sells new for well over $1000 ($1340 US)... maybe you could find a used one for under $1000. It's sharp, but lacks IS (or OS), which is very nice to have on such a long lens, especially when using it on a crop sensor camera.
- The Canon 300/4 IS has stabilization... sells new for well over $1000 ($1450), but can be found used at times for $850-900. Add a 1.4X teleconverter... a used Canon Mark II or a new Kenko 1.4X MC4 DGX might be found for around $150. This gives you two focal lengths (300mm and 420mm), plus it has very useful IS.
The 300/4 and 400/5.6 probably offer some of the best AF performance, which can be an important consideration shooting fast moving birds. The Canon 100-400 is pretty darned good, too... It's popular with airshow photographers and you can't get much faster moving subjects than that. I honestly have used the Sigmas, so can't really compare their AF performance. They do have HSM, which is Sigma's version of Canon USM and should help them give good performance... but I can't say from personal experience. I use the 300/4 IS and Canon 1.4X II combo myself and can vouch for it. I've used it with birds, larger wildlife, and sports, including pretty fast moving motor sports. As my longest handheld lens (or lens + TC) I currently use it mostly with 7D cameras, but have used it successfully with 50Ds in the past, which have virtually the same AF system as your 60D.
With f5.6 lenses (including an f4 lens with a 1.4X teleconvter added), you will likely be restricted to using only the center AF point of your 60D. Two of the Sigmas go to f6.3 at their longest focal length, but they "pretend" to be f5.6 and that allows AF to work. There is a work-around, but in general 1.4X teleconverters aren't as useful with f5.6 and slower lenses. (2X TCs pretty much need an f2.8 lens to be able to focus normally on a 60D. Plus 2X TCs "cost" a lot more image quality, need to be more carefully matched up with superior lenses.)
The camera is not supposed to be able to AF and if the camera detects the TC, it will turn off AF automatically. However, you can tape up a couple pins on the TC so that the camera doesn't know it's there and "fool" the camera into trying to AF. Don't expect the AF to work as well as it does when no TC is present... It will be slower and hunt more, especially in less than ideal light and/or lower contrast subjects.
- Canon 100-400 IS is normally well over $1000 used or new.
- Sigma 50-500 OS is also normally well over $1000 used or new.
The prices go up from there.