I don't have any experience with the lenses you have highlighted as possible choices but here are a few thoughts.
The Canon 70-300 IS USM determines your maximum budget but you "don't really want to be changing between my 18-55 and my 70-300 all the time". Within budget the only Canon zoom which seem to fit he bill as a general purpose lens with IS and a better telephoto capability than your 18-55 is the EF 28-135mm f/3.5-5.6 IS USM
. I don't believe that Gordon has reviewed this lens yet but you can see reviews at The-Digital-Picture.com
and SLR Gear
(click the links to read the reviews). From those reviews this lens appears to be good value and can, of course, be used on full-frame Canon cameras should you wish to go that route later on.
I imagine that the only reason you didn't mention this lens in your list is that you feel that 135mm doesn't get you close enough to those birds on the wall! Another way of looking at this is that at 135mm you are getting 45% of the linear resolution that you could achieve at 300mm with the EF 70-300mm f/4-5.6 IS USM - is that good enough? Bear in mind that you won't necessarily achieve a better result at 300mm compared with 135mm if that is achieved by buying a less sharp third party lens without image stabilisation.
You might then have enough left in the budget to buy the EF 50 mm f/1.8 II
. This lens is reviewed at The-Digital-Picture.com
and SLR Gear
. This lens is excellent value but whether the "buzzy" autofocus and slightly lumpy bokeh are a compromise too far only you can decide.
It seems that you are reasonably content with your macro work at the moment so maybe that upgrade could be deferred? If not then maybe the 25mm extension tube coupled with the EF 28-135mm f/3.5-5.6 IS USM?
Hope the above is useful, even if it only provokes a "that's rubbish" reaction which, nevertheless, helps you clarify you own thoughts!