Camera advice please - pretty please?

I have a Nikon D5100 and use a 55-300 lens for bird photos.  I know this is not really good enough as all the little darlings are usually too far away.  I am new to photography as well as birding (couple of years with this my first SLR).  I am going to put on here some photos - some I'm pleased with others disappointed.  It could well be that I am not any good, or that I am trying to achieve the impossible with my equipment.  I know I don't know my camera well and am trying to get to grips with the manual and also just to experiment with different settings.  I never use auto; usually aperture priority but recently tried manual to get faster shutter speed and then adjust the focal length for exposure.  I saw seymouraves post about his lumix.  I rang Wex photography in Norwich who said don't bother with anything like a bridge camera (sorry if the camera you were referring to is NOT a bridge … ) as I will lose quality.  What does anyone else here think?  I’ve also read whistling Joe’s posts on photography, really helpful.  I realise I don’t understand depth of field properly, only a vague idea really.  I can neither afford nor carry large lenses as when I go out there’s only me to carry it all and I have to carry my flask and sarnies as well – can’t sacrifice lunch!  I am happy to spend £500 on another camera if I will get good quality bird/wildlife images and then use my DSLR more for landscapes.  I was looking at the Canon Coolpix P900; SX60; Nikon P610.  My camera really doesn't like ISO over 640 and that's pushing it.  The 3 avocet were moving and so was I (in a boat), the stone curlews were a long way away ...  the shorteared owl was some distance away - the flight one took me by surprise; I think I just messed up on the blacktailed godwit.Thanks so much if anyone has any comments to help me improve or to adjust my expectations …. 

Oh, and I won't post the other 5000 shots ...

Jill