I have a Nikon D5000 with a Nikon 70-300VR, i am looking to go a bit further in zoom terms. I have been recommended 3 lens: Sigma 50-500 os Dg: Sigma 150-500 OS DG: Nikon AFS Nikkor 300mm f4d if-ed with a Nikon Teleconvertor Tc 14E11. Any body have any experience or recommendations of any of these lens or any other lens which would be suitable. I use the lens i have mainly for birds in flight and the odd static. All comments and advice very welcome.