Something else to consider with longer focal lengths, whether using a TC or not, is that good technique is required to achieve good pictures. The longer focal length means faster shutter speeds are required. Image stabilisation can help, but can slow down autofocus. It can also be difficult to get and keep the target bird in the viewfinder, as the angle of view gets smaller. It is easy for the focus point to slip off the bird onto the background. There is plenty of advice online about long lens technique. A tripod or monopod can be a great help in getting sharper shots, although these can can get in the way of getting birds in flight. Taking the camera out of auto mode is a must, so that you can balance shutter speed, aperture and ISO and achieve the best results. Shooting in raw, rather than JPEG, gives you more control for post processing (I use Adobe Lightroom). Basically, if you let the camera have control, rather than taking control yourself, you are less likely to get the results you want. There is a learning curve, but it is worth it.