In my post about nesting Least Grebes I said I would be showing photos of the distant birds taken with my 300mm lens and stacked 1.4X and 2.0X teleconverters. Here is a photo of one of the parents is about to feed a piece of crayfish to the chicks.
Vibration is a problem with such long focal lengths so you want to employ the fastest shutter speed you dare and any way to minimize shutter curtain or mirror slap. I used the camera’s electronic shutter for this.
In the second photo I was lucky to get a nice reflection of the blue sky. This was taken using only the 2.0X teleconverter. Both photos are cropped to about 6 MP from a 20 MP file. More than enough for printing on a magazine or book page.
Have you tried stacked teleconverters? It gets a bit technical but sometimes you just need the magnification!