I'm using a 16-300mm Tamron on my A68, I'm trying to photograph birds, would a Kenko 1.4x converter MC4 DGX multi coated be a good purchase to achieve more focal length? Or do you have any other recommendations? Thx
Firstly, teleconverters inevitably degrade the image quality to some extent, and for many lenses the results will be worse than if you just crop the image and magnify it. The only lenses which can be safely used with teleconverters without the images suffering too much are telephoto primes and professional-grade telephoto zooms (where the zoom range is typically a factor of 3 or less).
Some manufacturers try to enforce this by designing teleconverters so that they are only physically compatible with lenses which they deem suitable. For example Canon teleconverters have protruding front elements, so they will only fit on lenses which have recessed rear elements, and Canon ensure that the only lenses with recessed rear elements are those which they deem suitable for use with teleconverters.
Third party teleconverters such as Kenko don't comply with this convention. Their teleconverters don't have protruding front elements so they can be fitted on any lenses, not just the ones the manufacturers recommend. However, just because you can, it doesn't mean you should. The Tamron 16-300mm, with a nearly 20x zoom range, is certainly not going to work well with a teleconverter, and the images will be worse than without the teleconverter.
At this point you might be thinking that you'd be willing to suffer some loss of image quality in return for the convenience, but that brings us to the second reason: loss of light and loss of autofocus.
Your 16-300mm lens has a maximum aperture of f/6.3 at the 300mm end. Add a 1.4x teleconverter and you lose one stop of light, making it f/9. Your exposure time or ISO will have to be increased by one stop and you will almost certainly lose the ability to autofocus. (High end cameras like the A99 can autofocus with a maximum aperture of f/8, and maybe at a pinch f/9, but your A68 probably can't.) With a 2x teleconverter your maximum aperture is f/12.7, you've had to increase your exposure time or ISO by two stops, and there's no hope of autofocus whatsoever.
The obvious alternative to a teleconverter is simply to crop into the image to get the same effect as digital zooming (which is what cropping into the image in PP is!). Your 24MP sensor should still let you get A4 prints (at least) even if you use the full 1.4x teleconverter equivalent crop.