If possible, perhaps you could post a couple of shots of the birds you have captured.
"Bird" photography is a pretty generic term and could refer to photographing relatively tame birds who are siting on a fence or in a tree fairly close to the photographer. This is perhaps the easiest form of "bird" photography and really doesn't require too much in specialized equipment or techniques.
However, photographing birds in flight is a whole different ball game and usually requires shooting at relatively high shutter speeds. I like to shoot at least at 1/1,000 of a second. What ISO I need to use to achieve that shutter speed is dependent upon the brightness of the day and the maximum f/stop of my lens. Additionally, getting a decent image of a bird in flight usually requires a relatively long focal length.
Although I always try to work with the lowest possible ISO that will enable me to get sharp pictures, I would rather shoot with a higher ISO and achieve a shutter speed that is fast enough to freeze the bird in flight. I am far happier with a sharp image that is somewhat noisy than an image that is fuzzy because I wasn't shooting with a high enough shutter speed.
There is another parameter which will have an impact on the quality of your birds in flight imagery and that is the speed and accuracy of your auto focus - which in turn depend on the camera and lens with which you are shooting. Serious bird photographers often invest large sums of money in their equipment...
However, you really don't need to sell the farm or your first born child
in order to set yourself up for bird in flight photography but, you are highly unlikely to be particularly successful if you are using an entry level DSLR camera and kit lens...