Perhaps the easiest way to create a catchlight in the eye of a bird
photographed in low light is to use flash as fill, setting your flash to
between -1 and -3 stops of underexposure while setting the correct ambient
exposure.

But using flash can be a pain: batteries, brackets, cords, and the rest. And
with amazing ability of digital to capture color and detail in low light I
have found myself using flash less and less each year.

Here are some tips for creating natural looking catchlights in the eyes of
birds photographed in low light (without flash).

1-Use a tiny Clone Stamp. Increasing the brush Hardness to 50% is an option
as long as you remember to put it back to zero when you are done.
2-Choose a white area in the frame as your source.
3-If you take a close look at a natural highlight, you will see that they
are always irregularly shaped and that the most prominent one is almost
always in the top half of the eye on the side where the light is coming
from. (There are often two catchlights even when flash has not been used,
one directly from the sun, and one from a reflected sunlight (often from the
water). Many times there is a faint highlight in the eye that is visible
only at high magnification. In those cases, be sure to create your highlight
at that spot.
4-After carefully considering the placement of the highlight, use multiple
clicks with the Clone Stamp with the Align box unchecked while moving the
cursor a small bit with each click to give the highlight an irregular shape.

Many times it may be best not to create a catchlight.

Feel free to post an image here where you have created a highlight. I will
do the same as time permits.