The size of the bird has to determine the size of the spacing of the bars in every cage.
A small bird like a finch/budgie/canary will be safe if its in a cage with bars that are spaced 1/2 inch apart or 3/8's apart like in the photo below.
This is to assure the bird won't get its head/feet/body stuck between the bars of the cage when it climbs the bars.
This cage is larger and has larger bar spacing 3/4 inch.
This size of bar spacing is good for larger parrots like a macaw or african grey or cockatoo as they need a larger spacing for their beaks and feet to fit as they climb their cages all the time.
This 3/4 inch size of bar spacing would be very bad for a small bird like a budgie or finch or canary, as any small bird might try to squeeze through the bars which could harm or kill the little bird.
