I would say 3/4 inch bar spacing max. At one inch they probably be able to squeeze their head, maybe their whole body, through the bar which you don't want. My current corner cage is 3/4 inch. My boys have some thick heads compared to other Suns I've seen come into the bird store.
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I'd be careful with 1/2inch bar spacing if its from a no name brand. In my experience bars are usualy thinner than a 3/4 cage. After 4 years my boys ripped the bars off the bottom of their cheap 1/2 sleep cage.
I Recently bought this for their sleep cage, they share the top cage, my lovebird gets the bottom one. 1/2 inch bar spacing.
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