hole in tree

I have a 50' tall Norway Maple in the corner of my yard. It has hole in it at the intersection of its 3 major limbs. The hole sits about 18' high. The diameter of the hole is about 4" wide by 12" deep. Either birds or squirrels have used it I was told to cover up the hole with a piece of tin. But then i read to let it breath as closing it up will cause for more decay.

Any thoughts or suggestions are appreciated...

Leave the hole alone. Attaching tin would mean driving nails into the cambium, right? Much greater damage from the attachment than gain by the perceived protection of the tin cover.

You need to work on protecting the tree's health in general by avoiding obvious stressors like lawnmower damage, poor pruning, soil compaction, too much or too little moisture, insect infestations,...the list goes on.

You might wish to consider some support cabling, since the hole is almost certain to lead to advancing decay and it appears to be located where it will cause a likely failure. Crown thinning or perhaps even some crown reduction might be called for.

Have a qualified arborist give you an opinion. If you pay them for their time, you will be more likely to get a neutral opinion, rather than an opportunity to make a sale.