All those materials are kind of outdoor frinedly on their own, LOL. I assume you have mosaiced on wood? The wood especially exposed edges needs sealed within an inch of its life, I would also use some sort of varnish or sealant over the pine cones, and it wouldnt hurt to seal the grout over the rest of the piece. Since wood is the assumed substrate-outdoors it has a limited life as the wood will absorb moisture and swell and shrink in cycles which will eventually loosen the stuff you have adhered to it. You can leave it as is and enjoy it a year or 2, or seal it well and hope for a little more. A sheltered position will help.
The best solution is to make another one to replace it when it falls apart, you might like it even better! LOL

Yep, seal it within an inch of its life. I take birdhouses outside and spray the insides of them with Thompson's wood and deck sealer. Just put the nozzle right up against the hole in the birdhouse. Then for the outside of it, you could seal with Aqua Mix Premium Stone and Tile Sealer which is kind of expensive but goes a long way if you pour it into a spray bottle and apply it that way. If the birdhouse is under a porch roof it might last a long time.