
Are there drainage holes in the bird bath? Without them, moisture levels build up and can rot the bulb. It does take a while for begonia tubers to break dormancy but 2.5 months is excessive. I would poke around in the soil to locate one and then gently remove the soil on top of it. Be careful; the first shoots break easily. If there is no sign of emerging shoots then gently try to lift it. If there are no roots anchoring it in the soil, it's dead. Of course, all this assumes you live in southern US where temps have been warm for a while. Some of us are just getting there!