Some birds like to make their nests in chimneys. The best solution is to have a chimney cap installed that will not allow birds to get into the chimney. Some birds, mainly Chimney Swifts, choose the inside of chimneys to build nests and hatch their eggs. This generally occurs in the June-August timeframe. Once the babies hatch they get very noisy and sometimes can fall out of the nest into your fireplace. To have birds removed from your chimney it's best to call a chimney professional.