Natural causes may include failure to thrive and death from natural predation.
Many more birds, however, die as a result of human activity, including (but not limited to) hunting, predation by domestic or feral cats, pesticides, collision with buildings, towers and power lines, automobile strikes, and loss of habitat to the cancer of human "development". Of the 836 bird species listed under the Migratory Bird Treaty Act, fully a quarter are in serious population decline in the U.S.
To learn more, check out Leslie Kaufman's article here. Then try to imagine a world without birdsong.