Some suggestions:

- "O" for birth, "X" (or "+"?) for death

- Filled circle for birth, non-filled circle for death

- Expanding filled circle for birth, Shrinking filled circle for death (might not work well with low fps)

If you decide to go with color change, here's some combinations that I (red/green blind) have trouble with:

- dark red/dark green/brown

- purple/violet/pink/grey

- dark red/dark purple/dark violet

- blue/violet/purple

- yellow/warm green/light brown

- yellow/orange