Once you have raked new grass seed into the topsoil, the best way to keep birds from eating grass seed is to lay a mesh netting over the soil.  Mesh Netting deters birds from eating grass seed for two reasons:
First, birds are wary of getting their feet tangled in the mesh and have difficulty reaching tilled grass seed through the mesh netting.
In addition, mesh netting can prevent other critters, such as mice or rodents, from getting to your newly planted grass seed.