Move any existing bird feeders a good distance from the lawn, perhaps to the other side of your property, several days before seeding the lawn. With plentiful supply of food elsewhere, birds are less likely to investigate the lawn.
Rake the seeds into the soil when sowing. Seeds in the soil are not as easy for birds to pick out as seeds on the soil.
Lay mulch, such as straw, or plastic sheets over the lawn after sowing. This makes it difficult for birds to access the seeds. Remove plastic sheets after a few days to allow light through to the grass seedlings.
Make several simple scarecrows. Tie balloons, streamers, empty cans or anything else that is shiny and moves in the wind to posts. Toy windmills are also suitable.
Push the scarecrows into the ground at intervals in the new lawn. Untie the balloons, cans or streamers every day or every couple of days and replace with new items. For example, have shiny cans one day and streamers the next.
Position imitation predators around the site. A large toy snake laid on the ground or a plastic owl on one of the posts might scare off birds, at least for a while. Move the fake predators around each day.