maybe the birds have eaten the seeds?

You haven't said where you are in the world, but if in the UK, and somewhere that had plenty of heavy rain, its possible the seed was washed off or out. Best time to grow a lawn from seed is May or September, turfing can be done almost year round so long as the ground isn't frozen or very wet. This answer assumes you didn't apply any weedkillers or chemical feeds to the area immediately before sowing seed.