Bird food is that which is eaten by birds, often varieties of seeds, which is often used to feed domesticated poultry. However, there are many that also make or buy bird food to feed pet birds or use in bird feeders for those birds that are found in the wild. It can be either natural or commercial. There can be many different types of bird food, and different varieties to suit different types of bird.
So can gerbils eat bird seed?
Unfortunately it is not a good idea to do this. They have different nutritional requirements compared to birds and so the seed mix would not suit their needs.
If they nibble in some they will be fine but it shouldn’t be fed to them as a main staple food.