Wild gerbils get most of the moisture they need from the vegetables and small seeds that they eat. They aren't vegetarian though and will eat insects and carrion if they find it. Gerbils kept as pets should be given a fresh supply of water and a proper gerbil food to make sure they get all the nutrients they need to stay healthy.
Gerbils will explore and try out almost any foods and although they will live happily on a diet of ready mixed gerbil food, they will really enjoy having some variation with small amounts of other foods. As they are omnivours like us, they can eat anything we can eat, but they tend to prefer dry or crunchy foods. They are particularly fond of oily seeds like sunflower and linseed and they adore millet. They will also eat insects, but this is mostly down to an individual gerbil’s preference.
Gerbils need less fat in their diet than hamsters, so it is best to avoid giving large numbers of sunflower seeds if the mix already contains some. Sunflower seeds are a gerbil’s favourite and they will eat them to the detriment of other foods and can become too fat.
Ideal treats include: dried banana, small amounts of budgie or canary food, pumpkin seeds, millet sprays sold for birds in pet shops, cooked beans, crispy vegetables such as raw broccoli, small amounts of fruit, such as grapes (but break the skin so the gerbil can easily see the moist interior,) alfalfa and hay.
Always give small amounts of these treats and remove any fresh food (like grapes) as soon as the gerbils lose interest in it. If you leave it they will bury any uneaten food and it will start to decay causing a smell and other possible health hazards.
Although gerbils drink very little and can get most of the water they need from their diet, they should always have access to water, particularly so in the case of elderly or pregnant or nursing gerbils. It is best to use a water bottle as gerbils have a natural instinct to bury their water bowls.