Newly hatched ducklings feed almost exclusively on invertebrates. This includes everything from earthworms to small crustaceans, snails, and insect larvae and adults. Invertebrates provide a concentrated source of protein and fat required by a rapidly growing animal. By the time ducklings are about 2 weeks old, or one-quarter grown, they begin to consume a diet that is more adult like. In some species this is still mostly invertebrates; in others, it is mostly plant matter. Invertebrate availability influences where a hen will take her brood. Very often people find a mallard nest close to a particular wetland and then are concerned because the eggs hatched, but the brood never appeared on their wetland. Many folks think that this means that something killed the ducklings, but in many cases it simply means that the hen took the brood to another wetland that had richer invertebrate resources. Such wetlands may be as far as 3 miles away from the nest. The ducklings don't have to feed while they are hiking to the "brood pond" because they have a "yolk sack" that contains the remainder of the egg yolk. The yolk sack contains enough nutrient reserves to keep the ducklings going for about 3 days after they hatch. The need for invertebrates also explains why very young broods are seldom seen on large lakes. Because water levels are relatively stable on such lakes, nutrients do not recycle rapidly enough to support the large invertebrate populations required by broods. Also, such lakes are likely to hold fish populations that may compete with ducklings for the limited invertebrates available there. This is one reason that shallow marshes are so valuable to waterfowl production.