The threespine stickleback is always as much of a bully as a four-inch fish can be, but in the spring it turns a brilliant red and becomes more of a bully than ever. It is time for it to build a nest and rear some babies.
It gathers algae and scraps of plants and begins to weave a nest around the stems of water plants. It holds the nest together with a thread he spins from his kidney.
Sometimes it has to build three or four nests before it is satisfied. When it is, it chases a female stickleback into the nest, where she deposits her eggs. It then chases her out, fertilizes the eggs, and stands guard.
In about a week, the eggs hatch, and the baby fish emerge. Their father guards them carefully, and if one strays too far, it catches it in his mouth and spits it right back into the nest!