The black skimmer's uneven bill is an acquired advantage, not a birth defect. Its chicks hatch with upper and lower mandibles roughly equal in length, a shape suited to being fed by their parents rather than catching fish. The imbalance comes later. As the chick grows, its lower mandible lengthens more than the upper one.
That difference turns the bill into a remarkably precise fishing tool. Flying just above the water, the skimmer dips its longer lower mandible beneath the surface while keeping the upper mandible clear. Sensitive nerve endings help it detect contact with prey, prompting a rapid snap of the bill. Touch does the work. The bird can feed in dim light, when finding fish by sight would be harder.
The chick's early symmetry is no contradiction; it reflects a different way of getting food. Parents bring prey to the nest, so a hatchling has no immediate need for the elongated lower bill used in skimming. Differential growth gradually produces the adult proportions, although the exact pace varies as the bird matures. Like other birds, skimmers have a keratin covering over the bony core of the bill; that covering can wear and be renewed through growth. The adult's equipment is distinctive, but it is not a separate part attached at a particular moment. It is a change in proportion, made useful by the bird's flight and sense of touch. Above the water, the apparent flaw becomes a line drawn through the surface.
