The structure of the black skimmer’s beak is related to its specialised foraging technique, which is unique to skimmers (2) (4). A feeding black skimmer flies low over water with the beak open and the lower mandible partially submerged, ‘skimming’ the water. If the lower mandible comes into contact with a prey item, such as a small fish or crustacean, the upper mandible snaps down while the head and neck double back under the body, securing the prey, which may be swallowed in flight or taken back to land (2) (5). The long beak and relatively long neck allow the skimmer to maintain its body position just above the water surface while skimming (2), and the beak can be opened unusually wide so that the upper mandible remains clear of the water. The knifelike edges of the beak help the bird to grasp slippery prey, and the neck muscles are very strong, enabling prey to be pulled from the water as the skimmer flies past (4). Although the black skimmer may sometimes wade, it does not swim or dive (2). Since skimming is a tactile rather than a visual hunting technique, skimmers are able to hunt at night, when many fish species come closer to the surface and strong daytime winds often lessen (2) (5) (7). Skimmers are the only birds in which the pupil of the eye constricts to a narrow vertical slit, an adaptation that may achieve a greater reduction in the pupil than with a circular opening, protecting the eyes from the bright glare of sunlight on water and sand during the day (2) (8) (10).
A social bird, the black skimmer roosts and breeds in colonies ranging in size from a few to thousands of pairs, and is often found in the company of gulls and terns, from which it may derive some protection from predators (2) (3) (5) (8) (1). Large, successful colonies usually occupy the same nest site from year to year (5) (1). The species is monogamous, and both the male and female help to prepare the nest and raise the chicks. The nest is a simple, shallow scrape in the sand, into which two to five eggs are laid, hatching after an incubation period of around 21 to 26 days (2) (5). Both the eggs and the chicks are white and are well camouflaged against the sand (2) (8). The black skimmer chick is quite well-developed and able to leave the nest after about a week, with fledging occurring after 28 to 30 days (2). The first attempts at skimming are made within about two days of the first flight, but initial success is low, and the young black skimmer may be dependent on the adults for a further few weeks (5). Interestingly, the two mandibles of the beak are of equal length on hatching. The lower mandible grows continuously faster than the upper, so that by the time the chick fledges it is already nearly 1 centimetre longer (2) (5). Its growth is kept in check by wear against the muddy or sandy bottom in shallow water, or by breakage on hitting obstructions, meaning that beak length and shape is quite variable between individuals and over time (2). The black skimmer is thought to breed from around the age of one to three years, and may live for up to at least 20 years in the wild (2) (5).