Rynchops niger
black skimmer

Rynchops niger, or black skimmers, have a large range. They are found in the Nearctic and Neotropical regions. In North America, they can be found on the Atlantic and Pacific coasts of the United States. Black skimmers are seen as far north as New York and in the south along the coast of the Gulf of Mexico. They occurs along the west coast of North America, from California through Mexico. In South America, they occur virtually throughout the continent. They can be found in each coastal and land-locked country. In the Caribbean, R. niger can be seen as far east as the Virgin Islands and Grenada. ("The IUCN Red List of Threatened Species", 2006; Hammerson and Cannings, 2006)

Rynchops niger is a migratory species. Individuals from northern North America winter in the southern United States and South America or the Caribbean. Individuals from southern South America may winter farther north, but only as far as Costa Rica. Typically, northern populations of R. niger migrate south in November. (Burger and Gochfeld, 1990; Hammerson and Cannings, 2006)

Rynchops niger is primarily found in bays, estuaries, lagoons, mudflats, beaches, shell banks, spoil islands, and coastal marshes. Individuals nest mostly on sand, salt marsh mats, and dredge spoil. They often choose sites near terns, including Sterna dougallii and Sterna antillarum. (Burger and Gochfeld, 1990; Hammerson and Cannings, 2006)

Black skimmers have distinctive physical characteristics with respect to color and shape. The upper part of the body is black and the lower body and forehead are white. Black skimmers have short tails with white spots on them. They have a bright red-orange bill with a black tip. The lower mandible is longer than the upper mandible by 2 to 3 cm. The feet are bright red-orange and webbed. Rynchops niger juveniles are a mottled brown color and the sexes are indistinguishable from one another. Their upper and lower mandibles are of equal length after hatching but not after fledging. ("The IUCN Red List of Threatened Species", 2006; Burger and Gochfeld, 1990; Gochfeld and Burger, 1994; Hammerson and Cannings, 2006)

Black skimmers exhibit sexual dimorphism in that males tend to be larger than females. Males weigh about 365 g while females weigh 265 g. On average, individuals are about 46 cm long and have a wingspan of 112 cm. ("The IUCN Red List of Threatened Species", 2006; Burger and Gochfeld, 1990; Gochfeld and Burger, 1994; Hammerson and Cannings, 2006)

Black skimmers are unique in that they have a large pupil with a vertical slit. The eyes are often difficult to see because they are surrounded by feathers. (Gochfeld and Burger, 1994)

Black skimmers generally live to between 5 and 15 years of age in the wild, but have a maximum lifespan of 20 years in the wild. (Burger and Gochfeld, 1990; Gochfeld and Burger, 1994)

Black skimmers are a social species. They form colonies that consist of an aggregation of flocks containing both young and old birds. In colonies, males and females form pairs. Colony size can vary between a small number of pairs to several thousand pairs, but when colonies are large the birds tend to nest in clusters of less than 50 pairs. There is no social hierarchy but black skimmers tend to be highly territorial and defend their nest sites. The distance between nests is usually 95 cm, but when space is available nests can be up to 400 cm from the nearest neighboring nest. Individuals communicate with one another both vocally and through posturing displays. Black skimmers exhibit aggressive behaviors such as barking and posturing to defend a territory. They also engage in active and passive anti-predator behavior. Their unique foraging behavior is interesting and distinguishes them from other birds as they skim above the water with the long, lower mandible extended into the water to search for prey. They are successful nocturnal foragers but are also active during the day and at dusk and dawn. Black skimmers clean themselves regularly. They bathe in freshwater and exhibit preening behavior. When sleeping, they tuck their heads into the feathers on their back. (Burger and Gochfeld, 1990; Gochfeld and Burger, 1994)

Home Range

Black skimmers forage between 5.2 to 8 km from colony sites. (Burger and Gochfeld, 1990)

Black skimmers communicate with each other by both vocalization and displays. Their bark has been commonly described as a low-frequency bark like that of a dog. They bark as an anti-predator response, to warn their neighbors of potential danger, and also give a low-frequency bark to display aggression when neighbors intrude on their territory. The barks of males are of a lower frequency than those of females. Typical barking calls have a high frequency of approximately 6 kHz and the longest of these barks can last about 0.5 sec. These barks are often accompanied by head tosses and an aggressive upright posture. A head toss occurs when black skimmers quickly raise and lower the head and bill. In an aggressive upright posture, black skimmers extend their legs, body, and neck and hold the carpals away from the body. They may also open their bill without making noise.

Another aggressive posture is the low oblique. This posture may follow the upright aggressive posture but is less intense. It is often accompanied by head tosses and soft barks. In the low oblique posture they tilt the body down and extend the tail and wings upward. To avoid aggression when moving through a colony black skimmers walk with the bill held upright. When selecting a nesting location, males and females communicate by kicking sand to establish a scrape at that particular spot. This method of communication also allows their neighbors to know where the nest will be made. Black skimmers communicate vocally most often when they are in their own territories. (Burger and Gochfeld, 1990; Gochfeld and Burger, 1994)

The tactile senses of black skimmers are important in their foraging behavior. When skimming over the water at night, they use their lower mandibles to make contact with fish and other marine prey before catching them. Their vertical pupils may help their vision during nocturnal feeding or protect their retinas. (Burger and Gochfeld, 1990; Gochfeld and Burger, 1994)

Black skimmers are piscivores, their diet primarily consists of small fish from 4 to 12 cm in length. Fish that are smaller than 2 cm are fed to young birds. They also eat arthropods, such as crustaceans, and other marine invertebrates. (Gochfeld and Burger, 1994; Hammerson and Cannings, 2006)

Fish species eaten include Odonthestes argentinenesis, Brevoortia aurea, Anchoa marinii, Lycengraulis grossidens, Engraulis anchoita, Pomatomus saltatrix, Mugil cephalus, Fundulus heteroclitus, and Anchoa mitchelli. (Gochfeld and Burger, 1994; Mariano-Jelicich and Faver, 2006)

Black skimmers have a unique foraging behavior in which they fly low over the water while submerging their sharp, elongated lower mandible into the water. Once the prey comes into contact with the lower mandible, the upper mandible closes and the prey is captured. Their method of feeding allows them to be successful nighttime feeders. (Burger and Gochfeld, 1990)

Black skimmers exhibit anti-predator behavior both actively and passively. Individuals form colonies, and this group living helps them passively avoid predation through spacing and numbers. They also employ vigilance behavior as an antipredator mechanism. Individuals of pairs look in opposite directions, or multiple individuals in a colony are able to look in every direction for approaching predators. When threatened by a predator, individuals in a colony will fly and give warning calls that are spread by neighboring individuals until the alarm radiates throughout the colony. Although adults do not exhibit cryptic defense, chicks are cryptically colored and able to blend in to their typical surroundings. Rynchops niger juveniles hide in small scrapes near vegetation where they can blend in to their surroundings. (Burger and Gochfeld, 1990)

Black skimmers fall prey to many different types of animals. Mammals and predatory birds often eat adults and juveniles alike. Egg predation is also a problem for them, as even ants, like Solenopsis molesta, Lasius neoniger, and Tetramorium caespitum can be a problem to eggs. (Burger and Gochfeld, 1990)

Black skimmers are important predators in coastal ecosystems in North and South America. They may play an important role in regulating the populations of small fishes. Rynchops niger is also prey for a variety of larger predators. (Burger and Gochfeld, 1990; Gochfeld and Burger, 1994)

It has been argued that R. niger exhibits information parasitism by taking advantage of warning signals from nearby colonies of common terns like Sterna dougallii and Sterna antillarum. (Burger and Gochfeld, 1990)

Little is known of parasites that affect R. niger. External parasites are rare, but internal parasites have been identified. The flatworms Parvitaenia ibis (Cestoda) and Stephanoprora denticulata (Trematoda) were present in individuals affected by botulism. Rynchops niger young are often victims of diarrheal epidemics that affect 10 to 15 percent of fledglings. The organism responsible has yet to be isolated and identified. (Burger and Gochfeld, 1990; Gochfeld and Burger, 1994)

Black skimmers often establish colonies on sandy beaches that could be tourist destinations. Once a colony is established, it is rare for them to leave. For example, black skimmers established a colony in the parking lot at a Dow Chemical Company facility in Texas, making it inaccessible to employees of the company. (Burger and Gochfeld, 1990; Dow Chemical Company, 2006)

Black skimmers were commonly hunted for food before the twentieth century, but are no longer hunted. Their eggs are still valuable to collectors and for food. (Burger and Gochfeld, 1990)

Rynchops niger also promotes ecotourism. Breeding colonies attract birders. (Dow Chemical Company, 2006)

Black skimmers are abundant and not in any serious danger of decline. Therefore, the IUCN Red List has the species listed under "Least Concern." Black skimmers are also protected by the US Migratory Bird Act. ("The IUCN Red List of Threatened Species", 2006)

Black skimmers were once hunted nearly to extinction for food, but are not threatened by hunting anymore. The coastal habitat of R. niger makes these birds vulnerable to human interference through construction, recreational water activities, fishing, crabbing, clamming, and dredging. In addition, like many birds, black skimmers are susceptible to environmental contaminants. The presence of pesticides can lead to shell thinning in eggs while other pollutants can lead to the formation shell-less eggs. Oil pollution and high levels of metal contamination found in R. niger are concerns as well. (Burger and Gochfeld, 1990)