Rynchops niger

Geographic Range

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.

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.

• Biogeographic Regions
• nearctic nearctic
  • native native
• neotropical neotropical
  • native native

Habitat

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 .

• Habitat Regions
• temperate temperate
• tropical tropical
• terrestrial terrestrial
• saltwater or marine saltwater or marine
• freshwater freshwater

• Terrestrial Biomes
• desert or dune desert or dune

• Aquatic Biomes
• lakes and ponds
• rivers and streams
• coastal coastal
• brackish water brackish water

• Wetlands
• marsh marsh

• Other Habitat Features
• riparian riparian
• estuarine estuarine

Physical Description

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.

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.

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.

• Other Physical Features
• endothermic endothermic
• homoiothermic
• bilateral symmetry bilateral symmetry

• Sexual Dimorphism
• male larger

Reproduction

Black skimmers begin a courtship process once they arrive at a breeding colony. They form dense flocks and form pairs within about one week. Though individuals may change location in the group more than once, males and females are usually paired within a few days and establish territories. They are monogamous and males aggressively protect their mates. Black skimmers exhibit courtship feeding usually during the evening hours. In this process, a male will present a fish to the female. Once the female accepts the fish and holds it in her beak, the male mounts her and copulation occurs. The female will hold the fish in her beak during copulation and swallow the fish afterward. This is a distinct difference between black skimmers and terns , where females usually eat the fish before copulation. If a male cannot find his mate a fish, he may still be successful in courting her by presenting a stick or a leaf. Copulation may occur several times a day.

• Mating System
• monogamous monogamous

Black skimmers breed annually during the warmer months of summer, generally between the end of April until the beginning of September. Clutch size ranges between 4 and 5 eggs. Egg laying occurs over a span of about 8 days and it is uncommon for female black skimmers to lay multiple eggs on the same day. Incubation takes between 21 to 26 days while the time to fledging usually takes about 28 days. Because males are larger on average, they can take up to 31 days to fledge.

On average, females reach sexual maturity around 3 years of age. The youngest breeding female discovered was 11 months old. Males tend to reach sexual maturity around 4 years of age.

• Key Reproductive Features
• iteroparous iteroparous
• seasonal breeding seasonal breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual sexual
• oviparous oviparous

Both sexes of R. niger attend to the eggs during the period of incubation. Black skimmers protect and care for their chicks until the time of fledging, which can take place 28 to 31 days from hatching. Males tend to feed young chicks more than females, but both sexes feed their young. Rynchops niger chicks are protected from overexposure to the elements by their parents. Black skimmers are territorial and protective of their young and will attack other skimmers, including other fledglings from coming near their nest.

• Parental Investment
• altricial altricial
• pre-fertilization
  • provisioning
  • protecting
    • female
• pre-hatching/birth
  • provisioning
    • female
  • protecting
    • male
    • female
• pre-weaning/fledging
  • provisioning
    • male
    • female
  • protecting
    • male
    • female

Lifespan/Longevity

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.

Behavior

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.

• Key Behaviors
• terricolous
• flies
• diurnal diurnal
• nocturnal nocturnal
• crepuscular crepuscular
• motile motile
• migratory migratory
• territorial territorial
• social social
• colonial colonial

Home Range

Black skimmers forage between 5.2 to 8 km from colony sites.

Communication and Perception

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.

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.

• Communication Channels
• visual visual
• acoustic acoustic

• Perception Channels
• visual visual
• tactile tactile
• acoustic acoustic
• chemical chemical

Food Habits

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.

Fish species eaten include Odonthestes argentinenesis , Brevoortia aurea , Anchoa marinii , Lycengraulis grossidens , Engraulis anchoita , Pomatomus saltatrix , Mugil cephalus , Fundulus heteroclitus , and Anchoa mitchelli .

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.

• Primary Diet
• carnivore carnivore
  • piscivore piscivore

• Animal Foods
• fish
• insects
• mollusks
• aquatic crustaceans
• other marine invertebrates

Predation

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.

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.

• Anti-predator Adaptations
• cryptic cryptic

Ecosystem Roles

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.

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 .

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.

Economic Importance for Humans: Positive

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.

Rynchops niger also promotes ecotourism. Breeding colonies attract birders.

• Positive Impacts
• food food
• body parts are source of valuable material
• ecotourism ecotourism

Economic Importance for Humans: Negative

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.

Conservation Status

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.

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.

Additional Links

Encyclopedia of Life

Contributors

Tanya Dewey (editor), Animal Diversity Web.

Mithil Pandhi (author), Kalamazoo College, Ann Fraser (editor, instructor), Kalamazoo College.

References

Burger, J., M. Gochfeld. 1990. The Black Skimmer: Social Dynamics of a Colonial Species . New York: Columbia University Press.

Burger, J. 1981. Sexual differences in parental activities of breeding black skimmers. American Naturalist , 117 (6): 975-984. Accessed October 11, 2006 at http://links.jstor.org/sici?sici=0003-0147%28198106%29117%3A6%3C975%3ASDIPAO%3E2.0.CO%3B2-X .

Collins, C. "U.S. Fish & Wildlife Service" (On-line). Black skimmer (Rynchops niger). Accessed October 11, 2006 at http://www.fws.gov/bolsachica/BlackSkimmerprofile.htm .

Erwin, M. 1977. Black skimmer breeding ecology and behavior. The Auk , 94 (4): 709-717. Accessed October 11, 2006 at http://elibrary.unm.edu/sora/Auk/v094n04/p0709-p0717.pdf .

Gochfeld, M., J. Burger. 1994. Black skimmer. Pp. 1-29 in The Birds of North America , Vol. 3 (108), 1st Edition. Philadelphia, PA: American Ornithologists Union & The Academy of Natural Sciences of Philadelphia.

Groom, M. 1992. Sand-colored nighthawks parasitize the antipredator behavior of three nesting bird species. Ecology , 73 (3): 785-793. Accessed October 10, 2006 at http://links.jstor.org/sici?sici=0012-9658%28199206%2973%3A3%3C785%3ASNPTAB%3E2.0.CO%3B2-Y .

Hammerson, G., S. Cannings. 2006. "NatureServe Explorer" (On-line). Comprehensive Report Species - Rynchops niger. Accessed October 11, 2006 at http://www.natureserve.org/explorer .

Mariano-Jelicich, R., M. Faver. 2006. Assessing the diet of the black skimmer through different methodologies: is the analysis of pellets reliable?. Waterbirds , 29 (1): 81-87. Accessed November 06, 2006 at http://www.bioone.org/perlserv/?request=get-document&doi=10.1675%2F1524-4695%282006%2929%5B81%3AATDOTB%5D2.0.CO%3B2 .

Dow Chemical Company. 2006. "Dow Black Skimmer Colony" (On-line). Accessed October 25, 2006 at http://www.dow.com/facilities/namerica/texops/community/skimmer.htm .

2006. "The IUCN Red List of Threatened Species" (On-line). Rynchops niger. Accessed October 11, 2006 at http://www.iucnredlist.org/search/details.php/49209/all .