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Rynchops nigerblack skimmer

By Mithil Pandhi

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. ("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)

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. (Burger and Gochfeld, 1990; Hammerson and Cannings, 2006)

  • Average elevation
    0 m
    0.00 ft

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. ("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)

  • Sexual Dimorphism
  • male larger
  • Range mass
    265 to 365 g
    9.34 to 12.86 oz
  • Range length
    40 to 50 cm
    15.75 to 19.69 in
  • Average length
    46 cm
    18.11 in
  • Average wingspan
    112 cm
    44.09 in

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. (Burger and Gochfeld, 1990; Gochfeld and Burger, 1994)

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. ("The IUCN Red List of Threatened Species", 2006; Burger and Gochfeld, 1990)

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. (Gochfeld and Burger, 1994)

  • Breeding interval
    Black skimmers breed once yearly during the summer months.
  • Breeding season
    Breeding occurs between the end of April and early September.
  • Range eggs per season
    4 to 5
  • Range time to hatching
    21 to 26 days
  • Range fledging age
    28 to 31 days
  • Average time to independence
    6 weeks
  • Range age at sexual or reproductive maturity (female)
    11 (low) months
  • Average age at sexual or reproductive maturity (female)
    36 months
  • Average age at sexual or reproductive maturity (male)
    48 months

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. (Burger and Gochfeld, 1990)

  • Parental Investment
  • 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. (Burger and Gochfeld, 1990; Gochfeld and Burger, 1994)

  • Range lifespan
    Status: wild
    20 (high) years
  • Typical lifespan
    Status: wild
    5 to 15 years

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. (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)

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. (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)

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. (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)

  • 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. (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)

  • Anti-predator Adaptations
  • 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. (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)

Commensal/Parasitic Species
  • Parvitaenia ibis
  • Stephanoprora denticulata

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. (Burger and Gochfeld, 1990)

Rynchops niger also promotes ecotourism. Breeding colonies attract birders. ("Dow Black Skimmer Colony", 2006)

  • Positive Impacts
  • food
  • body parts are source of valuable material
  • 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. (Burger and Gochfeld, 1990; "Dow Black Skimmer Colony", 2006)

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. ("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)

Contributors

Tanya Dewey (editor), Animal Diversity Web.

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

Glossary

Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map

Neotropical

living in the southern part of the New World. In other words, Central and South America.

World Map

acoustic

uses sound to communicate

altricial

young are born in a relatively underdeveloped state; they are unable to feed or care for themselves or locomote independently for a period of time after birth/hatching. In birds, naked and helpless after hatching.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

brackish water

areas with salty water, usually in coastal marshes and estuaries.

carnivore

an animal that mainly eats meat

chemical

uses smells or other chemicals to communicate

coastal

the nearshore aquatic habitats near a coast, or shoreline.

colonial

used loosely to describe any group of organisms living together or in close proximity to each other - for example nesting shorebirds that live in large colonies. More specifically refers to a group of organisms in which members act as specialized subunits (a continuous, modular society) - as in clonal organisms.

crepuscular

active at dawn and dusk

cryptic

having markings, coloration, shapes, or other features that cause an animal to be camouflaged in its natural environment; being difficult to see or otherwise detect.

desert or dunes

in deserts low (less than 30 cm per year) and unpredictable rainfall results in landscapes dominated by plants and animals adapted to aridity. Vegetation is typically sparse, though spectacular blooms may occur following rain. Deserts can be cold or warm and daily temperates typically fluctuate. In dune areas vegetation is also sparse and conditions are dry. This is because sand does not hold water well so little is available to plants. In dunes near seas and oceans this is compounded by the influence of salt in the air and soil. Salt limits the ability of plants to take up water through their roots.

diurnal
  1. active during the day, 2. lasting for one day.
ecotourism

humans benefit economically by promoting tourism that focuses on the appreciation of natural areas or animals. Ecotourism implies that there are existing programs that profit from the appreciation of natural areas or animals.

endothermic

animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

estuarine

an area where a freshwater river meets the ocean and tidal influences result in fluctuations in salinity.

food

A substance that provides both nutrients and energy to a living thing.

freshwater

mainly lives in water that is not salty.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

marsh

marshes are wetland areas often dominated by grasses and reeds.

migratory

makes seasonal movements between breeding and wintering grounds

monogamous

Having one mate at a time.

motile

having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.

nocturnal

active during the night

oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

piscivore

an animal that mainly eats fish

riparian

Referring to something living or located adjacent to a waterbody (usually, but not always, a river or stream).

saltwater or marine

mainly lives in oceans, seas, or other bodies of salt water.

seasonal breeding

breeding is confined to a particular season

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

social

associates with others of its species; forms social groups.

tactile

uses touch to communicate

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

terrestrial

Living on the ground.

territorial

defends an area within the home range, occupied by a single animals or group of animals of the same species and held through overt defense, display, or advertisement

tropical

the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

visual

uses sight to communicate

References

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.

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.

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

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 A Poole, F Gill, eds. 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.

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