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Phalacrocorax atricepsimperial shag

By Miracle Davis

Geographic Range

The imperial shag Phalacrocorax atriceps is a migratory bird with eight subspecies ranging across South America. the imperial shag can be found in the Fuegan Mountains of southern Chile. Its range extends northward along the Pacific Coast, Antarctica, South Georgia, and west along the subantarctic convergence. On the Pacific Coast, the imperial shag ranges from the north of the Santa Maria Island of Portugal, to the Santa Cruz River of Argentina. Colonies can be located on irregular substrates and cliff faces. (Behn, et al., 1955; Rasmussen, 1994)

Habitat

Imperial shags are located in savannas and grasslands with temperatures around 23°C. They can be spotted as high up as 800 m on bare rock substrates along the shore and near areas free of shrubs in order to have a clear view of predators. During the breeding season, nesting colonies are found to range between 3 to 20 m above the high tide line. Nesting material for these birds includes marine algae, feathers, sticks and shell all sealed together with guano (feces). (Behn, et al., 1955; BirdLife International, 2014; Murphy, 1960; Punta, et al., 2003)

  • Range elevation
    800 (high) m
    2624.67 (high) ft

Physical Description

Imperial shags are endothermic diving birds that exhibit strong sexual dimorphism. The length of these birds range from 671 to 792 mm. They have blue eyes, a white breast, black and white cheeks, an orange-yellow nasal knob, and a dark brown bill. The ventral portion of the birds’ bodies is white while the dorsal region is a glossy black. Imperial shags have pink webbed feet. These birds are sexually dimorphic, with males being larger than females. Males have an approximate body mass of 2040-2560 g, while females weigh approximately 1590-2160 g. Males’ wingspans are 287-317 mm, while the wingspans of females are 269-298 mm. (Punta, et al., 2003; Svagel and Quintana, 2007)

  • Sexual Dimorphism
  • male larger
  • Range mass
    1590 to 2560 g
    56.04 to 90.22 oz
  • Range length
    671 to 792 mm
    26.42 to 31.18 in
  • Average wingspan
    269 to 317 mm
    in

Reproduction

Imperial shags are monogamous, generally forming a mating pair during the breeding season. While the mating behaviors of imperial shags are not well studied, European shags Phalacrocorax aristotel, which are members of the same genus, display known mating behaviors. Members of this genus use vocalizations and physical displays during courtship. Male shags attract mates by performing the dart-gape, in which they point their heads towards the sky moving it upward and forward with their bills open. During this movement, male shags ruffle their feathers to make themselves look bigger. When females approach males, the males will use a different display known as the throwback. They will align their neck almost parallel to their back and point their closed beak upwards while making vibrations with their gular pouch, which is an area of featherless skin on the throat. Female shags exercise choice; they are attracted to larger male shags because these males are capable of protecting the nest against intruders. The bow is another movement by male shags during courtship, in most cases the bow is used following the sitting-gape or the throwback. The movement of this display includes the male shags leaning forward with their head almost touching their feet, this shows their approval of other females.

The nests of imperial shags often are found on flat surfaces of rocks, created out of marine algae, feathers, sticks and shells. The nests resemble a miniature volcano with a concavity in the middle to hold eggs. During nest building, a lateral head movement called nest quivering is used. Shags conduct this movement while they hold the nesting material in their beaks and quiver their heads from side to side. This movement can be done at ground level while sitting or standing. Shaw (1986) reported that nests from imperial shags in Isla, Isable Chubut ranged between 8.4 ± 3.3 cm with a diameter of 35.6 ± 2.9 cm, the depth of the dent in the center for the eggs is approximately 2.9 ± 1.2 cm (Shaw, 1986; Snow, 1963)

Imperial shags are seasonal breeders that breed once yearly from early October to late November. Shaw (1986) conducted a study of imperial shags at Signy Island located in South Orkney Island. In this study 448 clutches were laid in 34 days in 1980 while 316 clutches were laid in 24 days in 1981 (Shaw, 1986).The difference in weather conditions between the years could be a factor in the differences in clutches laid. Imperial shags are iteroparous, and female shags typically lay an average of 3 eggs per clutch but can lay up to 5. In this study 98-99% of the clutches were 2 to 3 eggs. The eggs are a chalky-blue color. Imperial shags of Signy Island hatched after 28 to 31 days of incubation. Shag chicks hatch naked and helpless; as they grow, they have brown feathers. There have been no observations of when exactly they begin to grow feathers. Shaw (1986) sampled 20 female shags age 4-9 years had young with a birth mass of 42.0 ± 3.42g, while a sample size of 10 female shags age 10-11 had young with a birth mass of 38.5 ± 2.38g. In this study, these birds were ringed (banded) at 21 days and are recorded as fledged if they made it to 65 days. While both female and male shags reach reproductive maturity at 3 years of age, their time of independence has not yet been reported. (Shaw, 1986)

  • Breeding interval
    Imperial shags breed once yearly
  • Breeding season
    early October to late November
  • Range eggs per season
    2 to 5
  • Average eggs per season
    3
  • Range time to hatching
    28 to 31 hours
  • Average time to hatching
    29 hours
  • Range fledging age
    21 to 65 days
  • Range age at sexual or reproductive maturity (female)
    3 (low) years
  • Range age at sexual or reproductive maturity (male)
    3 (low) years

Imperial shags have adopted a biparental style of care for their young, with no gender-specific roles. When it comes to foraging during the breeding season, imperial shags have adopted a routine. Female shags forage early in the morning, while males forage in the afternoon, and both spend the night together in the nest. Both male and female shags share the responsibility of incubation. From the beginning of the egg-laying process, until young shags reach 15-20 days in age, they are always guarded in the nest by at least one parent while the other is foraging. How long imperial shags stay in their nest has not been reported. (Fowler, 1955; Harris, et al., 2013; Shaw, 1986; Snow, 1963; Svagelj, et al., 2012)

Lifespan/Longevity

According to Calderón et al. (2011) imperial shags have a lifespan of 10-15 years, with an annual adult mortality of 10-15%. Cobley (1992) studied 182 pairs of imperial shags in Signy Island in 1987. The annual mortality rate for this study was 14%. Shags older than 11 years showed a 0.04% decrease in survival rates annually. (Calderón, et al., 2012; Cobley, 1992)

  • Range lifespan
    Status: wild
    10 to 15 years

Behavior

While the behaviors of imperial shag are not well studied it is known that they are a diurnal species. However, European shags Phalacrocorax aristotelis, which are members of the same genus, may display similar behaviors. Many species of this genus spend their time diving, preening, foraging and flying. Preening is done routinely throughout the day. After young European shags have defecated, they are preened around the anus, but the dorsal surface of their bodies receive the most preening attention from their parents. Adults preen each other around the sides of their breast and the dorsal surface area. To dry their plumage, they flap their wings for four minutes. When bathing, adult shags violently beat the top of water with their wings while dipping their heads underwater.

When it comes to displaying aggressive behavior, male shags are dominant. Shags are very protective of their nest. The most aggressive behavior from this species can be seen at the nest site. When predators like the kelp gull Larus dominicanus and southern skua Stercorarius skua approach their nest, shags will stretch their necks and move directly at them while raising their tails and erecting their feathers to display aggression. Fights between shags consist of pecking of each other’s beak; sometimes one bird grasps the wings of the other. Aggressive behavior can also be seen in a shags’ young. In competition for food, when young shags are waiting to be fed they attack other young in the nest.

While tail-pulling is a form of communication that European male shags use to get the attention of the female visiting him, it can also be seen as a form of aggression between young European shags and their parents. In an observational study, young shags in Lundy, Bristol Channel were seen pulling the tail of their mother after being ignored during food begging (Snow, 1963). It is expected that imperial shags have similar behaviors as the European shag behavior described here. (Snow, 1963)

Home Range

Imperial shags are a territorial species when guarding their nest, typically only leaving to forage. Quintana et al. (2011) studied imperial shags in Punta Leon, Chubut, Argentina. They observed and measured the distance that both female and male shags traveled to forage. Their results show that both male and female shags foraged within 50 km of their colony. When traveling at sea, female shags extended a range of 605 km^2, while males covered 805 km^2. While shags are able to cover great distances, the actual territory size of this species has not yet been reported. (Quintana, et al., 2011; Quintana, et al., 2011)

Communication and Perception

Imperial shags find prey underwater by sight. White et al. (2007) noted that cormorant eyes are comparable to a human's underwater, unaided. Verheyden and Jouventin (1994) noted that imperial shags' sense of smell, while understudied, has been noted to show preference to stay away from the odor of oils spills.

While the communication of imperial shags are not well known, European shags Phalacrocorax aristotelis which are members of the same genus, are expected to share similar communication behaviors. Adult male shags are able to vocalize, while females are limited to hissing and clicking. Snow (1963) noted that adolescent European shags’ voices were indistinguishable until they are 35 days old. Day 35 is important because this is when the females begin to lose their voices, but they do maintain the ability to make a hissing noise during the greeting season. It is not noted why female shags lose their voices during this time. Snow (1963) describes the voice of a male shag as “kroak-kraik-kroke” during the breeding season. The call varies in robustness according to how far the bill is open. Vocalization is also used during courtship, along with touch. During courtship, touch is seen via tail pulling. Tail Pulling is a form of communication male shags use in order to get the attention of the female visiting him. Both male and female imperial shags have ways of communicating, such as throat-clicking (vocalization in mating by female), sitting-gape (posture for nest watching), the dart-gape and throwback (movement by male in courtship), nest quivering (head movement in nest building), alarm posture (posture before flight), and the bow (male posture of acceptance for female) which are only implemented as a behavioral aspect to certain situations. (Snow, 1963; Verheyden and Jouventin, 1994; Wanless, et al., 1999; White, et al., 2007)

Food Habits

Imperial shags are omnivores often found foraging socially in flocks of around 200. According to Casaux and Barrera-Oro (1993), the shags’ freshwater diet includes benthic organisms such as sandworms (polychaetes), snails and sea slugs (gastropods), scallops (bivalves), and squid (cephalopods). These same researchers found that fish (in 100% of stomachs observed), stones (in 100% of stomachs observed) and algae (in 78% of stomachs observed) are most frequently found in the stomach of imperial shags. (Behn, et al., 1955; Casaux and Barrera-Oro, 1993; Harris, et al., 2014; Michalik, et al., 2010; Murphy, 1960)

  • Animal Foods
  • fish
  • mollusks
  • aquatic or marine worms
  • Plant Foods
  • algae

Predation

Imperial shags are prey upon by mammalian and avian species. Among predators, kelp gull Larus dominicanus, dolphin gull L. scoresbii, and southern skua Stercorarius skua occasionally take eggs and chicks. Mammalian predators include the Patagonian ferret Lyncodon patagonicus, gray fox Lycalopex griseus, hairy armadillo Chaetophractus villosus and Geoffroy's cat Felis geoffroyi which are responsible for the deaths of many adult shags. (Punta, et al., 2003; Quintana and Yorio, 1998; Svagelj, et al., 2012)

Ecosystem Roles

Imperial shags are described as a keystone species, as omnivores that eat both fish and algae. This feeding is important because it helps maintain a balance across multiple levels in the food web. Imperial shags are one of the many sources of prey for kelp gull Larus dominicanus.

Imperial shags are prone to infection by a variety of parasites. The most common parasites are roundworms. The roundworm species found were: Contracaecum chubutensis, Contracaecum rudolphii, Cyathostoma phenisci, Contracaecum pelagicum. (Garbin, et al., 2008; Oyarzun-Ruiz and Muñoz-Alvarado, 2015)

Commensal/Parasitic Species
  • Roundworms Contracaecum chubutensis
  • Roundworms Contracaecum rudolphii
  • Roundworms Cyathostoma phenisci
  • Roundworms Contracaecum pelagicum

Economic Importance for Humans: Positive

According to BirdLife International (2014), imperial shags are used as food both locally and nationally and kept as pets internationally. (BirdLife International, 2014)

Economic Importance for Humans: Negative

There are no adverse economic effects of Phalacrocorax atriceps on humans. (BirdLife International, 2014)

Conservation Status

Imperial shags were listed as “Least Concern” on the IUCN Red List and do not have special status on US government lists. While no known conservation measurements are being taken at this time, threats and impacts of imperial shags were mentioned in Yorio et al. (1999), who studied seabirds along the Patagonian coast. They report that imperial shags are accidentally killed during trawling (a type of net used in commercial fisheries). Although numbers are not reported, imperial shags are found oiled and dead as a result of petroleum pollution on the Patagonian coast. (BirdLife International, 2014; Yorio, et al., 1999)

Contributors

Miracle Davis (author), Radford University, Cari Mcgregor (editor), Radford University, Zeb Pike (editor), Radford University, Karen Powers (editor), Radford University, April Tingle (editor), Radford University, Jacob Vaught (editor), Radford University.

Glossary

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.

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.

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

female parental care

parental care is carried out by females

food

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

herbivore

An animal that eats mainly plants or parts of plants.

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).

keystone species

a species whose presence or absence strongly affects populations of other species in that area such that the extirpation of the keystone species in an area will result in the ultimate extirpation of many more species in that area (Example: sea otter).

male parental care

parental care is carried out by males

migratory

makes seasonal movements between breeding and wintering grounds

molluscivore

eats mollusks, members of Phylum Mollusca

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.

omnivore

an animal that mainly eats all kinds of things, including plants and animals

oviparous

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

pet trade

the business of buying and selling animals for people to keep in their homes as pets.

piscivore

an animal that mainly eats fish

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

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 savanna and grassland

A terrestrial biome. Savannas are grasslands with scattered individual trees that do not form a closed canopy. Extensive savannas are found in parts of subtropical and tropical Africa and South America, and in Australia.

savanna

A grassland with scattered trees or scattered clumps of trees, a type of community intermediate between grassland and forest. See also Tropical savanna and grassland biome.

temperate grassland

A terrestrial biome found in temperate latitudes (>23.5° N or S latitude). Vegetation is made up mostly of grasses, the height and species diversity of which depend largely on the amount of moisture available. Fire and grazing are important in the long-term maintenance of grasslands.

visual

uses sight to communicate

References

Behn, F., J. Goodall, A. Johnson, B. Philippi. 1955. The geographic distribution of the blue-eyed shags, Phalacrocorax albiventer and Phalacrocorax atriceps. The Auk, 70/1: 6-13.

BirdLife International, 2014. "Phalacrocorax atriceps" (On-line). The IUCN Red List of Threatened Species 2014. Accessed March 25, 2016 at http://www.iucnredlist.org/details/22729686/0.

Calderón, L., W. Svagelj, F. Quintana, S. Lougheed, P. Tubaro. 2012. No evidence of extra-pair paternity or intraspecific brood parasitism in the imperial shag Phalacrocorax atriceps. Journal of Ornithology, 153/2: 399-404.

Casaux, R., E. Barrera-Oro. 1993. The diet of the blue-eyed shag, Phalacrocorax atriceps bransfieldensis feeding in the Bransfield Strait. Antarctic Science, 5/04: 335-338.

Cobley, N. 1992. Aspects of the Population Dynamics of Antarctic Blue-eyed Shags Phalacrocorax atriceps King (Ph.D. Dissertation). Durham University: Doctoral thesis.

Fowler, G. 1955. Stages of age-related reproductive success in birds: Simultaneous effects of age, pair-bond duration and reproductive experience. American Zoologist, 35/4: 318-328.

Garbin, L., J. Diaz, F. Cremonte, G. Navone. 2008. A new anisakid species parasitizing the imperial cormorant Phalacrocorax atriceps from the north Patagonian coast, Argentina. Journal of Parasitology, 94/4: 852-859.

Harris, R., R. Phillips, F. Quintana. 2013. Sexual segregation in timing of foraging by imperial shags (Phalacrocorax atriceps): Is it always ladies first?. Marine Biology, 160/5: 1249-1258.

Harris, S., A. Rey, F. Quintana. 2014. Strong temporal consistency in the individual foraging behaviour of imperial shags Phalacrocorax atriceps. Ibis, 156/3: 523-533.

Michalik, A., H. van Noordwijk, P. Brickle, T. Eggers, P. Quillfeldt. 2010. The diet of the imperial shag Phalacrocorax atriceps at a colony on New Island, Falkland/Malvinas Islands combining different sampling techniques. Polar Biology, 33/11: 1537-1546.

Murphy, R. 1960. Oceanic birds. Proceedings of the Royal Society of London. Series B, Biological Sciences, 152/949: 642-654.

Oyarzun-Ruiz, P., P. Muñoz-Alvarado. 2015. Imperial shag, Phalacrocorax atriceps (Aves, Phalacrocoracidae): New host for a respiratory parasite, Cyathostoma (Cyathostoma) phenisci (Nematoda, Syngamidae). Journal of marine biology and oceanography, 50/2: 353-358.

Punta, G., P. Yorio, J. Saravia, P. Borboroglu. 2003. Breeding habitat requirements of the imperial cormorant and rock shag in Central Patagonia, Argentina. Waterbirds, 26/2: 176-183.

Quintana, F., P. Yorio. 1998. Kelp gull Larus dominicanus predation on an imperial cormorant Phalacrocorax atriceps colony in Patagonia. Marine Ornithology, 26: 84-85.

Quintana, F., R. Wilson, P. Dell'Arciprete, E. Shepard, A. Laich. 2011. Women from Venus, men from Mars: Inter‐sex foraging differences in the imperial cormorant Phalacrocorax atriceps a colonial seabird. Oikos, 120/3: 350-358.

Rasmussen, P. 1994. Geographic variation in morphology and allozymes of South American imperial shags. The Auk, 11/1: 143-161.

Shaw, P. 1986. Factors affecting the breeding performance of Antarctic Blue-eyed Shags Phalacrocorax atriceps. Ornis Scandinavica, 17/2: 141-150.

Snow, B. 1963. The behaviour of the shag. British Birds, 56/3-5: 77-103.

Svagel, W., F. Quintana. 2007. Sexual size dimorphism and determination by morphometric measurements in breeding imperial shags (Phalacrocorax atriceps). Waterbird, 30/1: 97-102.

Svagelj, W., M. Trivellini, F. Quintana. 2012. Parental investment theory and nest defence by imperial shags: Effects of offspring number, offspring age, laying date and parent sex. Ethology, 118/3: 251-259.

Verheyden, C., P. Jouventin. 1994. Olfactory behavior of foraging procellariforms. The Auk, 111/2: 285-291.

Wanless, S., S. Finney, M. Harris, D. McCafferty. 1999. Effect of the diel light cycle on the diving behaviour of two bottom feeding marine birds: The blue-eyed shag Phalacrocorax atriceps and the European shag P. aristotelis. Marine Ecology Progress Series, 188: 219-224.

White, C., N. Day, P. Butler, G. Martin. 2007. Vision and foraging in cormorants: More like herons than hawks?. PLoS ONE, 2/7: e639.

Yorio, P., E. Frere, P. Gandini, W. Conway. 1999. Status and conservation of seabirds breeding in Argentina. Bird Conservation International, 9/4: 299-314.

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