The Two Terrors: Hunt of the Anhinga and Cormorant

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The Two Terrors|October 14. Two birds rule the underwater hunt, terrorizing fish and frog from cypress swamp to mangrove forest. The anhinga (aka snakebird, American darter, water turkey, Tupi, and devil bird) and the cormorant (aka crow-duck, lawyer, shag and Taunton turkey). Nature has not spared these birds its sovereignty over design, tuning form and function to a life submerged in the dark belly of the river.

The anhinga (in the U.S., MX and CAR the lone species is Anhinga anhinga1), roosts in river bottomlands, exploring the thickets and snags for rough fish from the slow waters of the Santee to Gulf river headwaters and westward to the Rio Grande2. The bird is dimorphic. Males have black plumage with white to silver accents and hackles that stand on its neck like the mane of a horse. Females are mostly black, but with pale brown plumage hanging like a shirt front that ends neatly at the breast. The double-crested cormorant (Phalacrocorax auritus) is a permanent resident of Florida, and the sole species in the state (neotropic cormorants, P. brasilianus, are rarely observed; P. auritus floridanus is the proposed Florida subspecies)3. Its range is vast compared to A. anhinga (the three remaining species of Anhinga are native to S.E. Asia, Australia and Africa)4, extending from the Greater Antilles to the Aleutian Islands and everywhere in-between2. While it frequents the entangled swamps of the anhinga, it makes its home in river estuaries. The bird is named for nuptial feathers that dress the head above the eyes in breeding males5.

The anhinga and cormorant are master anglers. Adult cormorants consume as much as a pound (27% body weight) of fish per day, including bream, catfish, gar and schooling species5,6. Both birds have overcome the adversity of swamp and stream by their own devise. This includes adaptations to plumage, bone and bill for submerged hunting13. In cormorants, wing size and barbicel density are reduced, decreasing buoyancy by increasing wing loading and water absorption7,8. Its wings are designed for pursuit through water—for chasing prey up tidal creeks, and deep-diving bay pilings. In anhinga, wing barbicels are absent, and bones lack perfusion, leaving them dense and wet6,7,9. When startled from the shore, anhinga drop to water with the grace of a stone. Craning their serpentine neck above the waterline, they float with the buoyancy of a sinking ship. Yet underwater, they levitate effortlessly, ambushing prey pushed into the lie by a shoal. A cormorant’s bill turns downward at its tip and is used to snare and secure prey scattered from their aquatic sanctuaries during pursuit (in raptors, curved bills are also used to dismantle). In anhinga, the bill is lanceolate (also egrets, herons, kingfishers), and contains barb-like serrations on the lower mandible10. It is used to impale and immobilize ambushed prey before bringing them to the surface for consumption. Structural changes to cervical vertebrae and muscle provide the bill its strike force11,12.

BIBLIOGRAPHY

  1. Mayr et al. 2020. The large-sized darter Anhinga pannonica (Aves, Anhingidae) from the late Miocene hominid Hammerschmiede locality in Southern Germany. PLoS One: https://doi.org/10.1371/journal.pone.0232179
  2. Cornell Lab of Ornithology, All About Birds. Recovered from the web on July 10, 2024. https://www.allaboutbirds.org/news/
  3. Brugger, K. 1995. Double-crested cormorants and fisheries in Florida. Colonial Waterbirds: p. 110-117. https://doi.org/10.2307/1521530
  4. Mayr et al. 2020. The large-sized darter Anhinga pannonica (Aves, Anhingidae) from the late Miocene hominid Hammerschmiede locality in Southern Germany. PLoS One: https://doi.org/10.1371/journal.pone.0232179
  5. U.S. Fish and Wildlife Service. Expanding Management of Conflicts Associated with Double-crested Cormorants. (Downloaded from the internet on date): FWS-Cormorant.
  6. Casler 1973. The air-sac systems and buoyancy of the anhinga and double-crested cormorant. The Auk. 90(2): 324.
  7. Cavitt et al. Feathers and feather facts. Weber State University. Recovered from the web on August 9, 2023. https://faculty.weber.edu/jcavitt/Feather1.pdf
  8. Lapsansky et al. 2022. High Wing-Loading Correlates with Dive Performance in Birds, Suggesting a Strategy to Reduce Buoyancy. Integrative and Comparative Biology. 62(4): 878.
  9. Ryan, P. 2007. Diving in shallow water: the foraging ecology of darters (Aves: Anhingidae). J. Avian Biology. https://doi.org/10.1111/j.0908-8857.2007.04070.x
  10. Beltzer A. 1982. Note on the diet of the snakebird, Anhinga anhinga 1776, Pelicaniformes:Anhingadae in the alluvial plain of the middle Parana river and some considerations about the digestive tract. Hist. Nat. Mendoza: 2 (137).
  11. Terray et al. 2020. Modularity of the neck in birds (Aves). Evolutionary Biol. doi: 10.1007/s11692-020-09495-w
  12. Owre O. 1967. Adaptations for Locomotion and Feeding in the Anhinga and the Double-Crested Cormorant. Ornithological Monographs. 6: 1-138.

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