Population viability analysis predicts long‐term impacts of commercial Sooty Tern egg harvesting to a large breeding colony on a small oceanic island. Inch, T., Nicoll, M.A., Feare, C.J., Horswill, C.
2024. IBIS. DOI: 10.1111/ibi.13326 VIEW

With coastal habitats and islands lacking large terrestrial animals, humans used to rely heavily on marine species including fish, turtles, sharks and seabirds for protein. According to Schreiber and Burger (2002), 96% of seabirds are colonial and ground-nesting making large numbers comparatively accessible. This and the predictability of seabird breeding colonies in both space and time have predisposed them to extensive harvesting and exploitation and even today seabirds and their byproducts including oily residue, skin, feathers and eggs continue to be used and consumed around the world.

One example of this exploitation is the collecting of bird feathers to supply the millinery (hat-making) trade. Between 1897 and 1914 around 3.5 million seabirds in the Pacific Ocean alone were killed to supply feathers to garner the hat-making industry (Schreiber and Burger 2002). Today, many cultures are still dependent on seabird eggs and the income this generates, and in combination with habitat loss, overfishing, invasive species and climate change are responsible for the ongoing global decline in seabird populations.

Despite advances in agriculture and aquaculture, egg harvesting remains pivotal to communities for both economic and cultural reasons. This is well illustrated by the long-term egg harvesting of Sooty Terns (Ornychoprion fuscatus) in Seychelles. Found in abundance, but facing drastic declines, Sooty Terns are long-lived pelagic seabirds found both in the Pacific and Indian Ocean. In the Western Indian Ocean, of the 46 known colonies of Sooty Terns eight are in decline and 14 have gone extinct (Mondreti et al. 2018).

Bird Island, in Seychelles, is one of the many islands that has witnessed the severe decline of Sooty Terns over the years potentially due to extensive egg harvesting. During the 1930s yolks of Sooty Tern eggs were separated, put in barrels and exported for the egg yolk market (Feare et al. 2007). With little control, the establishment of the yolk market led to an almost total exploitation and by the end of the 20th century around seven million eggs were harvested annually (Ridley and Percy 1958). Some two million Sooty Tern eggs reached the Caribbean markets from Seychelles alone (Cline et al. 1979). Sooty Tern excrements (guano), like many other marine birds, have also proven valuable. Rich in nitrates and phosphates, guano is the perfect fertiliser (Cline et al. 1979). In the 1890s, Bird Island exported about 20,000 tons of excrement (guano) as fertiliser to Mauritius (Ham 2022).

Despite the current population decline of Sooty Terns in Seychelles, they remain categorised as least concern on the IUCN and until 2020 were continuously harvested. New Sooty Tern survival estimates combined with population viability analysis generated in our paper show that the recent population decline has been predominantly driven by egg harvesting. Conservation actions such as habitiat management and a temporary ban are underway to try and reverse the declines seen both on Bird Island and more generally throughout Seychelles.

A gap in the demographic profile of Sooty Terns persists, which can only be addressed through continuous, long-term monitoring and mark-recapture programs. The lack of data on foraging habitats, temporal variation in breeding success, and estimates of dispersal rates continues to hinder our ability to understand and quantify the impacts of human-induced factors on both local and global colonies. Additional studies and satellite tracking are essential to gain a better understanding of the demographic profile of Sooty Terns and to inform conservation decisions.


Schreiber, E.A., Burger, J. 2002. Biology of Marine Birds.

Cline, D.R., Wentworth, C., Barry, T.W., Bartonek, J.C., Nettleship, D.N. 1979. Social and economic values of marine birds. Conservation of marine birds of northern North America. Wildlife Research Report 11. VIEW

Feare, C.J., Jaquemet, S., Le Corre, M. 2007. An inventory of Sooty terns (Sterna fuscata) in the western Indian Ocean with special reference to threats and trends. Ostrich 78:2. VIEW

Ham, A. 2022. A paradise with too many palm trees. BBC News VIEW

Mondreti, R., Priya, D., Gremillet, D. 2018. Illegal egg harvesting and population decline in a key pelagic seabird colony of the Eastern Indian Ocean. Marine Ornithology. VIEW

Ridley, M.W., Percy, L.R.C. 1958. The exploitation of sea birds in Seychelles (No. 25). HM Stationary Office.

Steadman, D.W., Plourde, A., Burley, D.V. 2002. Prehistoric butchery and consumption of birds in the Kingdom of Tonga, South Pacific. Journal of Archaeological Science 29:6. VIEW

Image credits

Top right: Sooty Tern © Patrick Kavanagh CC BY 2.0 via Wikimedia Commons.

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