Shearwaters are seabirds that belong to the family Procellariidae. There are around 30 different species of shearwater that are found across all of the world’s oceans. Shearwaters are medium to large seabirds with long, narrow wings. They get their name from the way they glide or “shear” through the air over water. Some of the most common species of shearwater include the Manx shearwater, sooty shearwater, great shearwater, and little shearwater.
Global Population Estimates
It is difficult to get an exact count of the global shearwater population since they are pelagic birds that spend most of their lives at sea. However, researchers have been able to estimate the population sizes for various shearwater species based on surveys of breeding colonies and at-sea observations.
One comprehensive global review published in 2016 provided the following population estimates for the major shearwater species:
Species | Population Estimate |
---|---|
Manx shearwater | 5,000,000 to 13,000,000 pairs |
Sooty shearwater | 20,000,000 pairs |
Great shearwater | Over 5,000,000 pairs |
Little shearwater | 1,000,000 to 10,000,000 pairs |
These global estimates indicate there are likely tens of millions of individual shearwaters worldwide. However, many shearwater populations face threats from introduced predators, habitat destruction, bycatch, and climate change. More research is needed to accurately monitor trends in shearwater numbers over time.
Population Trends by Region
Shearwater populations show different trends in different parts of the world. Here is an overview of population trends by region:
North Atlantic Ocean
In the North Atlantic, the Manx shearwater has an estimated current population of around 5 million pairs, most of which breed on islands off the United Kingdom. However, evidence suggests the UK Manx shearwater population has declined by about 28% since the 1980s due to lower breeding success, potentially linked to climate change impacts.
Great shearwaters in the North Atlantic, which breed on islands off eastern Canada, are estimated to have experienced significant population declines from the late 19th to late 20th century due to hunting and egg harvesting by humans. Their numbers seem to have stabilized more recently.
South Atlantic Ocean
Population data for South Atlantic shearwaters is more limited. On the Falkland Islands, breeding colonies of sooty shearwaters and Manx shearwaters are thought to be stable or increasing slightly in recent decades. However, the lack of long-term monitoring makes trends uncertain.
Indian Ocean
No thorough population estimates or monitoring programs exist for shearwaters in the Indian Ocean. Anecdotal evidence suggests some colonies (like sooty shearwaters on islands near Australia) may be declining due to invasive predators. But solid data on population trends is lacking.
Pacific Ocean
The Pacific Ocean holds some of the largest shearwater populations. On islands off New Zealand, there are millions of breeding pairs of sooty shearwaters. But these colonies have experienced massive declines since the 1970s, dropping by 50-90% due to predation by introduced mammals.
Shearwater populations on other Pacific islands show mixed trends, with some colonies declining while others are stable or increasing slightly.
Southern Ocean
In the Southern Ocean surrounding Antarctica, the short-tailed shearwater has an estimated total population of around 23 million. Breeding colonies in Australia crashed in the late 1990s, but have since partially recovered. Potential threats in southern Australia include habitat loss, introduced predators, and harvesting of eggs and chicks by indigenous communities.
Conservation Status
The International Union for Conservation of Nature (IUCN) Red List provides conservation status assessments for many shearwater species:
Species | IUCN Status |
---|---|
Manx shearwater | Least Concern |
Sooty shearwater | Near Threatened |
Great shearwater | Least Concern |
Little shearwater | Least Concern |
Short-tailed shearwater | Least Concern |
The sooty shearwater is the only species currently considered Near Threatened globally, mainly due to population declines in New Zealand. Other species are ranked as Least Concern, but many local populations and colonies are threatened or declining.
Research Challenges
There are several key challenges facing researchers attempting to study and monitor shearwater populations:
Inaccessible breeding sites
Most shearwaters nest on remote, hard-to-access islands. Reaching breeding colonies to count burrows and nests can be extremely difficult logistically.
Pelagic lifestyle
Shearwaters spend most of their lives far out at sea, only returning to breed on islands. Tracking their movements and getting population estimates at sea is very challenging.
Limited baseline data
For many shearwater species and populations, very little historical data exists on past population sizes. This makes assessing long-term population trends difficult.
Climate change impacts
Climate change affects ocean health and food webs that shearwaters rely on. The full impacts of climate change on shearwater populations are not yet clear.
Conclusion
In summary, there are estimated to be at least tens of millions of shearwaters globally across around 30 species. However, many populations face threats and have declining numbers, especially on islands with introduced predators. Sooty shearwaters and Manx shearwaters likely have populations numbering in the millions, while great shearwaters and little shearwaters may number over one million pairs each. More research is needed to provide up-to-date population estimates and monitor trends over time, especially in understudied regions like the Indian Ocean.
Going forward, increased efforts to eradicate invasive species on breeding islands and reduce seabird bycatch could help protect shearwater populations worldwide. Reducing threats from climate change will also be key to allowing shearwater numbers to stabilize or recover in the long run. With proactive conservation management, the future outlook for these incredible pelagic birds can be positive.