Least Storm-petrel Hydrobates microsoma


Justification of Red List Category
This species has a very large range, and hence does not approach the thresholds for Vulnerable under the range size criterion (Extent of Occurrence <20,000 km2 combined with a declining or fluctuating range size, habitat extent/quality, or population size and a small number of locations or severe fragmentation). Despite the fact that the population trend appears to be decreasing, the decline is not believed to be sufficiently rapid to approach the thresholds for Vulnerable under the population trend criterion (>30% decline over ten years or three generations). The population size is very large, and hence does not approach the thresholds for Vulnerable under the population size criterion (<10,000 mature individuals with a continuing decline estimated to be >10% in ten years or three generations, or with a specified population structure). For these reasons the species is evaluated as Least Concern.

Population justification
Brooke (2004) estimated the global population to number hundreds of thousands of individuals, possibly millions.

Trend justification
The population was previously suspected to be in decline owing to predation by invasive species on its main breeding islands. A successful long-term campaign to eradicate invasive species from these islands has secured these sites (Aguirre-Muñoz et al. 2011, Bedolla-Guzmán et al. 2017), and so the population trend is now tentatively assessed as stable.

Distribution and population

This species breeds on islands in the Gulf of California and on the western coast of Baja California (Mexico). Its range stretches from California (U.S.A.) to as far south as Colombia and Ecuador.


This species is marine and often pelagic, mainly occurring over warm waters away from land. Its diet is comprised mostly of small planktonic crustaceans, particularly the larvae of the spiny lobster. It feeds on the wing by pattering, and by surface-seizing whilst on the water. Breeding starts in June or July, when it forms colonies on small islands or rocky stacks; nests are placed in rock crevices (del Hoyo et al. 1992)


Previously introduced predators, principally house rats Rattus rattus and domestic cats Felis catus (Carboneras et al. 2018) negatively impacted the main breeding colonies (Bedolla-Guzmán et al. 2017), but a successful long-term campaign to eradicate damaging invasive species from these islands has secured these sites (Aguirre-Muñoz et al. 2011, Bedolla-Guzmán et al. 2017). It is not unlikely, however, that rats may recolonise the islands at some point in the future.


Text account compilers
Ekstrom, J., Calvert, R., Fjagesund, T., Butchart, S., Hermes, C., Martin, R., Miller, E., Stuart, A.

Recommended citation
BirdLife International (2022) Species factsheet: Hydrobates microsoma. Downloaded from on 16/05/2022. Recommended citation for factsheets for more than one species: BirdLife International (2022) IUCN Red List for birds. Downloaded from on 16/05/2022.