Justification of Red List Category
This species has an extremely 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.
The population is estimated to number at least 30,000 individuals (C. Devenish per D. Diaz in litt. 2008).
The population is suspected to be in decline owing to limited food availability.
The Nazca Booby is found in the eastern Pacific Ocean, from the Galápagos Islands (Ecuador) in the south to islands off Baja California (Mexico) in the north.
This species practices obligate siblicide.
Food limitation has been shown to limit clutch size in some individuals, even in years of average and good food availability (Clifford and Anderson 2001). The collapse in sardine populations around the Galápagos Islands caused the boobies to shift to feeding on lower quality flying fish, leading to a halving of annual breeding success and causing the population to start to decline (Tompkins et al. 2017). Predictions of rapid further warming around the Galápagos Islands would result in sardines being permanently excluded from the masked booby's foraging range, preventing the growth rate from recovering (Tompkins et al. 2017).
Text account compilers
Ekstrom, J., Fjagesund, T., Butchart, S., Martin, R., Calvert, R.
Diaz, D., Devenish, C.
BirdLife International (2019) Species factsheet: Sula granti. Downloaded from http://www.birdlife.org on 22/01/2019. Recommended citation for factsheets for more than one species: BirdLife International (2019) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 22/01/2019.