Population Size and Factors Affecting At-Sea Distributions of Four Endangered Procellariids in the Tropical Pacific

Abstract

We used at-sea censuses to quantify pelagic behavior, relationships between density and habitat variables, and to estimate population size of four taxa of procellariids in the eastern tropical Pacific: subspecies of Puffinus auricularis (Townsend's Shearwater, P. a. auricularis, and Newell's Shearwater, P. a. newelli) and subspecies of Pterodroma phaeopygia (Galapagos Dark-rumped Petrel, Pt. p. phaeopygia, and Hawaiian Dark-rumped Petrel, Pt. p. sandwichensis). All are considered threatened or endangered with extinction. Birds commuting from colonies ranged 650 km (P. a. auricularis) to 1,300 km (P. a. newelli and Pt. p. sandwichensis). P. a. auricularis foraged at upwelling fronts in the Costa Rica Current over the continental slope of Mexico, while P. a. newelli preferred ocean fronts in the Equatorial Counter Current. Unlike the shearwaters, petrel densities correlated most strongly with distance to the colony and wind speed, although the two petrels each foraged in distinctly different current systems. Foraging behavior of the shearwaters was energy demanding, but they frequented highly productive feeding areas and rested often. The petrels appeared to forage more opportunistically and conserved energy by flying more efficiently. Foraging behavior was consistent with wing morphology of each group. Pelagic population estimates (adults and subadults grouped) were 46,000 P. a. auricularis, confidence interval (CI) = 18,000 to 89,000; 84,000 P. a. newelli, CI = 57,000 to 115,000; 19,000 Pt. p. sandwichensis, CI = 11,000 to 34,000; and 88,000 Pt. p. phaeopygia, CI = 54,000 to 130,000. On the basis of breeder to non-breeder ratios in these and closely related species, we estimated breeding populations of 10,600, 18,000-19,000, 3,750-4,500, and 20,000 pairs, respectively. Calculation of minimal and maximum estimates indicated breeding populations larger than previously thought for P. a. auricularis, P. a. newelli, and Pt. p. sandwichensis, but similar for Pt. p. phaeopygia.