At‐sea distributions and abundance of tropicbirds in the eastern Pacific

Abstract

During spring and autumn 1980–95, we surveyed Red‐tailed Phaethon rubricauda, White‐tailed P. lepturus and Red‐billed Tropicbirds P. aethereus at sea in the Pacific between the coast of the Americas and 176°W. For the Pacific, we had complete coverage of the range of aethereus, but only partial coverage of that of rubricauda and lepturus. Six areas of higher density were indicated: three of rubricauda, two of lepturus and one of aethereus. After pooling data across years, the ‘abundance’ (total number including subadults and adults) estimate for rubricauda was 81 700 (boreal spring) and 86 500 (boreal autumn) birds. Abundance of each of the three rubricauda subpopulations differed between seasons by about 50% despite seasonal consistency when populations were grouped. Furthermore, high densities of rubricauda occurred at the edge of our study area, indicating that neither of the seasonal estimates for this species could be considered as total numbers. Abundance estimates of lepturus during the non‐breeding season (when these birds had dispersed from colonies) were 11 000 and 41 500 birds for northern and southern populations, respectively. Estimated abundance of aethereus during the boreal spring was 26 700 birds, and 33 400 during the boreal autumn. Because tropicbirds are attracted to survey vessels, we also estimated the abundance after excluding those recorded as flying in a steady direction, or having been attracted to the ship. Considering only stationary birds (i.e. those that could not have been attracted), our minimum estimates were 41 000 rubricauda, 15 750 aethereus, 28 000 southern lepturus and 6400 northern lepturus.

Pelagic habitat of seabirds in the eastern tropical Pacific: effects of foraging ecology on habitat selection

“…The non-linear nature of ocean dynamics, however, can limit modeling techniques with parametric constraints. Recently, non-linear habitat modeling techniques utilizing generalized additive models (Hastie & Tibshirani 1999) have been applied to marine vertebrate ecology with relative success (Swartzman et al 1995, Hedley et al 1999, Forney 2000, Stoner et al 2001, Clarke et al 2003, Spear & Ainley 2005. Generalized additive models (GAMs) extend classical regression by exchanging linear regression coefficients with non-parametric smoothing functions.…”

Storm-Petrels of the Eastern Pacific Ocean: Species Assembly and Diversity along Marine Habitat Gradients

“…Owing to the subductance of oceanic plates beneath the western portion of American continents, there are relatively few islands for breeding in this area. Species abundance and community composition in the avifauna are known to respond to these factors (e.g., Ribic and Ainley 1988;Ballance et al 1997;Oedekoven et al 2001;Spear et al 1995aSpear et al , 2001Spear et al , 2003Spear and Ainley 2005a;Vilchis et al 2006). 1).…”

“…Besides insights into ocean habitat use, behavior, and estimates of abundance (Clarke et al 2003), our at-sea studies were designed to provide data on the relative contribution of subadults and adults to the total population (e.g., Spear et al 2003, Spear andAinley 2005a). Besides insights into ocean habitat use, behavior, and estimates of abundance (Clarke et al 2003), our at-sea studies were designed to provide data on the relative contribution of subadults and adults to the total population (e.g., Spear et al 2003, Spear andAinley 2005a).…”

At‐sea behaviour and habitat use by tropicbirds in the eastern Pacific

“…rubricauda (hereafter ‘ rubricauda ’). The at‐sea distributions of these species in the ECP have been well described (Spear & Ainley 2005).…”

“…We determined tropicbird densities (birds per km 2 of ocean) while simultaneously monitoring oceanographic parameters. Our surveys included only part of the at‐sea ranges of rubricauda and lepturus in the Pacific, but had extensive coverage of the entire at‐sea range of Pacific aethereus (Spear & Ainley 2005).…”