Comparison of Ship and Aerial Surveys of Birds at Sea

Abstract

Field tests were made off California to compare estimates of seabird density and species composition resulting from simultaneous ship and aerial observations. In controlled survey experiments, significantly higher mean densities were calculated from aerial observations than from the ship. Under variable field conditions, however, densities derived from vessel surveys for five species groups were statistically indistinguishable from corresponding aerial figures. On ship surveys, 95-97% of all birds were identified to species, whereas from the air, specific identifications were made for 77-96% of the birds seen. About the same total numbers of species were noted from the two platforms, and reported species composition was similar. J. WILDL. MANAGE. 49(2):405-411 During the mid-1970's, the needs of managers for basic population inventories of seabirds led to several large scale, multi-year studies involving both shipand aircraft-based observations at sea (Briggs et al. 1981, Wahl et al. 1981, Gould et al. 1982, Vermeer et al. 1983, Blake et al. 1984). Survey methodology in openwater habitats previously had received little critical examination, so investigators devised a variety of field procedures and accumulated vast data bases, with only modest standardization between research groups. It has become apparent that aerial and shipboard methodologies should be evaluated and compared. Qualitative comparisons of strip censuses from vessels and aircraft have been made by Briggs et al. (1981), Wahl et al. (1981), and Gould et al. (1982), whereas limited quantitative comparisons, emphasizing littoral habitats, have been reported by King and Conant (1982) and Savard (1982). Generally, aerial surveys cover wide areas (102-104 km2) in a few hours' to a few days' time and have been reported to yield relatively low counts along coastlines when compared to counts from shore or a boat. Due to short sighting times, aerial counts are reported to produce relatively imprecise information concerning species composition. Because ship surveys cover comparable areas more slowly, they introduce the possibility of changes in bird populations during a single survey. However, they yield more precise identifications and have the advantage of facilitating collection of correlative oceanographic information. Both types of survey are thought to have i herent biases leading to error in determination of bird density: (1) birds may be overlooked or their numbers overor underestimated; (2) on ship transects inclusion of counts of flying birds may exaggerate apparent densities (Tasker et al. 1984); (3) ship-followers (e.g., gulls [Larus spp.] and fulmars [Fulmarus spp.]) or avoiders (e.g., loons [Gavia spp.]) may be represented differently from those that are neutral to the ship's presence. Glare, altitude, and the width of transect corridors can affect density and species composition estimates from airplanes (Briggs et al. 1985). In this paper we compare some aspects of strip transects made during ship and aerial surveys. We also examine survey data taken off southern California during 1975-78 to compare density estimates under normal (variable) field conditions. Though limited, these appear to be the first direct comparisons reported for the two types of survey in offshore areas. D. H. Dettman, K. F. Dettman, E. W. Chu, G. D. Farrens, M. O. Pierson, and M. L. Bonnell This content downloaded from 157.55.39.177 on Wed, 16 Nov 2016 04:33:30 UTC All use subject to http://about.jstor.org/terms 406 SHIP/AERIAL BIRD SURVEYS * Briggs et al. J. Wildl. Manage. 49(2):1985 assisted with fieldwork. Analyses were discussed with G. L. Hunt, Jr., M. Tasker, and S. M. Speich. D. W. Anderson, J-P. L. Savard, and K. Vermeer provided helpful comments on earlier drafts; and C. Hitchcock patiently prepared the manuscript. Financial support was provided by the U.S. Dep. of Inter., Miner. Manage. Serv. (formerly Bur. of Land Manage.) under contracts AA550-CT7-36 and 14-12-0001-29090 with the Univ. of California.