Nature Of Migration In The Species
Like all Procellariiformes, tied to land only in breeding season; free to seek food throughout oceans in northern winter. Evidence from birds sighted or collected at sea, on methodical tracks or concentrated in shipping lanes, shows that most birds winter in tropical waters. Little evidence from recovered banded birds.
Timing And Routes Of Migration
Atlantic Ocean, Spring
Returns via w. Atlantic to North American islands. Seen near Bermuda mid-March to mid-June, 20/h in April. Density increases April-June off se. U.S.. Many stranded on Florida and Georgia beaches late May 1991. On Kent I., New Brunswick, earliest recorded arrival 17 April; most arrive May, but prebreeders reach maximum latter half of July.
Atlantic Ocean, Fall
At Kent I., New Brunswick, prebreeder numbers decline in August; most prebreeders gone by September. Breeders and nestlings begin departure early September. Last birds leave November; chick hatched about 11 September departed 27 November. Many migrate east to European waters after breeding. Estimated 300,000-2,000,000 birds, far more than total European breeding population, in Bay of Biscay and adjacent waters September-February, peak numbers November-December. A 1962 nestling from Gull I., Newfoundland (47°15'N, 52°46'W), found dead on beach in sw. Spain, Jan 1963; 1986 breeding bird from Kent I., New Brunswisck, found dead in nw. France, Jan 1988. Most birds winter in tropical Atlantic, especially w. African waters. Strong southward migration also past Bermuda October-November, little studied.
Pacific Ocean, Spring
Observations of flocking birds at sea indicate that major migration begins March, but earliest arrivals at Farallon Is., California, and Little River Rock, California (41°02'N, 124°00'W) in last week of February. Usually most abundant seabird species over equatorial and subtropical surface waters in e. Pacific in spring and fall, slightly less abundant in cool La Niña years (Ribic et al. 1992).
Pacific Ocean, Fall
Increase in population density observed in central Pacific as early as September, initially comprising prebreeders from breeding colonies or northerly summering waters or failed breeders that left nesting colonies early; these 2 groups are indistinguishable.
Rare north of Aleutian Is. at any season. Common in summer not only near Canadian, U.S., and Mexican nesting colonies but also off Pacific coasts of Nicaragua, Costa Rica, Panama, and west and northwest of Galapagos Is. (obviously nonbreeders; Crossin 1974).
Winters mostly in central and e. Pacific tropical waters, but seen year-round in Gulf of Alaska. In e. Pacific variably abundant, up to >0.2 birds/km2 in wide area from about 20°N 122°W off s. California to 8°S 90°W south of Galapagos Is. and westward (knowledge limited by coverage of Pacific Ocean Biological Survey Program, 1963–1969; western tropical Pacific little studied). Most of these e. Pacific birds referable on basis of size to O. l. leucorhoa populations nesting from se. Alaska to s. California, alternatively called O. l. beali, but also include dark-rumped forms O. l. chapmani. Less abundant in central Pacific than in east, but densities of >0.08 birds/km2in wide belt between 174°W and 173°E from equator to 4°N, rarely south of 12°S. Several smaller high-density areas lie west and south of Hawaii. Most common storm-petrel in Hawaiian area in northern fall .winter.spring; replaced by Band-rumped Storm-Petrel in summer.