A critical review of the transoceanic migration of the blackpoll warbler

-Blackpoll Warblers (Dendroica striata) may reach their winter range in South America by migrating directly over the Atlantic Ocean from the northeastern United States (Drury and Keith 1962, Nisbet et al. 1963) and the Maritime Provinces (Richardson 1972, 1980), through the southeastern United States (Cooke 1904, 1915; Murray 1965, 1966a), or through both regions. Evidence obtained since the 1960s has been accepted as supporting the first of these hypotheses. My purpose was to provide a critical review of the evidence for and against these conflicting views. I examined the geographic distribution and the time of occurrence of Blackpoll Warblers during migration from the northern United States to the Caribbean islands, differences in mean body mass of samples in time and place from the northeastern United States, Bermuda, and Florida, and radar and visual evidence from over the sea. There is no unequivocal evidence that any Blackpoll Warblers nligrate over the sea directly from New England to South America, and all evidence is consistent with the hypothesis that Blackpoll Warblers migrate to South America through the southeastern United States. Received 24 October 1986, accepted 29 February 1988. DRURY and Keith (1962) and Nisbet et al. (1963) proposed that Blackpoll Warblers (Dendroica striata) normally migrate directly from New England to South America across the western Atlantic Ocean on their postbreeding migration. I (Murray 1965) questioned this interpretation and supported Cooke's (1904, 1915) view that Blackpoll Warblers reach their winter range by migrating through the southeastern United States. Although I pointed out that the data on migrant Blackpoll Warblers in coastal New Jersey (Murray 1966b, 1976, 1979) and in Michigan (Murray 1966a) were consistent with this hypothesis, other investigators have continued to support the transoceanic migratory route. Initially, Nisbet et al. (1963) suggested that Blackpoll Warblers migrated not only from New England directly to South America but from south of New England because Blackpolls were commonly observed throughout October and into mid-November at Bermuda, "long after the last departures from New England" (Nisbet et al. 1963: 133). Only 17 of 141 Blackpolls recorded at Bermuda in 1962 were alleged (Nisbet et al. 1963) to have departed from New England. Later, although Blackpoll Warblers occurred throughout the coastal southeastern states, Nisbet (1970) interpreted their low frequency in the samples from the Carolinas and their occasional association with storms in Florida and t-hp RBhamasz t-n mean t-hat fhkiy warp qty1pr blown ashore. To account for stragglers in the southeast, Nisbet proposed that the western edge of the overwater flight must pass within 160 km of southern Florida. The A.O.U. Check-list Committee accepted this view and summarized the fall migration of the Blackpoll Warbler as "mostly across northeastern North America to New England and the Maritime Provinces, thence at sea over Bermuda to the Lesser Antilles (north, at least irregularly, to Puerto Rico and the Virgin Islands) to northern South America (including islands north of Venezuela), also regularly in California" (A.O.U. 1983: 619). In contrast, I argued (Murray 1966a) that Blackpoll Warblers from the western portion of the breeding range follow a zigzag route, crossing the continent on a generally southeastward course until reaching the Atlantic coastal plain, then turning southwestward to Georgia and Florida before turning southeastward for the final leg of the flight. It now seems more likely that Blackpoll Warblers funnel from the breeding range to the southeastern United States, along with the general movement of migration (Gauthreaux 1980), and depart from somewhere between Cape Hatteras and northern Florida (Fig. 1). Details of distribution and movement in the southeastern United States, however, remain to be studied. The "transoceanic hypothesis" proposes that 8 The Auk 106: 8-17. January 1989 This content downloaded from 157.55.39.138 on Sun, 26 Jun 2016 07:50:16 UTC All use subject to http://about.jstor.org/terms January 1989] Blackpoll Warbler Migration 9 Fig. 1. The hypothetical postbreeding migration route of the Blackpoll Warbler through the southeastern United States. Hatched area shows breeding range in North America and wintering range in South America. Stippling shows actual and hypothetical distribution during migration. Short arrows over the sea show wind drift to Bermuda. B = Bermuda, A = Antigua. Blackpoll Warblers depart from the Maritime Provinces (Richardson 1972, 1980) and the "northeastern" United States (south to Cape Hatteras?) (Drury and Keith 1962, Nisbet et al. 1963, Nisbet 1970) and cross the Atlantic Ocean directly to their South American winter range. Some birds drop out at Bermuda and in the West Indies (Fig. 2). If the western edge of this movement passes within 160 km of southern Florida, as Nisbet (1970) suggested, then some Blackpoll Warblers must fly southwestward parallel to the coast (but over the ocean) before turning southeast toward South America (Fig. 2). The main difference between the two proposals is whether Blackpoll Warblers depart the United States from north or south of about the Virginia/North Carolina border. Because Blackpolls occur in the Carolinas in autumn and undoubtedly continue their migration from there, the question I address is whether Blackpoll Warblers normally depart the continent north of Cape Hatteras and migrate successfully on a direct transoceanic flight to their winter quarters in South America. Fig. 2. The hypothetical postbreeding migration route of the Blackpoll Warbler over the western Atlantic Ocean. Symbols as in Fig. 1. Long arrows over land indicate the hypothetical direction of flight.