Reorientation of nocturnal landbird migrants over the Atlantic Ocean near Nova Scotia in autumn

-The behavior of landbirds involved in the predominating SSW-W migration over and near Nova Scotia during autumn nights was studied by radar. Takeoff began 28 ? 5 min after sunset. Many landbirds flew from Newfoundland across Cabot Strait to Nova Scotia and from western Nova Scotia across the Gulf of Maine toward New England. Contrary to most previous night observations, some changed course to reach land or to avoid moving offshore. However, many crossed coasts at right or acute angles without hesitation or turning. Landbirds moved offshore on nights with offshore, alongshore, and even onshore winds. Landbirds flying SSW-W over the sea late in the night sometimes ascended and/or reoriented WNW-NNE toward land, especially when winds were unfavorable for continued SW flight and/or favorable for flight to the NW. Reoriented birds usually continued to arrive at the coast until noon, but the density of arrival declined steadily through the morning. Indirect but strong evidence indicates that directional cues additional to sight of land were often used for reorientation. No evidence of dawn reorientation by landbirds flying SE-SSW toward the West Indies was found. Received 15 November 1977, accepted 9 April 1978. LANDBIRDS that normally migrate at night often occur over the western Atlantic Ocean during autumn days (Scholander 1955). Three groups are distinguishable. First, many migrants that fly SW or WSW from near-coastal areas move offshore (Fig. 1) and are 200+ km from land at dawn. Second, some landbirds migrate south from New England and Nova Scotia to the West Indies and South America (Drury and Keith 1962; Drury and Nisbet 1964; Richardson 1972a, 1976; Ireland and Williams 1974; Williams et al. 1977). The Blackpoll Warbler (Dendroica striata) is the only species of landbird convincingly shown to follow this route (Nisbet et al. 1963, Nisbet 1970, Ralph 1975), but it cannot be the only species involved (Murray 1965). Third, landbirds of species that rarely migrate successfully over the ocean are often found far at sea, at Bermuda (Wingate 1973) and even in Europe (Alexander and Fitter 1955, Sharrock 1974)-often after periods of offshore winds. These birds may be windblown vagrants and/or individuals with maladaptive orientation systems (Ralph 1975). Landbirds considered to be nocturnal migrants are often seen on autumn mornings flying NW and N toward land and up to 160 km inland (Bagg and Emery 1960, Baird and Nisbet 1960, Able 1977), usually with NW headwinds. It has been suggested that migrants are often drifted offshore by NW crosswinds at night, change direction ('reorient') at dawn, and then fly NW or N toward land (Stone 1937, Bagg and Emery 1960, Baird and Nisbet 1960). Dawn reorientation also appears to occur in autumn over the Pacific Ocean off California (DeSante 1973) and over the northern Gulf of Mexico (Hebrard 1972), and is well known east and north of Britain (Lack 1963, Lee 1963, Myres 1964, Wilcock 1965). These British birds may also ascend at dawn, and ascent sometimes occurs without reorientation (Lee 1963). Dawn reorientation was not found during radar studies at Cape Cod (Drury and Keith 1962), but was evident on Nova Scotian radars (Richardson 1972a). This 717 The Auk 95: 717-732. October 1978 This content downloaded from 207.46.13.57 on Mon, 08 Aug 2016 05:34:25 UTC All use subject to http://about.jstor.org/terms 7 18 W. JOHN RICHARDSON [Auk, Vol. 95 0 ' _ / #,/ // S ~~~~~~~~~~~~~~~~~~~~~~~~5 0? N-| r; =~~~~~~~~~~~~~~~~~~~~~~~~~5~ =~~~~~~~~~~~~~~~~~~~~~~~~~~ e w f o met n ds G ulf o\