Neotropical Bird Migration during the Ice Ages: Orientation and Ecology

--Reconstruction f breeding habitat of North American Neotropical migrants 18,000 years ago and 9,000 years ago indicated major shifts in both location and composition of plant communities relative to present conditions. Increased vegetation in xeric areas may have compensated, at least in part, for the reduction in breeding habitat due to glaciation. Autumnal flights of Neotropical passerine migrants flying on constant headings from North America to Central and South America were simulated under present wind conditions and for winds during periods of glaciation at 18,000 and 9,000 years ago. The 155 ø average headings currently observed for Atlantic migrants were found to function well during periods of glaciation and may have been more generally useful during those times than at present. Received 15 June 1994, accepted 27 January 1995. CYCLES OF QUATERNARY GLACIATIOIq during the last 1.6 million years have influenced the development of present routes of both Neotropical and Paleotropical migration (Moreau 1972, Gauthreaux 1982). North American Neotropical migrants were most clearly affected because most of their present breeding grounds were covered by a permanent ice cap only 18,000 (radiocarbon) years ago (18 kya) and because the ecology of North America has undergone several major changes during the period of deglaciation (Imbrie and Imbrie 1979, Emery et al. 1988, Webb 1988). Modern species assemblages or plant formations are of recent origin in North America. Most developed after early Holocene warming: modern tundra first appeared at about 8 kya, boreal forest at 7 kya, and modern mixed forest at 6-8 kya. Only modern prairie, first appearing about 10 kya, and modern deciduous forest (12 kya) existed during periods of significant glaciation (Webb 1988). After their first appearance, plant formations increased in area and approached their present distributions only in the past 4,000 years (Van Devender 1986, Webb 1988). The present distributions of plant taxa were relatively rare events during the Quaternary and, like the interglacial periods with which they are associated, have existed for only 10% of the 100,000-year glacial/interglacial cycles (Ruddiman and Raymo 1988). These findings pose two major problems for the study of the evolution of bird migration. What behavior would allow bird species to track changes in habitat over thousands of years, while preserving the fitness of current bird populations over their short lifetime? Would the orientation system(s) used by birds under present conditions have been effective under past conditions? We examine whether fixed-heading orientation would be sufficient to guide Neotropical migrants during periods of glaciation. We first estimate breeding habitats of Neotropical migrants in North America at 18 kya (full glaciation) and at 9 kya (partial glaciation). We then simulate migratory flights at fixed headings for these birds in order to examine the constraints posed on orientation at those times in the past. The Neotropical migrants we discuss are a subset of the type A Neotropical migrants defined by the Research Working Group of Partners in Flight (1992). We consider only autumnal (southward) migration. We exclude diurnal migrants such as raptors, which are clearly influenced by local topography, and exclude birds moving southwest within North America. We focus on shorebirds and passerines flying south to southeast from North America to Central and South America. We refer to this subset of birds as Neotropical migrants. Several lines of evidence, including radar and radio-telemetry observations of migration, the experimental manipulation of captive birds, and simulated migratory flights, suggest hat at least some songbirds and shorebirds guide long-distance migrations by maintaining a fixed compass heading. Drift by winds produces a variable track, but by selecting synoptic weather conditions for take-off, the great majority of birds