Neotropical Bird Migration During the Ice Ages: Orientation and Ecology

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1-1-1996

Neotropical Bird Migration During The Ice Ages: Orientation And Ecology

Timothy C. Williams , '64 Swarthmore College, [email protected]

T. Webb III

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Recommended Citation Timothy C. Williams , '64 and T. Webb III. (1996). "Neotropical Bird Migration During The Ice Ages: Orientation And Ecology". Auk. Volume 113, Issue 1. 105-118. https://works.swarthmore.edu/fac-biology/254

This work is brought to you for free by Swarthmore College Libraries' Works. It has been accepted for inclusion in Biology Faculty Works by an authorized administrator of Works. For more information, please contact [email protected]. Neotropical Bird Migration during the Ice Ages: Orientation and Ecology Author(s): Timothy C. Williams, Thompson Webb and III Source: The Auk, Vol. 113, No. 1 (Jan., 1996), pp. 105-118 Published by: American Ornithologists' Union Stable URL: http://www.jstor.org/stable/4088939 Accessed: 27-04-2015 14:44 UTC

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This content downloaded from 130.58.65.20 on Mon, 27 Apr 2015 14:44:17 UTC All use subject to JSTOR Terms and Conditions The Auk 113(1):105-118, 1996

NEOTROPICALBIRD MIGRATION DURING THE ICE AGES: ORIENTATIONAND ECOLOGY

TIMOTHY C. WILLIAMS' AND THOMPSON WEBB III2 'Department of Biology, SwarthmoreCollege Swarthmore,Pennsylvania 19081, USA; and 2Departmentof GeologicalSciences, Brown University, Providence,Rhode Island 02912, USA

ABsTRAcr.-Reconstruction of 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 1550average 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.

CYCLESOF QUATERNARYGLACIATION during the ing the fitness of current bird populations over last 1.6 million years have influenced the de- their short lifetime? Would the orientation sys- velopment of present routes of both Neotrop- tem(s) used by birds under present conditions ical and Paleotropical migration (Moreau 1972, have been effective under past conditions? Gauthreaux 1982). North American Neotropical We examine whether fixed-heading orienta- migrants were most clearly affected because most tion would be sufficient to guide Neotropical of their present breeding grounds were covered migrants during periods of glaciation. We first by a permanent ice cap only 18,000 (radiocar- estimate breeding habitats of Neotropical mi- bon) years ago (18 kya) and because the ecology grants in North America at 18 kya (full glacia- of North America has undergone several major tion) and at 9 kya (partial glaciation). We then changes during the period of deglaciation (Im- simulate migratory flights at fixed headings for brie and Imbrie 1979, Emery et al. 1988, Webb these birds in order to examine the constraints 1988). Modern species assemblages or plant for- posed on orientation at those times in the past. mations are of recent origin in North America. The Neotropical migrants we discuss are a sub- Most developed after early Holocene warming: set of the type A Neotropical migrants defined modern tundra first appeared at about 8 kya, by the Research Working Group of Partners in boreal forest at 7 kya, and modern mixed forest Flight (1992). We consider only autumnal at 6-8 kya. Only modern prairie, first appearing (southward) migration. We exclude diurnal mi- about 10 kya, and modern deciduous forest (12 grants such as raptors, which are clearly influ- kya) existed during periods of significant gla- enced by local topography, and exclude birds ciation (Webb 1988). After their first appear- moving southwest within North America. We ance, plant formations increased in area and focus on shorebirds and passerines flying south approached their present distributions only in to southeast from North America to Central and the past 4,000 years (Van Devender 1986, Webb South America. We refer to this subset of birds 1988). The present distributions of plant taxa as Neotropical migrants. were relatively rare events during the Quater- Several lines of evidence, including radar and nary and, like the interglacial periods with radio-telemetry observations of migration, the which they are associated, have existed for only experimental manipulation of captive birds, and 10% of the 100,000-year glacial/interglacial cy- simulated migratory flights, suggest that at least cles (Ruddiman and Raymo 1988). These find- some songbirds and shorebirds guide long-dis- ings pose two major problems for the study of tance migrations by maintaining a fixed com- the evolution of bird migration. What behavior pass heading. Drift by winds produces a vari- would allow bird species to track changes in able track, but by selecting synoptic weather habitat over thousands of years, while preserv- conditions for take-off, the great majority of birds 105

This content downloaded from 130.58.65.20 on Mon, 27 Apr 2015 14:44:17 UTC All use subject to JSTOR Terms and Conditions 106 WILLIAMSAND WEBB [Auk, Vol. 113 can move reliably between breeding and win- METHODS tering areas each year (Williams and Williams We used the mapped pollen data from Webb (1988) 1978, 1990, Alerstam 1981, Stoddard et al. 1983, and Overpeck et al. (1992) to estimate bird habitats. Cochran and Kjos 1985, Alerstam et al. 1986, We labeled these habitats as desert, prairie, tundra, Richardson 1991, Williams 1991). Direction and coniferous forest, mixed forest, and deciduous forest some cue for the cessation of migration (dis- to describe structural features, but do not imply pres- tance, time, latitude, or habitat) appear to be ent species assemblages for each region in the past (Baker 1983, Webb et al. 1987, Thompson 1988, Webb genetically encoded (Wiltschko and Wiltschko 1988). We set the southern limit of breeding habitat 1978, Gwinner 1986, Berthold 1988, 1990). One for Neotropical migrants as the southern limit of kill- or more such vectors direct first-year birds to ing frost and set no northern continental limit. Breed- the general location of stopover and wintering ing habitat includes all forest types, wetlands, prairie areas, where they then search for suitable hab- and tundra, but excludes desert and permanent ice. itat. In subsequent years, birds return to these We considered Arctic coastal and North Slope areas local habitats using familiar landmarks or some as suitable habitat for shorebirds, but not Neotropical other orientation system operational within the passerine migrants. areas specified by the coded vector (Wiltschko To simulate migratory flights, we first created a and Wiltschko 1978, Williams and Williams 1990, wind matrix for North America representative of those winds actually used by Neotropical migrants. We then R. Wiltschko 1992). A variety of approaches have simulated flights through these wind fields with an shown that such fixed headings could be main- iterative computer program. Finally, we modified the tained by use of a compass based on the sun, wind matrix to estimate conditions during glaciation stars, wind, or magnetic field of the earth (Able and repeated the simulations with these wind con- 1980, Alerstam 1981, Wiltschko and Wiltschko, ditions. 1988). The methods for creation of a wind matrix under Although birds may use other techniques for present conditions are described in detail in Williams orientation (Rab0l 1978, Able 1980, Wiltschko (1991). The 50 latitude x 50 longitude matrix for pres- and Wiltschko 1988, 1991, Richardson 1991, ent conditions was an average wind velocity for those Wallraff 1991, R. Wiltschko 1992), orientation North American weather systems known to have sup- of and by a fixed compass heading is consistent with ported major autumnal migrations passerines shorebirds moving to the south and southeast as ob- available data for Neotropical migrants moving served by radar. Weather systems associated with such to south to southeast across North American movements were determined from radar observations Central and South America (Williams et al. in: Alberta (Richardson and Gunn 1971); Wisconsin 1977a, 1977b, Williams and Williams 1978,1990, (T. C. Williams and J. M. Williams unpubl. 1979 report Williams 1985). Radar observations of Neotrop- to Office of Naval Research); Ontario and Quebec ical migrants over the Caribbean, Bermuda, and (Richardson 1972); and along the North American over the North American eastern coast revealed eastern coast (Drury and Keith 1962, Richardson 1972, average headings close to 1550 (relative to true 1979,1980, Williams et al. 1977a, 1977b, Williams 1991). north) at all sites with no significant change of At all sites, south or southeast migrations likely to headings to compensate for wind drift (Wil- contain significant numbers of Neotropical migrants occurred west of a cold front, when barometric pres- liams and Williams 1978, Williams 1985). In sure was rising or steady and when winds were north- simulations birds move southeast across North erly, westerly or calm (see Richardson 1978). The con- America and over the western North Atlantic, currence of these factors was then considered to be where they encounter northeast trade winds criteria for identifying weather systems likely to pro- that drift them westward to Central and South duce Neotropical migratory flight in areas or at times America. The best available sample of the radar when we lacked radar observations. South and south- observations is from Antigua in the Caribbean. east migration that occurs under other, less-predict- At Antigua the average heading was 1530, with able conditions was excluded (Richardson 1978, 1990). an angular (standard) deviation of 26.90 (Wil- The wind matrix for current conditions was created liams and Williams 1978, Williams 1985). Mean by averaging 774 wind measurements taken at 850 mb (ca. 1,500 m altitude), and 595 surface-wind mea- headings observed at other windward islands, surements. This gave an estimate of winds close to from 1510 to 1570 Bermuda, and Miami ranged the average altitude of migrant birds over continental (Williams 1985). Airspeeds at Antigua showed areas (Eastwood 1967, Able 1970, Williams et al. 1977a). two peaks, one at 45 km/h, presumably passer- Wind data from 500 mb (ca. 5,000 m) were used to ines, and one at 75 km/h, presumably shore- modify surface data when 850 mb data were lacking. birds (Williams 1985). Flight at 5,000 m is rare over continental areas, al-

This content downloaded from 130.58.65.20 on Mon, 27 Apr 2015 14:44:17 UTC All use subject to JSTOR Terms and Conditions January 1996] Ice Age Bird Migration and Ecology 107 though it has been documented for shorebirds initi- etation and faunas without analog in today's ating transoceanic flight (Richardson 1979). Flight at plant and animal communities (COHMAP 1988, these altitudes at a constant heading has been simu- Overpeck et al. 1992). Analyses of past pollen lated over eastern North America by Williams (1991), distributions show that the major tree taxa re- is included but not in our simulations. sponded independently to past changes in cli- Matrix values for present winds in areas not pen- mate. Major pollen-producing trees in both Eu- etrated by North American weather systems were determined as follows. Winds over the western North rope and North America show changes in dis- Atlantic were average wind velocities, recorded at the tribution that are independent of other taxa altitude of birds, during radar observations of mod- (Huntley and Webb 1989). The scant data on erate or heavy autumnal bird migration in those areas Quaternary avian distribution also indicate that (Williams et al. 1977a, 1977b, McClintock et al. 1978, birds utilized habitats dissimilar from their cur- Williams 1985). Prevailing winds were used over rent species associations (Gauthreaux 1980). oceans south of 30?N. We limited our wind matrix to Since associations among plant taxa were latitudes 10'S to 70?N and longitude 50?W to 130?W ephemeral during the Quaternary, we used because we lacked adequate radar observations in physical and structural factors to identify breed- South America and the Arctic. ing areas for long-distance migrants. Summers Parameters for heading and airspeed used in the simulations were derived from more than 2,000 radar had to be sufficiently warm and wet to provide tracks of migrants obtained over Antigua for five mi- a large insect and fruit food source, and winters gratory seasons. For the simulation we used a heading sufficiently cold to greatly reduce these food of 1550 ? 250. Simulated airspeed was 35 km/h (10 sources. Severe weather conditions during the m/s), about one standard deviation below the mean nonbreeding season in these areas would se- airspeed of late season (passerine) migrants observed verely limit the resident populations of birds, with radar at Antigua, and represents the slowest mi- mammals and parasitic insects, thus reducing grants likely to make the Atlantic crossing (Stoddard competition, predation, and parasitism for et al. 1983). breeding migrants (see Cox 1985). Below we Winds at 9 kya and 18 kya were estimated by mod- give habitat and migratory-route reconstruc- ifying the present wind-velocity matrix in relation to the National Center for Atmospheric Research Com- tions for 18 kya and 9 kya and compare these munity Climate Model (NCAR CCM) simulation for with the present conditions. In each case we North America at 9 kya and 18 kya as given in Kutz- begin with a description of simulated climate bach (1987). NCAR CCM simulations were available and paleoecology to locate likely breeding hab- for January and July for these periods. A model for itat and then consider the effects of constant modification of the simulated wind patterns was ob- heading orientation on Neotropical migration tained by comparison of present winds observed dur- under these conditions. ing southeast migrations with present simulated storm 18,000 years ago.-Glaciation has been the tracks, high- and low-pressure centers, and resultant predominant climate condition for North winds for January and July. Simulated resultant winds along the present simulated January storm track ap- America for more than 80% of the past 900,000 proximated winds observed during autumnal migra- years (Ruddiman and Raymo 1988). At the height tion more closely than did those of present July sim- of glaciation much of the present breeding area ulations. The average location of January and July of Neotropical migrants north of 45?N was cov- high-pressure centers and the average velocity of Jan- ered by ice up to 3,300 m thick (Fig. 1C). The uary and July trade winds corresponded to those ob- principal exceptions were Alaska which was served during present autumnal Neotropical migra- largely ice-free, and possible ice-free areas in tions. We obtained January storm tracks, average lo- the Canadian far north and between the eastern cation of stabile high- and low-pressure centers, and and western ice domes. Both summer and win- resultant winds for January and July, from NCAR ter temperatures south of the ice were 2? to 10?C CCM simulations for 9 kya and 18 kya and substituted lower than these values in the model derived from present sim- present. Unglaciated areas in Alas- ulations to obtain a 50 x 50 matrix of estimated winds ka, as well as in northwestern, north-central, used by autumnal Neotropical migrants at 9 kya and and southeastern United States, were drier than 18 kya. at present; the Northeast and Southwest had wetter summers than present (Kutzbach 1987, RESULTS COHMAP 1988). Lowering of sea level by about 100 m exposed The climates of glaciated North America were large areas of new land. We suggest that sub- unlike any today and this condition led to veg- stantial additional habitat was available to Neo-

This content downloaded from 130.58.65.20 on Mon, 27 Apr 2015 14:44:17 UTC All use subject to JSTOR Terms and Conditions 108 WILLIAMS AND WEBB [Auk, Vol. 113

A B C~~~~~~~~~~~~~~~~~~~~A

500 ya 9 kya 18 kya

EI Aridareas E Breeding Ice 7 Shorebird Arid' aeshabitat Lihabitat only Fig. 1. Estimated breeding habitat for passerine and shorebird Neotropical migrants based on criteria given in text. Stippled area would support major shorebird breeding populations, but probably not passerines. Northern extent of breeding areas 9 kya and 18 kya assumes ice-free Arctic coastal areas and tundra structure similar to present. At 18 kya, land area was continuous with eastern Siberia and ice cap extended northeast to cover Greenland; these areas have been truncated. Margins of ice caps and coastlines from Denton and Hughes (1981) and from COHMAP (1988). Glacial lakes shown: Agassiz (9 kya), and Lahontan and Bonneville (18 kya). tropical migrants along the east and west coasts land (Jacobson et al. 1987). Northwestern North of North America, and in the Caribbean and America consisted of a subalpine open wood- Gulf of Mexico (see Fig. 2C). The extent of this land grading into tundra in ice-free Alaska (Bar- habitat is difficult to estimate as areas near the nosky et al. 1987, Engstrom et al. 1990). In ice would have been depressed due to its mass mountainous areas, the tree line was depressed (Denton and Hughes 1981, Peltier 1987), and 1,200 to 500 m, with alpine vegetation below Caribbean areas were subject to tectonic forces glaciated summits (Baker 1983). The increased that may have depressed land areas (Aubry et precipitation and lower temperatures in the al. 1988). As suggested by Emery (1967), the Southwest created large shallow lakes, such as nature of these lands either could have been Lake Bonneville (Fig. IC). Present scrub and estuarine, where sea level dropped below the desert areas in the United States were a mixed continental shelf, or bay and wetlands, where woodland. The Mexican highlands were drier the seas were shallow. Vegetation in these areas than at present (Baker 1983, Van Devender 1986, of the exposed continental shelf was similar to COHMAP 1988). that on nearby continental margins (Emery The total breeding habitat available for Neo- 1967). tropical migrants appears to have been less than Pollen distributions from eastern North at 500 years ago (ya) due to glaciation of almost America 18 kya show a great reduction in de- all of the present northern breeding range (Fig. ciduous forests compared to present. A pine for- 1). This loss may have been compensated some- est with some deciduous trees covered the what by the addition of excellent habitat in southeastern United States and probably much greatly expanded coastal areas, in areas near of the area of the exposed eastern continental pluvial lakes, and in the increased woodland shelf (Emery et al. 1967, Jacobson et al. 1987). areas of the Southwest. With reduced oppor- An open woodland with deciduous trees, grass- tunities for migration, some species may have es and prairie forbs covered the area of the Unit- become resident in expanded Caribbean land ed States gulf states. Sedges and forbs indicating areas. During periods of maximum glaciation, open to treeless vegetation, dominated the species that at present breed in boreal forest northeastern United States. South of the ice in may have been able to compete successfully in the midwestern United States, enough spruce the generally coniferous, cooler habitats over trees grew to yield a vegetation similar to park- much of the continental United States. There

This content downloaded from 130.58.65.20 on Mon, 27 Apr 2015 14:44:17 UTC All use subject to JSTOR Terms and Conditions January 1996] Ice Age Bird Migration and Ecology 109

130 120 110 100 90 80 70 60 50 130 120 110 100 90 80 70 60 50 7 010 701

\ \ 40 \ ~ 55O~ 1550 40

130 120 110 100 90 80 70 60 50 130 120 110 100 90 80 70 60 50 _ I I I I I 5| | 70- 70 ~~-. _ \" 1 Ice ~~~~habita:t9//3 Wind d+ >-

E 1L * Bs13 0 ? - m/ 60 9~~~~~~~~~~~~~~~~~010

130 120 1 10 100 90 80 70 60 50

Fig. 2. Simulated migration routes and breeding habitats for hypothetical Neotropical migrant passerine moving from breeding grounds in North America to wintering areas in northern South America at three time periods: Present, 9 kya, and 18 kya. Simulated tracks of birds flying at airspeed of 35 km/h (10 m/s) with fixed headings as indicated;position plotted at 1-h intervals. Representativewinds of the 50 x 50 wind matrix used for simulations shown by solid arrows. (A) For present conditions, winds are average for synoptic conditions selected by present autumnal migrants moving southeast over eastern North America. Breeding habitat shown meets criteria described in text. For simulations at (B) 9 kya and (C) 18 kya: winds and breeding habitats estimated from global climate simulations; area of ice cap from Denton and Hughes (1981). All panels show tracks of birds passing over island of Antigua on average heading observed in Caribbean (155?) and for 130? and 1800. Tracks also shown for birds using headings of 155? from extreme eastern and western portions of breeding area. Additional breeding areas exist(ed) north and west of geographical limits of figures. Degrees of latitude and longitude given at panel margins. Mercator projection distorts relative areas and distances, but shows direction of tracks and winds accurately.

This content downloaded from 130.58.65.20 on Mon, 27 Apr 2015 14:44:17 UTC All use subject to JSTOR Terms and Conditions 110 WILLIAMSAND WEBB [Auk, Vol. 113 may have been considerable habitat for birds by the ice cap. A large high-pressure center such as Blackpoll Warblers (Dendroicastriata) in dominated winds over central Canada creating Alaska 18 kya, depending on the northward relatively light wind in this area. The average extension of conifers into the tundra of that storm track split into two. A northern track, period and the composition of the tundra, which arched up over north-central Canada and down differed from that of today (Webb 1988, Engs- along the northeastern coast, creating strong trom et al. 1990). Much of the Nearctic was prob- northwest winds in northeastern Canada. A ably ice free in summer and was suitable breed- southern branch of the storm track crossed the ing habitat for nonpasserine migrants (Johnson southern United States at about 40?N and then and Herter 1990). turned north along the coast to join the north- To facilitate comparisons of past and present ern track. The southern storm track created migration routes, we simulated the breeding strong northwest winds behind cold fronts over area of a hypothetical Neotropical migrant pas- most of North America between 30?N and 50?N serine that today moves from Canada and Alas- (Fig. 2C). These winds were more favorable for ka to South and Central America (Fig. 2). The southeast migration in the western and central breeding area of this migrant is similar to that United States than are present wind conditions of the taxa that Mengel (1970) termed taiga (Fig. 2A; Kutzbach 1987). Birds continuing fixed wood-warblers. Alaskan areas (shown in Fig. 1) heading flight over the Atlantic encountered are not included in Figure 2. At 18 kya (Fig. northeast trade winds south of Bermuda, which 2C), the breeding area was shifted far to the drifted them westward toward Central and south of the present distribution (Fig. 2A). The South America. Simulated migrants moving northeastern portion of the range of the hy- along the west side of Central America found pothetical migrant was restricted to a narrow similar but less extensive west winds which strip of exposed continental shelf between the drifted them eastward toward South America ice and the sea. An alternative interpretation of (Fig. 2C). breeding area is to restrict the wood-warbler to At 18 kya both passerines and shorebirds the much smaller areas of spruce located in the passing over Antigua could have used the same midwestern states. constant headings as they do today (Fig. 2 A In this and the following simulations, we first and C). Passerines on headings of 1550 to 1800 investigate whether the headings of 1550 ? 250 would have arrived from the narrow north- that reliably bring birds over Antigua on their eastern seacoast, as there were no suitable pas- way to South America at present would have serine habitats north of 50?N in eastern North been functional in the past. We also plot tracks America. Passerines on headings of 1300 could with a heading of 155?departing from the west- have come from as far as Alaska. Birds breeding ern and eastern limits of the hypothetical north of the ice 18 kya would make two nonstop breeding range to see if these headings are migrations. The first, over the ice cap, would broadly applicable in North America. In Figure take about 90 h for the slowest passerines (35 2 we use an airspeed of 35 km/h (10 m/s), equal km/h) and about 50 h for shorebirds (75 km/ to the slowest passerines currently migrating h). The over-water flight would have taken about over Antigua. Previous work has shown birds the same time as at present, up to 110 h for slow with low airspeeds to be the most constrained passerines and 60 h for shorebirds. in their use of fixed-heading orientation (Stod- Of particular interest are flights from Alaska dard et al. 1983, Williams 1991). We also sim- on headings centered around 1550 (the most ulated flights at airspeeds of 75 km/h (21 m/s) westerly track in Fig. 2C), and flights from the near the average for shorebirds flying over An- area bracketed by the eastern and western 1550 tigua. These simulated tracks (not shown in Fig. tracks in Figure 2C. These areas constituted the 2) were less deflected from their heading and great majority of breeding area 18 kya, and Fig- took less time to complete their flight (see Stod- ure 2C shows that fixed heading flights at about dard et al. 1983, Williams 1991). Tracks are shown 1550, coupled with favorable winds at 18 kya, from the most northerly point, but tracks ini- could have provided a rapid and efficient route tiated from a more southerly point will follow to wintering areas in South America, Central the same path, given that there is no effect of America and the Caribbean. For the great ma- previous experience in the simulation. jority of migrants to South America, these tracks Wind patterns at 18 kya were greatly affected would have taken birds over the (probably)

This content downloaded from 130.58.65.20 on Mon, 27 Apr 2015 14:44:17 UTC All use subject to JSTOR Terms and Conditions January 1996] Ice Age Bird Migration and Ecology 111 much expanded land areas of the Caribbean and packrat (Neotoma) middens from western North Central America, a logical initial route between America indicate closed-canopy coniferous for- North and South America. Migrants at this time ests in coastal regions and at higher elevations might have used fixed headings not to make of the northwest, with grasslands in the drier long flights over open ocean or inhospitable lower elevations (Baker 1983). The flora of the arid lands, but to direct movements over well- southwestern United States was similar to the vegetated continental areas or to guide rela- present, but forests extended further upward in tively short flights over water between land ar- altitude due to warmer temperatures and fur- eas. ther downward due to wetter summers. Forest 9,000 years ago.-Nine thousand years ago a habitat covered a larger area than at present in reduced North American ice cap covered north- most areas (Thompson 1988). Deserts, such as eastern North America (Fig. 1B), but was rapidly the Chihuahua Desert, were grasslands due to receding. As perihelion (closest approach to the the increased summer monsoon (Van Devender sun) occurred in July, summers were warmer 1986). than at present except for areas near the ice. Most of ice-free North America would have Simulated January temperatures indicated the been excellent breeding habitat for migrants. distribution of killing frost would be similar to The additional forest and grassland in the the present (Kutzbach 1987). Ice-free coastal ar- Southwest probably increased breeding popu- eas probably existed along the northern and lations of passerines and some shorebirds in eastern margins of the ice due to the advection that area compared to present conditions, while of warmer air above a stabile anticyclone over winters would have been sufficiently cold to the ice cap. In the Northwest (including Alaska) favor migrants over residents. Loss of coastal the North Central, and the Northeast, summers and wetland habitats under the ice cap (Fig. 1B) were drier than at present. Due to a strength- may have been equaled by a gain in similar ened summer monsoon, the southern United habitat along the exposed continental shelf, States were significantly wetter than present. around glacial lakes, and along the multitudes The average storm track dipped southward from of rivers and streams draining the melting ice western Canada to well below the Great Lakes cap. Species currently nesting in tundra may and then ascended northward along the eastern have found structurally similar although warm- United States coast and out to sea. This pattern er habitats in the Southwest and near the melt- provided strong northwest winds west of cold ing ice. Tundra and coniferous forest habitats fronts over most of the continent (Fig. 2B). were probably reduced compared to present Weaker winds were found in the Southwest and conditions, but there may have been a slight winds during migration were more northerly increase in mixed forest and deciduous forest in the south-central United States than along habitats compared to the present. A general shift the storm track (Fig. 2B). Thus, winds over the in breeding habitat to the south and west is southern United States generally were less fa- likely, due to increased moisture in the South- vorable than 18 kya. The relatively small ice cap west and elimination of habitat in the Northeast reduced sea level about 10 m below present under the ice cap. (Kutzbach 1987). This probably resulted in an The breeding area of the hypothetical mi- increase in lagoon and wetland habitat in areas grant in Figure 2B is similar to that for present of continental shelf that were exposed (Emery conditions except for areas of permanent ice. 1967). The melting ice created extensive inland The southern boundary extends further south lakes such as lake Agassiz (Fig. 2B). Winter tem- than at present in the eastern United States due peratures were similar to those at present. to greater presence of confers in that area 9 kya. Deciduous forests dominated eastern North At 9 kya, the headings of migrants observed America, but the association of taxa differed from at present over Antigua would have brought the present (Jacobson et al. 1987). A mixed co- birds from much the same areas of ice-free North niferous-hardwood forest extended north from America as they do today (Fig. 2B). Birds breed- the Great Lakes. Prairie forbs, sedges, and grass- ing north of the ice 9 kya would have taken es were found in central North America, but less than 45 h to cross the ice, even for the were associated with pines and spruces in the slowest passerines (35 km/h), while the over- north and with deciduous trees in the south. water flight would take up to 110 h as at present. Pollen distributions and macrofossils from Headings observed at present in the Carib-

This content downloaded from 130.58.65.20 on Mon, 27 Apr 2015 14:44:17 UTC All use subject to JSTOR Terms and Conditions 112 WILLIAMSAND WEBB [Auk, Vol. 113 bean could have been used from almost all North DISCUSSION American breeding areas 9 kya. The retreat of ice in the eastern portion of North America and Changes in climate during the most recent the shift in the center of breeding distribution deglaciation of North America resulted not only to the north and east would have resulted in a in major changes in the area covered by vege- greater percentage of birds moving over the tation, but also in the species composition of Atlantic at 9 kya than during periods of full plant formations. During the past million years, glaciation. present conditions were rare, representing not 500 years ago.-The present time is one of the norm, but the extreme development of bo- maximum deglaciation. Boreal forest and tun- real forest and desert habitats. During the re- dra replaced most of the remaining ice cap, pro- maining time, glaciation covered much of the viding further habitat for Neotropical migrants. northern breeding areas currently used by Neo- Grasslands and desert expanded in the South- tropical migrants. Our analysis indicates that west and central United States due to a drier increased vegetation in southern and coastal ar- climate (Kutzbach 1987, Thompson 1988, Webb eas may have compensated for some of this loss 1988, Thompson et al. 1993). The total area avail- if northern species could compete effectively in able for breeding as judged by our criteria, in- these habitats. The present expansion of tundra creased relative to 9 kya (Fig. 1A and B), but and taiga in North America is exceptional, and the removal of tree species from prairie areas, the abundance of species exploiting those areas the retreat of forest areas in the Southwest and was reduced significantly during periods of gla- the increase in desert in the Southwest, may ciation. have reduced the density of Neotropical pas- The fauna of North America reacted to cli- serine migrants that could be supported by these mate change in a variety of ways. Some animals, habitats. Northern shorebird habitat appears to such as southern reptiles, showed little change be near maximum at 500 ya (Jarvinen and Vais- in distribution during the past 20,000 years, in- anen 1978). Total populations of boreal passer- stead they adapted to the changing conditions ines and shorebirds at 500 ya may have been of the area (Van Devender 1986). Although there near maximum for the entire glacial cycle (giv- are few avian fossils from the Quaternary en that present habitat for Neotropical migrants (Gauthreaux 1980), it is probable that birds, due has been greatly reduced by human activity from to their high mobility and observed common the levels at 500 ya). Winds, winter tempera- occurrence of vagrancy, changed distribution tures, and average storm tracks at present are with the migration of plant species over North similar to those at 500 ya and 9 kya (Kutzbach America. This conclusion is consistent with the 1987). observed low rate of Quaternary speciation in The expansion of habitat in the north and birds as compared with mammals (Mengel 1964). east in the past 9,000 years and the reduction Gauthreaux (1980) and Huntley and Webb (1989) in habitat in the Southwest resulted in a north- independently concluded that the movements eastward shift in the center of breeding distri- of birds and of trees differed only in the time butions. The usefulness of fixed-heading ori- scale of their migrations. As a description of the entation for Neotropical migrants is more con- phenomenon this is true, but birds are able to strained at 500 ya (and at present) than at any direct their migrations, while trees are not. Thus, other time in the glacial cycle. Birds departing the type of orientation system used by birds in from the eastern portion of the breeding migration may well have implications for grounds on headings less than 1550 (illustrated changes in avian species distribution. by the most eastern track in Fig. 2A) would fly Flexibility in habitat selection would benefit too far east over the Atlantic to make landfall birds during periods of major changes in plant in South America (Stoddard et al. 1983). Birds formations. Habitat selection by birds includes departing the far western breeding range over both inherited preferences and reaction to prox- the Pacific on headings greater than 1550 also imate factors (Hutto 1985,- Sherry and Holmes would fail to make landfall in South America. 1985). Selection of both breeding and wintering Thus, although fixed-heading orientation would sites appears directly influenced by competition be successful from all of the breeding range, (Cody 1985, Cox 1985, Sherry and Holmes 1988, permissible headings are severely restricted in 1989, Holmes et al. 1989, Sherry 1990). Our anal- the eastern portions and somewhat restricted in ysis agrees with that of Cox (1985); habitat se- the far western portions of that range. lection in Neotropical migrant birds is more

This content downloaded from 130.58.65.20 on Mon, 27 Apr 2015 14:44:17 UTC All use subject to JSTOR Terms and Conditions January 1996] Ice Age Bird Migration and Ecology 113 likely to be the result of interspecific competi- liams et al. 1977a, 1981). These latter flights may tion than dependence on specific plant species constitute the search phase of orientation and, or plant associations. Such specific dependence rather than being the result of errors, may be a would be difficult to reconcile with relatively specific adaptation to Quaternary conditions stable avian species in the face of major changes (Richardson 1982, gives alternative interpreta- in vegetation. Our estimates of breeding habi- tions). tats indicate that Neotropical migrants, even Searching for suitable habitat, necessitated by those presently restricted to boreal forest breed- a fixed-heading orientation, may itself have been ing areas, would have not have fluctuated as adaptive during periods of rapid climate change. greatly in abundance as did the plant species Both birds and mammals could track the changes which they now exploit. During periods of max- in plant distribution during the Quaternary, yet imum glaciation, sufficient additional habitat most bird species survived, while many mam- was available in presently unsuitable areas to mal species did not (Cox 1985, Sutcliffe 1985). compensate at least in part for the loss of areas An effective system for discovering new, often covered by ice. If this view is correct, the pres- distant habitats and reevaluating old ones may ent observed reduction in Neotropical migrant have made a crucial difference. Current studies populations (Hagan and Johnston 1992, Gill indicate that, for several species of passerine 1994) could be greater than that due to glacia- and shorebird migrants, successful adults re- tion. turn to the same breeding, stopover, and win- Fixed-heading orientation, coupled with dif- tering areas, while first-year birds and unsuc- ferential reaction to weather as a stimulus for cessful adult breeders disperse widely (Sherry migration, appears well suited to coping with and Holmes 1985, 1989, Myers et al. 1987, John- changing Quaternary ecology. Although the ex- son et al. 1989). In most cases, birds that have act routes would vary from year to year, our found recently opened, underutilized breeding simulations indicate that such a system would sites, wintering areas, or stopover sites could reliably direct birds over oceans, ice caps, and easily recoup the cost of a few hours of flight. continental areas. Despite the major shifts in Present rather than past conditions appear to breeding distribution shown in Figure 1, the pose the greatest stress on a fixed-heading ori- southeast headings shown in Figure 2 contin- entation system in our simulations. Consider- ued to bring the great majority of migrants to ing that for 80 to 90% of the past 900,000 years Neotropical wintering areas at all times tested. North America has been in a state of glaciation Some cue indicating cessation of migration must (Van Devender 1986, Ruddiman and Raymo be specified as well as direction. If this is dis- 1988), present conditions might be considered tance or time as suggested by Gwinner (1986), to be a rare period in avian evolution, posing then this parameter would have to change for a relatively brief stress on a system largely birds making longer flights. If the cue were evolved under different conditions. Our sim- instead a function of latitude, as would be the ulations suggest that constant-heading orien- case for the elevation of the sun or stars, or tation with a mean heading of 1550 could have inclination of the geomagnetic field (see evolved during periods of full glaciation. At Wiltschko and Wiltschko 1992), the system could that time it would have guided flights from function at all times in the Quaternary without breeding areas in southern North America over change. Rather than directing birds to a specific the Caribbean Sea and Gulf of Mexico to win- location, this orientation system would bring tering grounds in the Caribbean Islands, Cen- them to a large general area more than 100 km tral America, and South America. As the ice in diameter (Williams 1991). Birds would then retreated, the same orientation system would have to search within this area for suitable hab- direct birds on longer and longer over-water itat (Williams and Williams 1978, 1990, Wiltsch- flights. During subsequent glaciations, the sys- ko and Wiltschko 1978, 1991). tem also would serve to cross the ice caps them- Radar observations of Neotropical migration selves, as is currently observed over Greenland reveal both large-scale broadfront migrations as (Alerstam et al. 1986). The over-water and over- predicted for fixed-heading orientation and ice flights would result in energy savings and (usually under other weather conditions) flights reduced risk of predation compared to flights of large numbers of birds in directions inap- around the ice caps or around seas (Williams et propriate for the seasonal migration (Drury and al. 1977a, Johnson and Herter 1990, Williams Keith 1962, Gauthreaux and Able 1970, Wil- and Williams 1990)

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Orientation by means of a fixed southeast tem, such as fixed-heading orientation, allows heading is not the only system used by Neo- a species to track habitat changes over thousands tropical migrants (in the broad sense; Richard- of years, we still must account for its selective son 1991). Birds moving south to southwest advantage during the reproductive lifetime of through North America to Central America a bird. Behaviors that are beneficial to a species would have to use south to southwest headings, are not favored in evolution unless they are also and Richardson (1979) observed large numbers beneficial to the individual (Krebs and Davies of shorebirds departing the Canadian Maritime 1987). A generalized, nonspecific orientation coast on headings too far east to allow successful system coupled with extensive habitat search flight to South America without a change in must also have raised the average migrant's fit- orientation. Laboratory experiments with ori- ness above that of a system which reliably re- entation cages indicate multiple vectors con- turned it to its natal location and to specific sisting of fixed heading and distance segments stopover and wintering areas. A general char- for Palearctic-African and Australian migra- acteristic of Quaternary ecology is that local tions (Wiltschko and Wiltschko 1991, Munro et habitats change relatively rapidly both in space al. 1993). Genetic analyses of these vectors show and in time. Fires, floods, storms and severe that avian populations can shift both direction seasonal weather create a fluctuating patchwork and distance significantly in only a few gen- of species associations and successional stages. erations (Berthold 1990). These experiments Raw data from pollen-core sites show that plant suggest an alternative to our hypothesis of a associations consisted of a matrix of species relatively fixed, general heading; bird species changing irregularly from site to site. Only when could follow the displacement of specific plant many sites are averaged do we find continuous species or species associations or specific inver- variation over large areas (Baker 1983, Jacobson tebrate fauna (Hutton 1984). In this view, mi- et al. 1987). Palynological analysis on a high- gration is accomplished by route-specific in- resolution time scale shows many rapid changes structions, and the present migratory routes of in species composition at a single site, some birds reflect the past as well as present location changes being as rapid as historic deforestation of the critical habitats for that bird species. Each from intensive, widespread lumbering (Jacob- successive stage of glaciation would require al- son et al. 1987). Records of lake or river sedi- tering the genetic system for the migratory route ments, shoreline positions, water temperatures, or at least the addition of a new segment to the snowfall amounts, and volcanic or organic ash encoded route. Some anomalies in present mi- deposits all show variations at the finest dis- gration routes could be explained by the per- cernible time intervals that are comparable to sistence of all or portions of past migration routes many of the average variations at intervals of in the species' genetic code (Rab0l 1978, Sharpe hundreds or thousands of years (Kutzbach 1981, 1978, Gill 1994). Our analysis of North Ameri- Street-Perrott and Smith 1983, Kutzbach and can vegetation history, the probable benefits of Street-Perrott 1985, Jacobson et al. 1987, Engs- less precise orientation systems, and the dem- trom et al. 1990, Gear and Huntley 1991, Kol- onstration of the efficacy of a simple fixed-head- termann and Gorelick 1992, Wang and Lewis ing system suggest that such route-based, pop- 1992). Climate simulations and studies of Neo- ulation-specific orientation systems should be tropical areas suggest similar variation for win- rare in Neotropical migrants (as defined for our tering areas of migrants (COHMAP 1988). South simulations). This conclusion is supported by America and the Amazon basin appear to have the circular normal distributions of headings supported unstable associations of species swept around a southeast mean observed for Neo- at intervals by fire and flood as recently as a tropical migrants over the western North At- few thousand years ago (Meggers 1975, Colin- lantic Ocean (Williams 1985). The relatively vaux 1987). Under such conditions, it appears broad spread of headings around this mean (SD that the same factors of unstable climate and > 250) is consistent with birds facing uncertain competition that favor habitat search at present conditions in both winter and breeding areas. would also have been effective in increasing We suggest that a more precise orientation sys- fitness in the past. An average individual shift tem (including one with less variation in fixed of as little as 0.5 km per year is sufficient to headings) would not benefit migrants. account for a population shift of 10,000 km in If it is true that a generalized orientation sys- 20,000 years. We conclude that fixed-heading

This content downloaded from 130.58.65.20 on Mon, 27 Apr 2015 14:44:17 UTC All use subject to JSTOR Terms and Conditions January 1996] Ice Age Bird Migration and Ecology 115 orientation and search within a goal area would and migration of thrushes: A telemetry study. benefit avian populations during the lifetime Illinois Nat. Hist. Surv. Bull. 33:297-330. of an individual and would also allow a species CODY,M. L. (ED.). 1985. Habitat selection in birds. to track large-scale vegetation changes over Academic Press, Orlando, Florida. 1988. Climatic thousands of years. COHMAP MEMBERS. changes of the last 18,000 years: Observations and model simulations. Science 241:1043-1052. AcKNowLEDGMENTs COLINVAUX,P. 1987. Amazon diversity in light of the paleoecological record. Quat. Sci. Rev. 6:93- We thank:J. Kutzbachand P. Behling for climate- 114. model simulations; and S. Johnson, R. Merz, W. J. Cox, G. W. 1985. The evolution of avian migration Richardson,J. Weiner,J. Williams,and W. Wiltschko systems between temperate and tropical regions for their comments on drafts of this paper. This re- of the New World. Am. Nat. 126:451-474. searchwas supportedby NSF grant ATM87-13981 to DENTON,G. H., AND T. J. HUGHES. 1981. The last Brown University and by the SwarthmoreCollege great ice sheets. Wiley Interscience, New York. FacultyResearch Fund. DRURY,W. H., ANDJ. A. KEITH. 1962. Radar studies of songbird migration in coastal New England. Ibis 104:449-489. LrrmAruRECIIED EASTWOOD,E. 1967. Radar ornithology. Methuen, ABLE,K. P. 1970. A radar study of the altitude of London. nocturnal passerine migration. J. Field Ornithol. EMERY,K. 0. 1967. Estuaries and lagoons in relation 41:282-290. to continental shelves. Pages 9-11 in Estuaries (G. ABLE, K. P. 1980. Mechanisms of orientation, navi- Lauff, Ed.). American Association for the Ad- gation, and homing. Pages 283-373 in Animal vancement of Science, Washington, D.C. migration, orientation, and navigation (S. A. EMERY,K. O., A. S. MERRILL,AND E. R. M. DRUFFEL. Gauthreaux, Jr., Ed). Academic Press, New York. 1988. Changed late Quaternary marine environ- ALERSTAM,T. 1981. The course and timing of bird ments on Atlantic continental shelf and upper migration. Pages 9-54 in Animal migration (D. J. slope. Quat. Res. 30:251-269. Aidley, Ed.). Cambridge Univ. Press, Cambridge. EMERY,K. O., R. I. WIGLEY,A. S. BARTLErr,R. MEYER, ALERSTAM,T., C. HJoRT,G. H6GSTEDT, P. E. J6NSSON, ANDE. S. BARGHOORN.1967. Freshwater peat on J. KARLSSON,AND B. LARSSON. 1986. Spring mi- the continental shelf. Science 158:1301-1307. gration of birds across the Greenland Inlandice. ENGSTROM,D. R., B. C. S. HANSEN, AND H. E. WRIGHT, Medd. Gr0nland, BioScience 21:3-38. JR. 1990. A possible younger Dryas record in AUBRY,D. G., K. 0. EMERY, AND E. UCHUPI. 1988. southeastern Alaska. Science 250:1383-1385. Changing coastal levels of South America and GAUTHREAUX,S. A., JR. 1980. The influences of long- the Caribbean region from tide-gauge records. term and short-term climatic changes on the dis- Tectonophysics 154:269-284. persal and migration of organisms. Pages 103- BAKER,R. G. 1983. Holocene vegetational history of 174 in Animal migration, orientation and navi- the western United States. Pages 109-127 in Late gation (S. A. Gauthreaux, Jr., Ed.). Academic Press, Quaternary environments of the U.S. Vol. 2, The New York. Holocene (H. E. Wright, Ed.). Univ. Minnesota GAUTHREAUX,S. A., JR. 1982. The ecology and evo- Press, Minneapolis. lution of avian migration systems. Avian Biol. BARNOSKY,C. W., P. M. ANDERSON,AND P. J. BARTLEIN 6:93-198. 1987. The northwestern U.S. during deglacia- GAUTHREAUX,S. A., JR.,AND K. P. ABLE. 1970. Wind tion: Vegetational history and paleoclimatic im- and the direction of nocturnal songbird migra- plications. Pages 289-321 in North America and tion. Nature 228:476-477. adjacent oceans during the last deglaciation (W. GEAR,A. J., AND B. HUNTLEY.1991. Rapid changes F. Ruddiman and H. E. Wright, Eds.). The geology in the range limits of Scots Pine 4000 years ago. of North America, vol. K-3. Geological Society of Science 251:544-547. America, Boulder, Colorado. GILL,F. B. 1994. Ornithology, 2nd ed. W. H. Free- BERTHOLD,P. 1988. The control of migration in Eu- man, New York. ropean warblers. Pages 215-249 in Acta XIX Con- GWINNER,E. 1986. Circannual rhythms. Springer gressus Internationalis Ornithologici (H. Ouel- Verlag, Heidelberg. let, Ed.). Ottawa, Ontario 1986. National Museum HAGAN,J. M., III, AND D. W. JOHNSTON(Eds.). 1992. of Natural Science, Ottawa. Ecology and conservation of Neotropical migrant BERTHOLD,P. 1990. Genetics of migration. Pages 269- landbirds. Smithsonian Institution Press, Wash- 283 in Bird migration: Physiology and ecophys- ington, D.C. iology (E. Gwinner, Ed.). Springer Verlag, Berlin. HOLMES,R. T., T. W. SHERRY,AND L. RErFsMA.1989. COCHRAN, W. W., AND C. G. Kjos. 1985. Wind drift Population structure, territoriality and overwin-

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orientation and celestial cues in migratory ori- orientation: Magnetic compass orientation of entation. Pages 16-37 in Orientation in birds (P. GardenWarblers (Sylvia borin) after a simulated Berthold, Ed.). Birkhauser Verlag, Basel. crossing of the magnetic equator. Ethology 91: WILTSCHKO,W., AND R. WILTSCHKO. 1992. Migratory 70-74.

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