AN ANALYSIS OF UNUSUAL FLIGHTS OF NEOTROPICAL MIGRANTS TO NORTHEASTERN NORTH AMERICA 
Figure 6. Pressure isobars (3 millibar spacing) and selected measured or estimated wind directions and speeds (in knots) at an alti- 
tude of approximately 1500 m (850 mb) between the evening of 2 April and the morning of 4 April 2009 at some coastal and offshore 
sites. Based on charts from <http://nomads.ncdc.noaa.gov/ncep/charts>. Jamaica and the tip of Yucatan Peninsula are near the low- 
er boundary of the maps, and the island of Newfoundland is at the upper boundary. 
Figure 7. Pressure isobars (3 millibar spacing) and selected measured or estimated wind directions and speeds (in knots) sites at an 
altitude approximately 1500 m (850 mb) between the evening of 6 April and the of morning 8 April 2009 at some coastal and off- 
shore sites (cf. Figure 4 caption). 
birders, mostly from North America. These 
are downloadable as spreadsheet files by 
species, year, month, state, and latitude/ 
longitude. We downloaded all lists that in- 
cluded Indigo Bunting and Summer Tanager 
in March and April 2009 from all the East 
Coast states south of New York and from 
Louisiana, Mississippi, and Alabama. Consid- 
erable editing for our purposes was needed. 
We narrowed the selections to eBird (<www. 
ebird.org>) submissions for dates between 25 
March and 15 April. To better understand ef- 
fort, as number of birding hours, we also 
sought all counts made during the interval by 
downloading those that included one or more 
of the following common regional species: 
Mourning Dove (Zenaida macroura). North- 
ern Mockingbird (Mimus polyglottos) , Ameri- 
can Robin (Turdus migratorius), Northern 
Cardinal (Cardinalis cardinalis) , and Red- 
winged Blackbird (Agelaius phoeniceus). We 
eliminated duplications by different observers 
in the same group, place, and date and also re- 
moved all designated and apparent lists from 
feeders and backyards, for which hours of ef- 
fort are probably not comparable with those 
from other surveyed sites. We then divided 
the resulting daily counts of Indigo Buntings 
and Summer Tanagers by the total number of 
birding hours in each state on that date. Fi- 
nally, to increase sample sizes from the rela- 
tively short stretch of Gulf Coast states, we 
combined counts and efforts from Louisiana 
and Alabama south of latitude of 31° and the 
Florida Panhandle west of 83° W longitude. 
During the first half of April, Indigo 
Buntings and Summer Tanagers were almost 
absent in East Coast counts south of New Jer- 
sey, but both had arrived in good numbers 
along the Gulf Coast, with the former species 
considerably more common than the latter 
(Figure 4). There were several apparent peaks 
of numbers along that coast, including Indigo 
Buntings on 3 April, the day before their fall- 
out much farther to the northeast, and Sum- 
mer Tanagers prominently on 8 April, the day 
of their fallout. These results suggest that 
some Indigo Buntings deposited along the 
Gulf shore on 3 April could have continued 
that night to the northeast but that Summer 
Tanagers in the 8 April fallout and the pulse 
that arrived that day on the Gulf had both 
originated from a source farther south. 
Another striking difference between the 
nights of 3-4 and 7-8 April is evident on 
Nexrad radar images from the southeastern 
United States (Figure 5). At 10 p.m. EDT, 
when night migrants would be aloft not too 
distant from their points of departure, there 
was almost no apparent migration on the lat- 
ter compared with the former night. This, too, 
suggests that some migrants that reached the 
Gulf Coast 3 April were being displaced far- 
ther eastward during night of 3-4 April, 
whereas the fallout birds of 8 April had large- 
ly bypassed the Gulf Coast. 
Meteorological patterns 
Atmospheric analysis charts of pressures and 
winds in North America from the U. S. Na- 
tional Oceanic and Atmospheric Agency 
(<http://nomads.ncdc.noaa.gov/ncep/charts>) 
were used to assess meteorological conditions 
on nights preceding the 4 and 8 April fallouts 
(Figure 6, 7). We assumed that birds were fly- 
ing at roughly 1500 m altitude (850 mb pres- 
sure). At this height, wind directions follow 
pressure isobars quite closely. 
Prior to both fallouts, there were deep and 
extensive low-pressure systems centered near 
or approaching the Great Lakes, producing 
westerlies in the Gulf and Caribbean as well as 
strong southwesterly airflow off the Atlantic 
coast. Two evenings before both fallouts, any 
bird departing from the southeastern United 
States or farther south could have been pro- 
pelled far over the ocean, with no wind assis- 
tance back toward the Atlantic Coast south of 
the northeastern United States and Atlantic 
Canada. But there were distinct differences in 
the meteorological patterns between the peri- 
ods (cf. Figures 6, 7). On the night of 2-3 
April, there was southerly airflow in the 
southern Gulf of Mexico and the Caribbean, 
whereas on the night of 6-7 April, winds were 
northerly in the Gulf and weak (with high 
pressure) in the southern Caribbean. 
Discussion 
We assume that departures took place two 
evenings before the initial fallout days in Nova 
Scotia, as departures the nights before, even 
given the wind speeds indicated in Figures 6 
and 7, would require unsustainable flight 
speeds for passerines. Evidently, migration 
was pronounced along the Florida Panhandle 
during evening of 3 April, following an in- 
crease that day of Indigo Buntings along the 
Gulf Coast. The apparent drop in numbers of 
Indigo Buntings along the Gulf Coast between 
3 and 4 April, and their virtual absence during 
the next few days along the coasts of the 
southeastern United States, suggest that the 
birds had left Central America and Mexico 
(they only rarely winter in South America), 
along with other migrants, on the night of 2-3 
April but were then displaced eastward over 
VOLUME 63 (2009) • NUMBER 3 
367 
