Solar Radiation, Day Length, and Climate Variables
We calculated solar radiation, day length, temperature, precipitation,
and elevation values for each species by extracting spatial data from
the distribution shape files used to calculate migratory distance. Data
were extracted separately for breeding (May-July) and non-breeding
(Nov-Feb) seasons. Although nonbreeding and partial migrant birds may
reside in the same location for 12 months, we extracted values from the
same periods for all species for consistency. The solar radiation and
day length datasets were acquired from the NASA Langley Research Center
Atmospheric Science Data Center Surface meteorological and Solar Energy
(SSE) web portal supported by the NASA LaRC POWER Project (Nasa 2008).
We estimated radiation as the average insolation incident on a
horizontal surface per month (hereafter solar radiation) over the course
of a year in units of kWh/m^2/month. We estimated daylight hours as
the average daylight hours a species experiences per month (hrs/month).
We separated solar radiation into radiation experienced in the breeding
and winter ranges separately and combined for an overall average. We
also created a new variable to estimate total solar exposure by
multiplying solar radiation by day length. We extracted ten climatic
variables from WorldClim 1.4 (Hijmans et al. 2005) at 2.5 minutes
resolution. We extracted breeding and nonbreeding range values from
maximum, minimum, and mean temperature, precipitation, and altitude data
sets. Temperature is provided and degrees Celsius x 10. To extract solar
radiation, day length, and climate variables, we generated 10,000 points
randomly within each distribution map polygon. We extracted data from
each variable layer at each of the 10,000 points for the breeding and
winter months, in the appropriate polygon for each species. We then
calculated the mean value for each variable in the breeding and winter
distributions.