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.