To visualize the model Exo2 predicted northern-allele frequency distribution for the entire region we used the NGB 2904 Spatial Analyst ordinary kriging function in ArcMap 10 to create an interpolated continuous grid surface. We then used the resulting interpolated surface to create allele frequency contours in 0.1 increments with the 0.5 isocline representing the center of the cline. Our results demonstrate that the northern-allele of the WW1 locus is strongly correlated with both altitude and latitude. Sites dominated by individuals with the northern-allele are characterized by low summer temperatures and a short growing season. This suggests it may be a marker associated with local adaptation for alpine breeding in Fennoscandian willow warblers. The use of our expanded dataset also demonstrated that lowland acredula populations in Finland had a significantly higher frequency of southern than northern-alleles, similar to those in coastal Norway where habitats tend to be milder and lower in altitude. Further, our results confirm that the distribution of the two alleles is better explained by our climate model than by the distribution of the subspecies. Lowland coastal breeding acredula have a high proportion of the southern allele and mountainous breeding trochilus in Southern Norway have high proportion of the northern allele. These patterns suggest that climate is a selective force for WW1 independent of subspecies and their migratory behaviour. Although the environmental model found climate and growing season conditions as significant predictors of the northern-allele distribution, these variables are highly correlated with both latitude and altitude. Therefore, these variables may simply reflect the conditions found in these habitats and serve as a useful proxy for other factors that represent a possible agent of selection. Birds that breed at high latitudes and altitudes often experience greater daily and seasonal climate stochasticity and extremes in climatic conditions, such as temperature, wind, precipitation, and snow cover than those in other habitats. For example, differences in the timing of summer snow melt can vary by as much as one month between high and low elevation habitats. Further, years of exceptionally harsh conditions and frequent severe storms may make it difficult for potential breeders to acquire food, increase individual thermoregulatory costs, and limit opportunities to breed during the short summer season encountered at these latitudes and altitudes.