The correct answer is - 12-December.
If the bird population and its numbers are the indicator for the temperature of the months, than the twelfth month of the year, December, would be the coldest month. The reason for that conclusion is that in December there's not a single bird, while in all the other months there are. The graph though is kind of contradictory because the next lowest temperatures would be in the months of January, March, and August, and to get some of the lowest temperatures in both January and August is almost impossible,