Nat believes that it is the unusually hard winter weather that has caused the birds to behave strangely.
In the beginning of the story, we notice that Nat is concerned about the number of birds that have descended upon his little town. Nat's farmer neighbor thinks that the winter will be a hard one, and Nat agrees with him. The blustery cold east wind is unusually forceful, and Nat hypothesizes that this will be a bitter winter, where the dreaded black frost will be a definite problem. White frost is usually formed when the temperatures are low and the humidity is relatively high. It can be seen as thin coatings of ice on plants and trees.
On the other hand, black frost is formed when the temperatures are low and the humidity is extremely low. This black frost results in plants freezing and dying (turning black). So, Nat thinks that the sudden dawning of bad winter weather is the cause of the birds' strange behavior.