According to Nat, what is odd about winter so far this year? What do you think the description of the cold and black ground early in the story foreshadows?

Nat notes it has been a very mild autumn until the weather turns overnight on December 3, and it becomes cold and winter arrives suddenly. He notices great flocks of birds have been arriving on the peninsula since the fall, and they are incredibly restless. They seem to dive and wheel in the sky continuously, and they do not seem interested in eating from the rich autumnal soil. Instead, they constantly take to the skies, as if they sense death is coming.
The cold and black ground, covered with frost, symbolizes the impending death of humans and the land. The sky becomes leaden, and the hills, which had shone in the sunlight the day before, suddenly become black. Winter has arrived in a single day. The darkness of the ground and sky foreshadows the beginning of the birds' attacks and the demise of humans as a result.

In the opening paragraphs of "The Birds," Nat notices that the winter is so far quite different than that of previous years. The birds are more "restless," for example, and appear to be unsatisfied, even when they feed. In addition, there are more birds than usual, a fact which is supported by the observations of the farmer, Mr Trigg.
After the attack on Nat's house, Du Maurier describes the cold ground and how it has the "hard, black look of frost." This, perhaps, foreshadows Nat's attempt to bury the dead birds which have attacked his family. The ground is too hard to dig, however, and Nat is forced to take the birds to the beach. While he is there, he sees the gulls riding the waves and realises that the birds will return to attack his home. He must now take action to safeguard his home so that his family is protected.