In "The Birds," what birds looked like white caps on the sea?
This description first comes just after the episode where Nat tries to bury the bodies of the birds that tried to enter the house the night before at the beach, and is unable to. Note the description that the story gives us of what Nat sees as he looks out towards the sea and how the sheer number of birds are emphasised:
Then he saw them. The gulls. Out there, riding the seas.
What he had thought at first to be the white caps of the waves were gulls. Hundreds, thousands, tens of thousands... They rose and fell in the trough of the seas, heads to the wind, like a mighty fleet at anchor, waiting on the tide. To eastward, and to the west, the girls were there. They stretched as far as this eye could reach, in close formation, line upon line.
The fact that Nat at first took them to be the "white caps of the waves" indicates how many there are. But the numbers, and the direct references that compare them to a military force clearly foreshadow the attack that is to come. Note the simile that compares them to a "mighty fleet at anchor" and how the girls are described as being "in close formation, line upon line." The implication is clear. The gulls are described as being a threatening, overwhelming military force that will overwhelm their enemy.