In terms of Daphne duMaurier's story "The Birds," I would look at the massing gulls riding the waves in two ways.
On a simple level, as the bird mass upon the rising waves, the reader is given the sense of an impending natural disaster, such as a tsunami or tidal wave, which gains strength before crashing down upon coastal lands, causing widespread devastation. The single image of the birds gathered on the wave provide an image, or impression, of imminent disaster, in the form of foreshadowing.
However, on a deeper level, what duMaurier may have tapped into, which makes her story more horrific, and her foreshadowing more impressive, is found in the following information about the social habits of gulls.
Gulls—the larger species in particular—are resourceful, inquisitive and intelligent birds, demonstrating complex methods of communication and a highly developed social structure. For example, many gull colonies display mobbing behaviour, attacking and harassing would-be predators and other intruders.
Our main character, Nat, is an observant man; he might well see something a little more unnatural as these birds congregate. That there is an underlying "plan" among these creatures could, in the author's mind, be attributed to the following:
[They are] intelligent birds, demonstrating complex methods of communication and...highly developed social structure...[including] 'mobbing behavior.'
These descriptions provide the reader with the personification of an enemy which has a collective social identity, the ability to communicate, and a mob mentality.
With this information, the story becomes more frightening because it is more deeply based upon facts than one might initially believe. In light of this perception, the birds amassing on the waves could be seen as the mobilization of military forces in an impending attack, accentuating a sense of foreshadowing to the reader.