Some notable instances of alliteration in "The Birds" come at the very start of the story, as Nat observes the birds' restless movements over the sea.
Black and white, jackdaw and gull, mingled in strange partnership, seeking some sort of liberation, never satisfied, never still. Flocks of starlings, rustling like silk, flew to fresh pasture...
In these two sentences, the sibilant /s/ sound appears eight times at the start of words, three times in immediate succession ("seeking some sort"). The repetition evokes the sound of countless birds' wings. There's also a second, less pronounced set of alliteration in the second sentence, with the /f/ sound repeating three times (repeated /f/ sounds are also often considered to form a kind of sibilance). A few sentences later, Nat moves to observing the birds in flight to observing the birds that stand on the beach.
Oystercatchers, redshank, sanderling, and curlew watched by the water’s edge; as the slow sea sucked at the shore and then withdrew, leaving the strip of seaweed bare and the shingle churned, the sea birds raced and ran upon the beaches.
Here, too, are additional, smaller forms of alliteration ("watched by the water," "withdrew"; "bare," "birds," "beaches"; and "raced and ran") woven with the more striking /s/ alliteration: "the slow sea sucked at the shore;" "the strip of seaweed bare and the shingle churned." As in the first example, alliteration not only makes the sound of du Maurier's prose pleasing to the ear, it also amplifies what is being described. The overlapping sounds call to mind the overlapping waves on the beach and the overlapping birds, many in number and various in kind. This, in turn forms an image that lingers with the reader and grows more and more sinister as the story progresses.