1. The siren signalled the approach of an ambulance.
  2. Many kind of birds fly south at the approach of winter.

Both are taken from Cambridge Dictionary while I was searching the meanings of "approach".

In the first sentence no preposition has been placed before "the approach" while in the later "at" is placed before it. I cannot when to use "at" and when not to use.

  • Just a side note, are you 'stuck' on the word 'approach'? :) – Maulik V Apr 9 '16 at 7:10

Everything depends on the context and the verb preceding the noun considered.
Approach is a direct object in the first sentence.You don't have to use a preposition. Signal [WITH OBJECT] is used to indicate the existence or occurrence of (something) by actions or sounds.

The Community could signal (What?) displeasure by refusing to cooperate

1.The siren signalled (What?) the approach of an ambulance.

2.Birds fly south (When?) at the approach of winter(when they feel winter coming)

Approach here is a part of an adverbial phrase of time.

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