Peter insists that he will lay down his life for Jesus. Jesus answers by predicting that Peter will deny him three times before the cock crows.
The rooster's call gives the prophecy an ordinary sound from daily life that will later become unforgettable.
Roosters were familiar in the ancient Mediterranean world, and their crowing was associated with the late-night or predawn hours. Mark's Gospel uses a more specific double-crowing formulation, while John simply says 'before the cock crow.' Attempts to eliminate the bird by claiming that 'cockcrow' must refer only to a Roman trumpet signal are unnecessary; the ordinary avian meaning is entirely natural, even though 'cockcrow' could also function as a recognized division of the night.
The everyday time marker makes the prediction immediate. Peter is not warned of failure in some distant future but before the night gives way to morning.
When the rooster later sounds, an ordinary feature of dawn becomes the audible confirmation of Jesus' foreknowledge and Peter's collapse under pressure.