Mar 14:30 And Jesus saith unto him, Verily I say unto thee, That this day, even in this night, before the cock crow twice, thou shalt deny me thrice.

The cock crow and what it is.

The cock crow has been thought of as being the voice of a bird, but those that think that are mistaken.  Jewish law forbids there to be poultry within the Holy City.  The hour of 3 AM was called cock crow.  The sign of the changing of the guard at 3 AM was signaled by the blowing of a trumpet. The word for trumpet call in the Latin is gullicinium, which means cock-crow.  The reason that there would of been two blowing of the trumpets was that the city was overly populated due to the Passover.  One in one direction, and one in the other.

Now the Romans divided each day into three-hour segments and the ones at night were called watches.  The first night watch began at 6 PM.  The Jews often used an abbreviated form when talking about these watches.

Mark 13:35 shows this.  Mar 13:35 Watch ye therefore: for ye know not when the master of the house cometh, at even, or at midnight, or at the cockcrowing, or in the morning:
Notice we have the even or evening, midnight, the cockcrow and the morning.  So we have all four time frames at night.  This should explain the cockcrow so that you have a good understanding of it.