RequiredFieldValidation question

On a web form I have 5 textbox controls that all require input.  I am using a RequiredFieldValidation control with each text box.  When the form loads focus is placed on the first textbox.  If I tab through the textboxes without having typed in anything no validation occurs.  If I were to hit the space bar once inside any of the textboxes, then tab away, validation occurs.  If I were to type in some text then delete the text leaving an empty textbox, validation occurs.  Shouldn't validation occur the second I tab from the initial text box without having first to enter anything into the textbox?
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