Here are the field marks that make this a Lesser Scaup (as opposed to a Greater Scaup, which we also have here):

• The tufty peak above and behind the eye
  
• The vermiculations on the otherwise white flanks
  
• The darker pattern on the back
  
• The appearance of the eye being closer to the crown of the head
  
• The black on the tip of the bill not extending beyond the "nail" (although it's really hard to see on this photograph)