Data were collected to test the idea that male mallards guard their female partners during the time when they can be fertilized (critical period). Males maintained closer proximity, and followed their partners more during the critical period. Females were chased by other males significantly more during the critical period, and male partners were more aggressive to such intruders, than at other times. Most attempts at extra-pair bond matings were by paired males. Unpaired males appeared more intent on forming pair-bonds. The sex ratio varied over the egg-laying period, but was always male biased.