Vigilance while foraging incurs costs such as a reduction of the time available for feeding or a reduction in instantaneous intake rate. Vigilance has two major components: scan rate and scan duration. We investigated the cost of these components for the instantaneous intake rate in Mallard. We first modelled the relationship between food availability and instantaneous intake rate (i.e. the functional response), and looked at the relationship between the residuals of this relationship and scan rate and scan duration. Both scan rate and scan duration were negatively correlated with the residuals, but the cost associated with an increase in each component of vigilance differed. The average time spent in vigilance by ducks was 5 sec per minute: 5 scans of 1 sec decreased the maxi-mum instantaneous intake rate by 2.4% whereas one scan of 5 sec decreased the maximum intake rate by 6%. This shows that an increase in the duration of scans has a greater effect than an increase in their frequency and confirms that vigilance not only affects the time spent feeding but also feeding efficiency through instantaneous intake rate. We explored the potential consequences of such costs in the time spent foraging on a daily basis.