Testing for peripheral vigilance: do birds value what they see when not overtly vigilant?

Previous studies showed that dark-eyed juncos, Junco hyemalis, gain only partial information when their flockmates detect threats, and that they can detect threats peripherally even when they are feeding intensely. Here, we tested how this 'peripheral (head-down) vigilance' interacts with overt (head-up) vigilance and the collective detection of threats. Low barriers were used to prevent juncos from seeing to the side while pecking for food on the ground; a bird with its head raised could easily see over the barriers. Juncos and other ground-feeding birds preferred to feed without the barriers. When the barriers were present, birds generally took longer scans but spent similar periods between scans. Also, scanning patterns were less variable when the barriers were present. Following the threat-induced departure of a single bird, nondetectors were more likely to initiate scans when feeding among barriers. Barriers prevented feeding juncos from seeing the early stages of threat-induced departures of flockmates and thus impeded the collective detection of attack. The results suggest that juncos value peripheral vigilance and that scanning patterns are affected by interactions between peripheral vigilance and overt scanning behaviour.