Simple SummarySentinels monitor their surroundings from vantage points for early detection of predators and rivals. The presence of multiple sentinels in a group may allow sentinels to relax their vigilance, especially if sentinels monitor different areas at the same time. We investigated sentinel behavior in groups of the Florida scrub jay (Aphelocoma caerulescens). Sentinels in this species turn their heads frequently to monitor different areas for potential threats. As predicted, we found that sentinels turned their heads less frequently in the presence of other sentinels. However, multiple sentinels tended to gaze in the same direction at the same time more often than predicted by chance alone. Gaze synchronization reduces the efficiency of collective detection by increasing the amount of time that some areas are not monitored by any sentinel. Despite the benefits of the presence of other sentinels, our results highlight the limits to collective detection when multiple individuals are vigilant at the same time.AbstractSentinels can detect predators and rivals early by monitoring their surroundings from vantage points. Multiple sentinels in a group may reduce the perceived predation risk by diluting the risk and increasing collective detection, especially if sentinels monitor different areas at the same time. We investigated sentinel behavior in groups of the Florida scrub jay (Aphelocoma caerulescens). Sentinels in this species turn their heads frequently to monitor different areas for threats. As predicted, we found that sentinels turned their heads less frequently in the presence of other sentinels. Multiple sentinels, however, tended to gaze in the same direction at the same time more often than predicted by chance alone. Gaze synchronization reduces the efficiency of collective detection by reducing visual coverage at any one time at the group level. Despite the benefits of the presence of other sentinels, our results highlight the limits to collective detection when multiple individuals are vigilant at the same time.