Adult Yellow-crowned Night-herons face in opposite directions at the nest

Vigilance is especially important in colonial socially monogamous birds during the nesting season as nest materials, offspring, and mates are vulnerable to theft, depredation, and extra-pair copulations, respectively. We found that when both members of a mated pair of Yellow-crowned Night-herons (Nyctanassa violacea) were at the nest they faced in opposite directions in 73% of observations, which was significantly more often than would be expected by chance (P < 0.0001, χ²₁ = 33.3). This behavior may improve vigilance against intruders from all directions. When an extra-pair conspecific was present at the nest of a mated pair, members of the pair were significantly more likely to orient in the same direction towards the conspecific rather than face opposite directions. In 95% of all cases in which an extra-pair conspecific was present, at least one member of the mated pair faced it, indicating that extra-pair conspecifics are perceived as threats by nesting pairs.