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.
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Vol. 119 • No. 4