Seasonal shifts in the daily vocal activity of a nocturnal tropical bird, the great eared nightjar ( Lyncornis macrotis )

Abstract

Many tropical birds occupy permanent territories and use vocalizations for year-round defense, yet they typically exhibit a seasonal peak in vocal activity associated with breeding. In this study, we used five years (2020–2024) of passive acoustic monitoring (PAM) data from southern Vietnam to analyze the monthly and daily rhythms of vocal activity in a nocturnal tropical bird, the great eared nightjar (Lyncornis macrotis). This sedentary species produces a distinctive “pee-ouw” call, and its vocal activity increased from 2020 to 2024. We found that the species vocalized year-round but exhibited a prominent peak from April to June, i.e., from the late dry season to the early wet season, coinciding with the probable breeding period. The daily rhythm of vocal activity varied by month. During the peak period (April–June), nightjars vocalized throughout the night. During other months, vocal activity was bimodal, characterized by two short peaks – one after sunset and another just before sunrise. We hypothesize that the all-night activity from April to June may be related to breeding, primarily serving mate attraction and pair bonding. In contrast, the bimodal pattern during the rest of the year may be associated with territorial defense.