Vocalization and Behavior of the Sooty-Headed Bulbul (Pycnonotus aurigaster), the Streak-Eared Bulbul (Pycnonotus conradi), and the Stripe-Throated Bulbul (Pycnonotus finlaysoni) (Pycnonotidae, Aves) in Southern Vietnam

The bulbul family Pycnonotidae is an Old-World group of tropical birds that plays important roles in tree seed dispersal. The behavior and vocalization of bulbuls have been studied only in a few species. The objective of the present study is to describe, for the first time, the acoustic and social behavior of three species of bulbuls of the genus Pycnonotus: the sooty-headed bulbul (P. aurigaster), the streak-eared bulbul (P. conradi), and the stripe-throated bulbul (P. finlaysoni). Field studies were conducted from November 2021 to May 2022 in Cat Tien National Park, Dong Nai Province, Vietnam, where all three species are sympatric and tend to inhabit disturbed biotopes. We analyzed “observations,” that is, a single record of a bird or a group of birds of a given species. During each observation (about 800 in total), the behavior of the birds was described and, when possible, vocalizations were recorded. The total duration of phonograms of all three species amounted to about 227 min. The sooty-headed and streak-eared bulbuls are social birds that maintained pairs throughout the year, but often, especially during the nonbreeding season, gathered in larger groups. Pair members and individuals from neighboring pairs maintained acoustic contact with each other all the time. To do so, they used calls: broadband calls (noise sounds without clear structure, but with a wide continuous frequency spectrum) emitted by streak-eared bulbuls, and tonal calls (musical sounds showing one clearly defined frequency) by sooty-headed bulbuls. Both these species were only rarely observed singing, this possibly being due to reduced territorial behavior in these birds. The stripe-throated bulbul differs from the other two in singing more frequently, along with producing calls, and song could be used for territorial purposes. We assume that territorial behavior is better expressed in stripe-headed bulbuls than in the other two species. However, even stripe-throated bulbuls did not react to playback-simulated territorial intrusion in our experiments. We assume that vocalizations play important roles in the communication of the three species because we noted these sounds or others in almost all observations. Moreover, we observed a general increase in vocal activity at the beginning of the breeding season (February–March). In addition, during this time, bulbuls were most often those to sing.