## 1 Introduction
We believe that direct observation of the vertical and horizontal soundscape of vocalizations among birds would also contribute to this field, as well as to bioacoustic analysis of bird behavior. There is initial work on 3D localization of bird songs using multiple microphone array units [10] and observation of nocturnal birds with a single microphone array unit based on the azimuth-elevation estimation [17]. However, we still need to investigate how HARK or HARKBird can estimate both azimuth and elevation angles of bird songs to capture the dynamics and structures of the soundscape of bird songs. In particular, a systematic evaluation of the localization accuracy of elevation angles and an estimation of the structure of soundscape in a realistic situation where multiple bird species are vocalizing are important for the practical use of the system. This paper aims to demonstrate a systematic evaluation of the localization accuracy of azimuth-elevation angles of replayed bird vocalizations in a practical forest environment, and show an example field observation of the structure and dynamics of birdsong soundscape. For this purpose, we use a self-developed 16-channel microphone array, called DACHO, using HARK and HARKBird. [PERSON] et al. [18] used the same microphone array to conduct spatiotemporal analysis of acoustic interactions between great reed warblers (_Acrocephalus arundinacus_). They conducted a 2D localization of their vocalizations using multiple arrays and estimated the location of two individuals' song posts with mean error distance of \(5.5\pm 4.5\) m from the location of observed song posts. They then evaluated the temporal localization accuracy of the songs by comparing the duration of localized songs around the song posts with those annotated by human observers, with an accuracy score of average 0.89 for one bird that stayed at one song post. However, the localization accuracy of songs in the elevation angle was not evaluated, and thus a systematic analysis of the accuracy of elevation angle estimation in field conditions would supplement and strengthen our knowledge about the application of robot audition techniques to ecoacoustic research. We used a single microphone array unit because it is a minimal system and its cost is low for field deployment. We think that sound source localization is useful to passively monitor auditory behaviors of rare or nocturnal birds. Localized results can be used to estimate the abundance and the distribution of those birds. The high portability and low deployment cost are both essential in such a case. First, we evaluated the accuracy in estimating the azimuth and elevation angles of bird vocalizations replayed from a loudspeaker on a tree, 6.55 m above the height of the array, from different horizontal distances in a forest. The results showed that the localization error of azimuth and elevation angle was equal to or less than 5 degrees and 15 degrees, respectively, in most of cases when the horizontal distance from the array was equal to or less than 35 m. We then conducted field observation of vocalizations to monitor birds in a forest. The results showed that the system can successfully capture how birds use the soundscape horizontally and vertically. This can contribute to bioacoustic and ecoacoustic research, including behavioral observations and study of biodiversity. The organization of the paper is as follows: We firstly introduce two cases of experimental trials: a speaker test and field observation of soundscape dynamics of bird vocalizations, and introduce the sound source localization method based on HARK and HARK Bird in Section 2. Then, we show experimental results of the two trials in Section 3, and finally summarize and discusses the significance of the findings and their implications for further contribution to ecoacoustics and related fields in Section 4.