## 1 Introduction  
Songbirds are one of the study targets for the purpose of ecoacoustic research [1, 2]: an interdisciplinary science that investigates natural and anthropogenic sounds and their relationship to the environment across multiple scales of time and space [3], as well as bioacoustic research. This is because their vocalizations (1) can tell us a suite of useful information about the environment for monitoring, (2) have rich and complex variety of structures [4], which are used as benchmark problems for classification tasks (e.g., BirdCLEF [5]), and (3) enable bird individuals to interact in complex ways, behaving as complex systems [6]. There are several approaches for using microphone arrays to localize bird vocalizations. [PERSON] et al. recently surveyed applications of acoustic localization using autonomous recording units in terrestrial environments [7], and pointed out that ecologists will make better use of acoustic localization; it can collect large-scale animal position data with minimizing the influence on the environment if recording hardware and automated localization and classification software are more available, and their algorithms are improved for outdoor measurement. Some of these studies focused on both azimuth and elevation estimation [8; 9; 10]. [PERSON] et al. developed a 4-channel microphone array unit by combining two stereo microphones and evaluated the accuracy to estimate azimuth and elevation angles of replayed songs of a few species [9]. The results showed that most of sounds were estimated within 12 degrees of the true direction of arrivals (DOA) in the azimuth angle and 9 degrees in the elevation angle within a range of at least 30 m. It was also discussed that the DOA estimation may improve the ability to assess abundance in biodiversity surveys. However, the experiment was conducted in an open space, and the elevation angle was limited to \(-10\) to 15 degrees from horizontal. As a different approach, [PERSON] et al. constructed a microphone array system to estimate 3D position of flying songbirds with a wireless microphone array. The system was consisted of four 7-m poles arranged in a 25 m square, and each pole had two microphone channels that are placed on top and bottom of the pole. They adopted a triangulation method based on time-of-arrival differences of a sound recorded at these microphones to cross-correlate and estimate sound position. They showed that both broadcasted bird calls and calls of natural migratory birds were successfully triangulated with the accuracy and estimated accuracy of less than 3 m. In addition, there is increasing interest and development for sound event localization and detection (SELD) of various environmental sounds using microphone arrays and ambisonic microphones [11]. We expect that practical experimental analyses of natural sounds such as bird vocalizations in forests can further contribute to better use of such microphone array-based techniques in natural fields. We have been proposing that robot audition techniques [12], especially an open source software for robot audition HARK (Honda Research Institute Japan Audition for Robots with Kyoto University), can contribute to bioacoustics and ecoacoustics. It not only provides the DOA estimation of sounds, but also allows us to separate them and perform further signal processing on them, even in real time. We developed HARKBird, a collection of Python scripts for localizing bird songs in fields using HARK [13]. Previously, we confirmed the effects of playback of conspecific song on song or call responses by measuring the changes in their localized direction [14] and changes in their 2D position using a set of microphone arrays [15]. It is recognized that data characterizing the vertical structure of vegetation are becoming increasingly useful for biodiversity applications as remote sensing techniques such as radar and lidar become more readily available [16].