In this poster, we describe the 2-D acoustic array design for robust acoustic beamforming used in woodpecker localization sensor networks. Although woodpecker calls are not narrow-band signals, due to the rugged spectral power density of woodpeckers, the beampattern of the 2-D acoustic array has side lobes that are nearly as tall as the main lobe. With strong multi-path effects of sound propagation in the woods, the sidelobe can easily grow taller than the main lobe and thus causes large error in the beamforming based target direction estimation. We propose to choose the microphone spacing in the 2-D acoustic array according to the narrow-band beamforming of the dominant frequency of the woodpecker calls. This design greatly improves the robustness of target direction estimation using acoustic beamforming.