Bird sound detection from real field recordings is essential for identifying bird species in bioacoustic monitoring. Variations in the recording devices, environmental conditions and the presence of vocalizations from other animals make the bird sound detection very challenging. In order to overcome these challenges, we propose an unsupervised algorithm comprising two main stages. In the first stage, a spectrogram enhancement technique is proposed using a multiple window Savitzky-Golay (MWSG) filter. We show that the spectrogram estimate using MWSG filter is unbiased and has lower variance compared to its single window counterpart. It is known that bird sounds are highly structured in the time-frequency (T-F) plane. We exploit these cues of prominence of T-F activity in specific directions from the enhanced spectrogram, in the second stage of the proposed method, for bird sound detection. In this regard, we use a set of four moving average filters that when applied to the enhanced spectrogram, yield directional spectrograms that capture the direction specific information. We propose a thresholding scheme on the time varying energy profile computed from each of these directional spectrograms to obtain frame-level binary decisions of bird sound activity. These individual decisions are then combined to obtain the final decision.