This study addresses the critical need to understand and measure the significance of large old trees. Such understanding has been lacking because tree canopies are complex and traditional assessments typically have coarse resolution. This research mapped more than 100,000 branches from millions of laser samples using lidar technology and recorded bird behaviour through years of field observations. Comparing trees reveals that the vast majority of suitable branches are found only in large old trees. On average, large old trees had 383 metres of horizontal or dead branches preferred by birds, while medium trees had very little and young trees had none.