Bird lung mean HU density

I have a dataset from turkeys (bird). I’d like to measure the volume of the lungs at different ages of the animals. In the literature there is an average HU value for some species lungs but not for the turkey. What is the optimal method to calculate a mean HU value for an organ? I’d like to concentrate on the lung tissue not the vessels in the lung.
So far I did the following.

  1. I make a new segment with the segment editor form the lung on some consecutive slices.
  2. With the segment statistic I calculated a mean HU value for this segment.
    What should be the next step for lung volume measurement? What value should I use for further treshold segmentation?

Thanks in advance for your comments,