I was cleaning out one of my tree swallow boxes a few weeks ago shortly after the little swallows fledged, and found at the bottom an unhatched egg. Deciding to further my knowledge of the bird world even more, I carefully removed the egg to take some careful measurements. These will be listed at the bottom, right after some other measurements taken from Cornell Lab's All About Birds online field guide.
According to the website, an adult tree swallow is anywhere from 4.7-5.9" in length, and weighes 0.6-0.9 oz. While I did not have a tree swallow to measure, I did compare my egg measurements to those of Cornell Lab, and here is what I found:
Me: Egg weight- 1/8 oz. Cornell: No data
Me: Egg length- 7/8" Cornell: 7/10--8/10"
Me: Egg width- 1/2" Cornell: 1/2"--6/10"
My measurements were very close, which pleases me. Now does an old egg weigh differently than a fresh egg? I have no idea, but quite possibly it does. But I had only an old egg to weigh, and did the best with what I had. If the difference between a fresh/old egg is different, then at least I know the weight of an old tree swallow egg! That is all I have, I just thought that I would share my findings.