I took 20 measurements of the egg using calipers on the broken edges to try and avoid the curve and the average was 0.31 mm (standard deviation of 0.02).
According to USGS Biological and Ecotoxicological Characteristics of Terrestrial Vertebrate Species Residing in Estuaries ‘Peregrine Falcon’ pre DDT shell thickness is generally around 0. 35-36 and post DDT it dropped by 10-22 % to 0.27-0.29.
So our Swift’s eggs would seem to be quite thin. This may reflect the high percentage of starlings and pigeons in their diet as contaminants tend to accumulate particularly strongly in these species.
I’ll see what it costs to have the eggshell (which is now in pieces) tested for contaminants.
Comments are welcome.