(first published in the CBO Annual Report for 1975)
While walking along the NW shore with Andrew Fawcett yesterday I found a Herring Gull egg which on examination was seen to be completely double shelled. The outer shell was hard and typically coloured, as also was the inner shell. A curious feature was that the inside membrane of the outer shell had normal development of blood vessels indicating that some connection must have been present between that and the embryo. Although one egg of the clutch had hatched I felt that for the emerging chick to break two shells would tough going.
As one can imagine this evidence was received back at the building with some scepticism and would not be entered in the log but for finding a further egg today. Again while walking with Andrew Fawcett (just good friends) I found a Herring Gull egg containing an obviously dead but hatching chick, this time on the south shore. The chick had partially emerged and died in its effort to be liberated from the shell. 70% of the outer shell still remained intact and again both shell layers were hard and typically coloured. The membrane of the outer shell also contained normal blood vessel development, and close examination revealed an air space between the shell layers – probably about 2 – 3 mm at the deepest part.
This space may well have been the intermediate 'lung' space which enabled gaseous diffusion to take place satisfactorily across both shells – certainly in this case the chick was in every way quite normally developed.
No member of the party had witnessed such an occurrence before and it is curious that two should be found in separate nest sites over the one weekend. Any further observations or theories of how it could happen would be appreciated.