The size and shape of kittiwake eggs are influenced by several factors, in particular by the position of the egg in the laying sequence, the breeding experience of the female and a possible inherited effect which tends to make the same female lay similar eggs in successive years. The adult kittiwake becomes larger towards the north of its range (Bergman's Rule) and probably as a consequence of this, the eggs also tend to be larger towards the north.
In general, as female kittiwakes become older, their eggs become shorter and broader while the shape index (breadth × 100/length) and volume also tend to increase. These changes become less marked as the birds become older but there is no indication of the breadth and egg volume becoming smaller in older birds although very few birds over eight years of breeding age were studied.
It is shown that several Laridae laying clutches of three eggs lay a final egg which differs markedly from the first two in both shape and size. In the Kittiwake, this difference (as nieasured by the variance) increases with age but no such difference occurs in clutches of two eggs.
The use of the egg characteristics as a means of determining the age composition of breeding females in populations or colonies is discussed and applied to two colonies of kittiwakes where the age composition was unknown. It is suggested that, according to the species and the clutch size, the shape index and the variance of the breadth within a clutch are probably the most useful characters for such a method.