Alvarez F. (2003) Relationship between egg size and shape of an Iberian population of Common Cuckoo Cuculus canorus and its host. ARDEA 91 (1): 127-129

The hypotheses of increased egg strength and incubation efficacy were tested in relation to egg size and shape of Common Cuckoos Cuculus canorus parasitising Rufous Bush Chats Cercotrichas galactotes in southern Spain. The found positive relationship between weight and elongation of Cuckoo eggs (larger and therefore weaker eggs are more elongated, while small ones are more spherical) is in accord with absence of puncture-egg-ejection by the host and supports the hypothesis of optimization of heat transfer from the female to the egg during incubation