The structural-function hypothesis provides an alternative to signalling-based predictions to explain the remarkable diversity observed in avian eggshell colour. According to the hypothesis, protoporphyrin, the common pigment of visible speckles, lubricates and thus strengthens the shell and simultaneously moderates gas transfer across it. Correlational evidence for the structural-function hypothesis in form of a coincidence of both shell thinning and reduced evaporation with eggshell speckles comes from a restricted set of species with limited calcium supply or little nest predation and no need for camouflage of the eggs. Here, we investigate whether protoporphyrin-based pigmentation similarly affects a species with cryptically marked eggs and ample dietary calcium, the black-headed gull, Larus ridibundus. Although shell thinning of speckles occurred, this effect was minimal compared with thinning through embryonic growth. Furthermore, speckled and plain areas of the shell did not differ in water vapour conductance through the shell. We conclude that protoporphyrin speckling does not fulfil a structural function in gull eggs. Instead, during shell formation where the protoporphyrin of speckles is deposited in place of calcite it could inflict a structural cost. We propose that the mechanical and water vapour conductance functions of shell speckling need to be evaluated as separate hypotheses and that both functions could, in fact, negatively affect each other.