Jan., 1910 
A DEFENSE OF OOLOGY 
21 
but a single set, or the very remarkable “purplish eggs” of the California Jay 
(. Aphelocoma calif ornica californica) which are so entirely different from the usual 
brown-spotted green specimens. I happen to have found but two sets of this type. 
Mr. H. A. Snow of Newark also terms them rare, while Mr. H. W. Carriger states 
he has found them commonly in the vicinity of Sonoma. In 1894 I collected a set 
of eggs of the Black-headed Grosbeak ( Zamelodia melano cep halo) of a type I have 
never seen since, having an almost white ground color lightly spotted with bright 
rusty-brown. In 1890 I took an egg of the California Towhee (Pipilo crissalis 
crissalis) which for glossiness I have never seen equaled among eggs of this 
species. In fact the texture and thickness of egg shells should also be lookt into 
more carefully; for it seems that altho the food of birds is widely different, yet 
each species is enabled to secrete sufficient material for a protective covering for 
Fig. 11. TWENTY EGGS OF THE BREWER BLACKBIRD, SELECTED FROM NINE SETS TO SHOW 
VARIATION IN COLOR AND SHAPE; SOMEWHAT REDUCED IN SIZE 
the embryo. Whether this lime-shell material is in some cases derived solely from 
food, or from water alone is an interesting question. In some sections, as the 
higher Sierras, the water being snow-water is almost pure. Analysis of the foods 
of birds would perhaps show what percentage of lime could be obtained from them. 
From my own observation I would say that the thickness of egg-shells seems solely 
due to the intention of Nature, for I have failed to discern any difference in eggs of 
the same bird from different localities; while on the other hand, as in the case of 
the Cliff and Rough-winged Swallows, we have both hard and soft-shelled eggs 
in birds of the same family — breeding in the same locality and living on the same or 
very similar diet. I am not aware whether it is a common characteristic of them 
or not, but the eggs of the Rock Wren ( Salpinctes obsoletus ) which I took on the 
