Page:The Zoologist, 4th series, vol 5 (1901).djvu/134

110 or may not be cracked by the blow." This is true; but if the nut is broken by the bird, then it has been hacked. The Nuthatch habitually breaks nuts.— (Bloxham, Oxon).

Note on the Weight and Specific Gravity of some Common Eggs.—It is with diffidence and some hesitation that I record the following details of some few eggs that I have weighed, and of which I have also taken the specific gravity, during the past egg season. It is very probable that the average weights of the eggs of the commoner species have been already recorded, and it is difficult for the dweller in the country, with few opportunities of consulting the literature of a subject, to be aware of what has been done. He is therefore restricted to the recording of facts of observation honestly, and with the greatest care and accuracy of which he is capable; and he must leave to those with larger material and better opportunities for reference at their command the deductions which may be drawn from the facts which he has put on record. This is perhaps the less to be regretted when one recognises the danger, so difficult to avoid, of being more or less unconsciously influenced by the exigencies of an imperfectly formed theory while recording the facts which may or may not support it.

The weighings given below were made on a chemical balance turning with ·001 gramme, and may be taken as accurate. The specific gravities must not be taken as absolute, but are accurate when compared inter se, the source of error being the weight and specific gravity of the fine india-rubber ring and thread used to support the egg while being weighed in water. It would doubtless have been desirable to have weighed a clutch of each species, but this was impossible in the time at my disposal; in two instances, however—Turdus merula and Vanellus vanellus—three eggs from the same nest were dealt with, and it will be seen that the variation in weight and specific gravity is very noticeable. Again, if the weights of eggs of the same species are to be compared, they should perhaps be weighed at the same age, though whether any appreciable change takes place until after the bird has begun to sit regularly is doubtful. In all the cases given, except three—Alauda arvensis, A. phragmitis, and Sylvia sylvia—the eggs were taken before the whole clutch was laid, and in none except the last named had the process of incubation apparently commenced. With these exceptions, however, the specific gravities may be taken as comparable. It was, of course, to be expected that the weights of eggs even from the same nest would vary, but the considerable variation in the specific gravities was unexpected, and may perhaps be partly due to a varying amount of salts in the shell and contents of the egg, caused by a greater or lesser amount of lime in the diet of the parent during the time the eggs are being matured; or it may possibly be partly due to difference