I902. 237 



OBSERVATIONS ON THE WEIGHTS OF BIRDS' 



ECarCjO* 



BY N. H. FOSTER. 



To the h'ish NaHiralist of November, 1901, Mr, W. H. Work- 

 man contributed a paper giving the result of his observations 

 on the weights of birds' eggs. During this year I have been 

 giving some attention to the subject; and now append the 

 measurements and weights of those I have been able to procure. 



As is well known, an ^%% loses weight during the process of 

 incubation, and as a result of my observations I have come to 

 the conclusion that this loss amounts to about 15 per cent, of 

 the weight of the fresh ^%%. Such being the case, I have 

 given the weights of fresh eggs, but in those cases where I 

 have been unable to procure eggs in this condition, the state 

 of incubation is noted. 



The eggs of all our birds also present differences in size in 

 the same species — accounted for, no doubt, by the age and 

 state of health of the bird — and where I have obtained clutches 

 showing a considerable variation, I have given the measure- 

 ments and weights of a large-sized and small-sized clutch. 



My list includes 56 species, almost all of which were taken in 

 Co. Down, and I regret having failed this season to obtain nests 

 containing eggs of the following : — Ring Ousel, Wheatear, 

 Dipper, I^ong-tailed Tit, Tree Creeper, Pied Wagtail, House 

 Martin, Siskin, Linnet, Twite, Corn Bunting, Barn Owl, 

 Heron, Mute vSwan, Shoveller, Teal, Wigeon, Tufted Duck, 

 Stock Dove, Partridge, Ringed Plover, Woodcock, Redshank 

 and Great-crested Grebe, all of which breed in this neighbour- 

 hood or within easy distance of it. 



Each of these sets is a clutch from one nest, except where 

 the letters " a " and " b " signify two separate clutches. 



MisseIy Thrush {lurdus viscivorus). 



