i62 THE NESTS AND EGGS OE BRITISH BIRDS. 



excavation where the rocks overhang ; sometimes on the 

 smooth, bare face, but invariably under a projection of 

 some kind. The nest is made externally of mud, a semi- 

 globular structure, shaped like half a basin, in some 

 cases more elongated than others. This mud is brought 

 in small pellets and built on piece by piece, sometimes 

 with bits of straw intermixed, to give it better adhesive 

 qualities. The narrow, but rather wide entrance-hole is 

 at the top, cither at the front or on one side. The inside 

 of this mud shell is lined with dry grass and feathers. 

 The birds are very much attached to their breeding- 

 places, and will build nest after nest in one chosen spot, 

 as soon as they are removed. 



Range of egg colouration and measurement : 

 The eggs of the House Martin are four or five, rarely six 

 in number. They are pure and spotless white, smooth 

 in texture, and somewhat glossy. Average measurement, 

 77 inch in length, by "54 inch in breadth. Incubation, 

 performed by both sexes, lasts thirteen days. 



Diagnostic characters -. The larger size and more 

 polished surface of the eggs of the House Martin dis- 

 tinguish them from those of the Sand Martin, the only 

 species with which they are likely to be confused. The 

 nests of the two species^ however, are very difterent. 



