REPRODUCTION— NIDIFICATION 189 



inch in depth, the walls of this fragile structure do not exceed an 

 eighth of an inch in thickness. In this tiny space a single egg 

 is laid which, in most species of the genus, appears to be incub- 

 ated in the usual manner, though at this time, owing to the large 

 si'/.e of the bird, the nest becomes invisible. But one species, 

 the Javan Tree-swift {M. /o//^<^//^('//f//s), builds so flimsily — the 

 walls of the nest being no thicker than parchment — that the 

 sitting bird has to rest her weight upon the bough, to the side of 

 which the nest is attached, contriving so that the abdomen is in 

 contact with the egg. 



We have already alluded to the fact that many birds breed 

 in colonies, often of vast extent, as in the case of Penguins, Alba- 

 trosses, Flamingoes and the members of the Gull tribe, which 

 may often be numbered by millions. Here the nests, often but 

 the rudest structures, are all separate, though they may be 

 divided only by a few inches. Smaller colonies are formed by 

 many species, notably by the Swifts and Swallows, where the 

 nests, sometimes to the number of several hundreds, are fre- 

 quently so crowded as to touch one another. And thus we find 

 the way paved for the evolution of a yet more intimate relation 

 between associated nests. Yet but two species appear to have 

 adopted the practice. One of these is a Weaver-bird {Ploceida)^ 

 the other the Sociable Grosbeak of Africa {PJiilJiatcerus socius). 

 The enormous umbrella-shaped, thatch-like masses which these 

 birds combine to erect in trees have been known to contain as 

 many as three hundred and twenty-seven separate nests, but 

 whether the whole is produced by the packing together of separ- 

 ately constructed nests, or whether the nesting tunnels and 

 chambers are driven through after the common foundation has 

 been built, does not seem to be known ! 



Probably this arboreal " warren " is formed by the interlock- 

 ing of separate nests, since this method of building is also 

 practised by the Wax-wing of San Domingo {Dul/is dominlms), 

 several pairs of which will often join their nests, made of twigs, 

 into a circular mass. 



The huge nests of the Osprey and Secretary-bird present 

 us with yet another aspect of colony formation among birds ; 

 for both species permit small Passerine birds to build their nests 

 within the outer walls of the castles of their overlords, much in 



