NESTS AND NEST-BUILDING IN BIRDS 185 



of trees which serve as support ; the outer surface or ' ' ex-um- 

 brella," has the appearance of a thatched roof, suggesting the 

 hut of a native, while the under side is perforated by entrance- 

 holes corresponding to the number of nests in use. These aggre- 

 gated nests are frequently used as refuge, and are occupied year 

 after year, the mass growing by an extension of the "eaves," 

 until they are thrown down or the tree gives way under the 

 weight. It may prove that in this instance there is really no 

 mutual aid rendered either in building a common foundation or 

 in thatching a common roof, but that the nests though closely 

 aggregated, are strictly individual units; upon this question 

 observations at the present are conflicting. 



We now come to consider the great division of individual, 

 primarily adaptive nests, which are either mainly excavated, 

 in earth {i, a, i of table 2), the more primitive fashion, or in 

 wood (i, a, 2). The nest-chamber, however formed, is apt to 

 be finished by a certain though \'ariable amount of constructive 

 effort, by the addition of a lining {i, a-a^). In many cases, as 

 in sand martins, the lining is reduced to an insignificant layer 

 of feathers, dry grass, or rootlets, and suggests the relic, or 

 echo of a time when other methods prevailed, and a nest was 

 formed more completely upon the increment plan, though this 

 interpretation, it is true, might seem to conflict with other facts. 



It is to be further noted that birds which commonly excavate, 

 such as many of the kingfishers, woodpeckers, and titmice, on 

 occasion cut short or omit this work, and use a natural cavity 

 with little or no change instead. Thus the American chickadee, 

 which is often found in class i, a, 2, not infrequently neglects 

 the work of excavation entirely, and drops back to class 2, bK 

 Again bluebirds and hornbills adapt natural cavities to their 

 needs, but in different ways, the former always constructing 

 within them a more or less elaborate cup on the increment 

 plan, while the former, as we have seen, merely reduces the 

 opening, and smears it with a sticky secretion. Most of the 

 woodpeckers which drill holes in wood in order to secure their 

 insect prey, also drill other holes which they use as nests ; accord- 

 ingly it is not surprising to find that the rufous woodpecker of 

 India (Micropterus phaeoceps) j\vhich riddles the mounds of 

 termites for its food, will occasionally drill them to make a 

 nest; nor is it strange that certain insect eating kingfishers 



