REPRODUCTION— NIDIFICATION 185 



We must pass now to a brief survey of such nests as are 

 built of mud, wherein it will be seen that the selection of this 

 material has been independently made by the most diverse 

 orders of birds, some of which have acquired the most wonder- 

 ful skill and ingenuity in the use of this material. 



Mud is employed, as the foregoing pages have shown, by 

 many birds either to form a cement in holding together other 

 materials, or more remarkable still, to serve as a balance to 

 nests suspended by a single strand. Originally used as a 

 cement only, it is easy to see how, when the normal stable 

 materials were hard to come by, mud alone came to be em- 

 ployed. But whether the shapes of the mud-nests of to-day 

 are replicas of the older nests built of a mixture of materials, 

 or whether the evolution in intricacy which is to be met with 

 is independent of such models, we cannot say. 



The simplest types of mud-nests are cup-shaped, or rather 

 basin-shaped structures such as those built by the Grey Strut- 

 hidea and Conorax of Australia on the branches of trees ; or 

 of the huge columnar piles hollowed at the top built by the 

 Flamingo in swamps. The mud-nests of the Swallow and 

 Martin need no description here, but it is to be noted the other 

 mud-building members of this tribe show a wonderful ingenuity 

 in the construction of their nurseries, some of those of the Cliff- 

 swallows of the Genus Pctrodiclidon, for example, building retort- 

 shaped nests of no little beauty. It would seem, however, that, 

 like the Swallows generally, they originally built in holes, in- 

 asmuch as a North American species, P. pyrrkouota, will fre- 

 quently build in holes, around the entrance of which they 

 construct a rim of clay. Probably the size of this rim was 

 increased in proportion to the smallness of the hole, until at 

 last the entire nest came to be of mud whenever holes were 

 wanting. The nests of Australian Cliff-swallows are said to 

 be constructed by several birds working together, one remaining 

 inside the nest and receiving the pellets as they are brought by 

 its companions. The entrance tubes to these nests are often 

 eight or nine inches in length. But observations are wanting 

 as to whether these birds build their nests on this principle of 

 mutual help, or whether they are polygamous, each nest being 

 built by the male and his mates. But the most noteworthy 

 of nests of this kind is that built by the Oven-bird {Fiiniarins) 



