OF EASTERN PENNSYLVANIA. 193 



The interior is chiefly lined with feathers, but, 

 occasionally, with a mixture of feathers and soft 

 grasses, especially Poa anmia. It is probable that 

 the saliva of the birds materially aids in the agglu- 

 tination of the particles of mud. The fact that the 

 nests crumble to pieces with great facility, is no 

 argument that this fluid is not utilized. They 

 adhere so firmly to their supporting surfaces as to 

 require considerable physical effort for their de- 

 tachment. While intact they will endure the 

 peltings of winter. The force requsite to separate 

 them from the surfaces of contact, undoubtedly 

 tends to diminish the force of adhesion resident in 

 the mud-particles, and thus renders the structure 

 exceedingly fragile, which has led some to believe 

 that saliva is not used. The small twigs which 

 compose the nest of the Chimney Swift, it is well- 

 known, are held in situ by a viscid secretion 

 elaborated by that species. Experience has taught 

 us that this secretion looses its adhesive property 

 in time, even when the nests are kept in either moist 

 or dry situations. It is difficult to preserve a nest 

 intact fora sixth-month. This is readily accounted 

 for. Clay and earth of any kind being more 

 porous and brittle than wood, it is a reasonable 

 presumption that after the lapse of time when the 

 saliva has lost much of its virtue, that it will yield 

 with great readiness to natural forces. In fine, 

 the saliva would mingle with the moisture. of the 

 clay, which moisture would be imparted to the 

 clay, even in the dryest places, and be equally 



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