246 The N aturaliU in La Plata. 
bird can pass in and out without turning round. 
Another species scoops a circular hollow in the soil, 
and builds, over it a dome of fine woven grass. It 
should be mentioned that the nesting habits of only 
about fifteen out of the sixty-five species comprised 
in this genus are known to us. In the genus 
Furnarius the oven-shaped clay structure is known 
to be made by three species ; a fourth builds a nest 
of sticks in a tree ; a fifth burrows in the side of a 
bank, like a kingfisher. 
The explanation of the most striking features 
of the Dendro colap tidge, their monotonous brown 
plumage, diversity of structure, versatile habits, 
and the marvellous development of the nest-making 
instinct which they exhibit is to be found, it appears 
to me, in the fact that they are the most defenceless 
of birds. They are timid, unresisting creatures, 
without strength or weapons ; their movements are 
less quick and vigorous than those of other kinds, 
and their flight is exceedingly feeble. The arboreal 
species flit at intervals from one tree to another ; 
those that frequent thickets refuse to leave their 
chosen shelter ; while those inhabiting grassy 
plains or marshes study concealment, and, when 
forced to rise, flutter away just above the surface, 
like flying-fish frightened from the wmter, and, 
w^hen they have gone thirty or forty yards, dip into 
the grass or reeds again. Their life is thus one of 
perpetual danger in a far greater degree than with 
other passerine families, such as warblers, tyrants, 
finches, thrushes, &c. ; while an exclusively insect 
diet, laboriously extracted from secret places, and 
inability to change their climate, contribute to make 
