NESTS AND NEST-BUILDING IN BIRDS 257 



amount of material may be ten times greater than is commonh' 

 used, and such proceedings undoubtedly render them safer from 

 the annoyances of other birds. It appears like a measure to 

 secure protection; at any rate it is an adaptation to surround- 

 ing conditions. 



Whenever a bird of this group (see table II, of part II, No. 

 3. i. b.) makes a nest of odd or irregular form, it would be well 

 to carefully examine the site, to which it may be merely a clumsy 

 adaptation, and to note in particular the size and character of 

 its inner wall, before hastily inferring that the bird was blunder- 

 ing in the dark, through inexperience, or the loss of its " copy 

 book," so to speak. The variation may prove to be unimportant, 

 the inner wall (character No. i of the analysis given above) 

 conforming strictly to the specific type. 



The size and " perfection " of the nest is subject to a number 

 of variables (see p. 177 of part I), some of which are hard to 

 determine. When the first egg is laid before the nest is com- 

 pleted, the building instinct is liable to subside, and a some- 

 what scamped or imperfect nest to result. Echoes of this instinct, 

 however, are sometimes perceived many days after the eggs 

 have appeared and incubation is well advanced. Thus the 

 great herring gulls are likely to add fresh materials to their 

 nests at any time, and I have seen this bird while brooding her 

 eggs, reach down with her bill, pull a little fresh grass by 

 the roots and drop it on the nest wall or tuck it underneath her 

 body; the white-bellied martin will return feathers which have 

 blown from its box, even after its young are fledged, and both 

 eagles and hawks occasionally bring a fresh spray of evergreen 

 or seaweed to their eyries, but we should not be justified in 

 referring such acts to the building impulse, without a knowledge 

 of all the factors which mold conduce at other times and under 

 other conditions. Thus the gulls are constantly pulling and 

 carrying about what looks like nesting material, whether they 

 are building a nest or not, and whatever the condition of their 

 eggs or young. 



A case has indeed been recorded in which the walls of a hum- 

 mingbird's nest were gradually raised from the time the eggs 

 were laid until the young were fledged, when the structure had 

 grown to more than twice its original size. The behavior of 

 the gull just noticed should throw some light on such acts, and 



