150 CALIFORNIA ACADEMY OF SCIENCES [Proc. 4th Ser. 



Residents of Mendocino City said the tree mice lined their 

 nests with hair, but this we failed to confirm. 



As remarked above, feces were found in quantity through- 

 out the entire nest structure, except in the inner nest cavities, 

 although a few feces are present in a mass of inner nest cavity 

 material which was saved and brought to the Museum. 



Even this much of a localization in the deposition of excre- 

 ment is interesting, as it seems to indicate an early stage in 

 the development of an instinct of sanitation. 



(5) Large nests the work of years 



Observations have already been made concerning the ex- 

 tremes of size displayed by different nests. For example, 

 Todd records noting a structure the size of a robin's nest 

 (1891, p. 241), or even smaller (in True, 1891, p. 304). 

 Our party observed nests which were as much as two or three 

 feet in height and three feet in transverse diameter. A sig- 

 nificant point regarding these larger nests is that only their 

 upper portions were in use. 



Clay reports that the family nests showed signs of being 

 renewed at least annually and possibly at even more frequent 

 intervals. Deserted beds were found under or alongside 

 those which were occupied. The type of branching at the 

 point where the nest was located was a most important fac- 

 tor in determining the form of the nest. Some nests were 

 built wide; in these the deserted inner cavities would be on 

 a level with the occupied one. Others were built high rather 

 than wide; in these the deserted beds would be superimposed 

 one above the other. It seems clear that each nest becomes 

 larger year by year. Apparently the discarded resin ducts of 

 the fir leaves with the twigs remaining after the tree mouse 

 has eaten oH the fleshy portions of the leaves are immediately 

 incorporated into the nest mass. This, in itself, would cause 

 a continuous increase in size. Whether there is a new nest 

 actually built on top of the old mass every year is unknown, 

 but it is not improbable that some such regular addition is 



constructed. 



(6) Desertion of nests 



Under this head Todd (1891, p. 241) says: *Tor some 

 reason which I have not been able to discover, these nests 

 seem to be frequently changed or deserted, from the fact that 



