196 NORTH AMERICAN FAUNA [No. 55 



what the function of a squirrel's bushy tail in enabling them to keep 

 their balance. 



In California Wilder has discovered several of the males living in 

 burrows under logs and woodpiles on the ground under the trees 

 where their food is obtained, but this is where they live in compara- 

 tively low trees with nests in the branches often not far above the 

 ground. In Oregon the mice have not as yet been found on the 



f round, although tracks in snow at the base of the trees were thought 

 y Todd to have been made by tree mice. Hundreds of traps set on 

 the ground on logs and low branches have failed to secure any speci- 

 mens, but this may in part be due to the fact that the mice refuse 

 the ordinary trap baits used. 



The nests of the tree mice are of various sizes from the new or 

 freshly built nests of the males, the size of ordinary mouse nests, to 

 the old breeding nests in large trees that have been used and built 

 up for many years until they attain the size of a peck or half -bushel 

 measure. In fact, some would apparently fill a bushel basket and 

 have the appearance of permanent houses on as elaborate a scale as 

 a wood rat or muskrat house. The smaller nests are often well out 

 on the branches away from the trunks and at first are a mass of clean 

 dry twigs and leaf fibers with a hollow cavity inside where the mouse 

 sleeps and eats the leaves from fresh green twigs brought in nightly 

 for food. The nests are largely built up from the leaf fibers left as 

 food refuse and slowly grow in size and solidity, those on firm and 

 broad foundations eventually becoming large and solid, while those 

 in young timber are smaller and fresher in appearance. The nests 

 built high up on large limbs or forks of old trees, 60 to 100 feet from 

 the ground, are the oldest and largest. They are usually close to 

 the trunk and in some cases surround the tree trunk and rest on a 

 whorl of 5 or 7 limbs. 



These large structures are really houses, containing in some cases 

 as many as five nests in excavated cavities connected by burrows or 

 tunnels, winding spirally up and down through the .mass and open- 

 ing out on the top of the supporting branches or close to the tree 

 trunk. The main mass of these old houses consists of packed and 

 settled leaf fibers and twigs with which the mouse pellets of years 

 have become compacted into an earthy mass more or less solid and 

 substantial. There is no reason why one of these houses should not 

 continue for hundreds of years, or as long as the tree lives, and some 

 already have the appearance of great age. Usually they are rounded 

 above and more or less dome shaped and apparently shed water dur- 

 ing the rainy season and keep the inmates dry and comfortable. 



Some of the larger houses have been started on old hawks' or 

 squirrels' nests as shown by large sticks at the base which the mice 

 could not have brought together. Such strong foundations have 

 undoubtedly been a factor in the size and endurance of the tree 

 mouse houses. 



When the house is disturbed the mice usually slip out quietly along 

 the branches or up the trunk of the tree and if possible escape to the 

 next tree, but they rarely come down the trunk to the ground unless 

 forced to. They run slowly but securely along the tops of the 

 branches and from one tree to another over the interlacing tips, hid- 



