1 14 ILLINOIS ACADEMY OF SCIENCE 



streams, the beaver-dams are conspicuous objects. The 

 dams are built of tag-alder principally, with poplar and 

 birch. The tag-alder is an almost constant border tree along 

 the streams. I have seldom seen any indication that its bark 

 is used for food, but it is extensively cut to form the dams. 

 These dams are usually rather loose structures, the chinks 

 of which are filled in by floating debris that is entangled in 

 the brush cut and piled by the beavers. Many of the dams 

 have the appearance of being constructed by the accumulation 

 of the stripped poles and logs that have floated down from 

 the beaver pond and lodged in the pile of alder brush which 

 initiates the dam. The dams usually bend down stream and 

 will vary from a few yards in length to many hundred feet. 

 The long dam in the accompanying map was 256 feet in 

 length. Morgan records dams three times this length. The 

 dam varies in height from a few inches to several feet. I 

 recall one dam on the branch of the Huron River that was 

 something over 300 feet long and some 12 feet high at its 

 highest point. In the beaver pond the houses are built and 

 near the house in the fall there is accumulated a pile of brush 

 made up ordinarily of poplar branches, — that is the store 

 of food for winter. 



Beavers are vegetarians and feed on bark and tender 

 twigs of the poplar, occasionally on birch and other trees, 

 but the poplar is the staple article of food. Populus tre- 

 muloides and grandidentata are the common forms in north- 

 ern Michigan. Succulent roots and tender shoots of many 

 of the plants growing along and in the streams are also eaten 

 during the spring and summer. In the fall, most activity 

 is seen in the beaver colony, for then the dam and houses 

 are repaired and the winter stores accumulated. The pop- 

 lar trees are cut while the beaver sits upon its haunches, 

 supported in part by the tail, and the stump is usually from 

 16 inches to 20 inches in height. Poplars from 4 inches to 

 6 inches in diameter seem to be the favorites, but much larger 

 trees are cut down up to 18 inches. Larger cuttings are re- 

 ported, but I have never observed them in the northern 

 woods. The branches are trimmed off the trees, the trunks 

 cut into convenient lengths, three feet to six feet long and the 

 branches and logs are transported to the pond to be piled 

 in the heap of winter food. Not infrequently one finds a 

 stump showing one cutting running halfway through it, 

 which had evidently been abandoned by a beaver, then a 

 second beaver has finished cutting down the tree, but instead 

 of completing the cut started by the first beaver, he made a 

 new one three inches to four inches above the first partial 

 cut. In several instances, trees had been cut off by the 

 beavers, but instead of falling the butt of the tree had slip- 



