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After following such a line once or twice, the notes will probably not 
need to be referred to except where traps are changed to better locations. 
The date of running each trap-line should be recorded and it is of interest 
to record bait used and animals taken at each trap. Such notes on trap- 
lines will be extremely useful in writing up field notes on mammals, and 
particularly valuable in checking fluctuations from year to year. Counting 
up the results of “trap-nights” is the best method known for estimating the 
comparative numbers of small mammals in the same area from year to 
year, or the relative abundance at any time. One trap set for one night 
counts as one “trap-night”, and fifty traps set every night for a week, as 
three hundred and fifty “trap-nights.” 
The number of traps used will depend entirely upon conditions. Some 
commercial fur trappers in bush country where the snow does not drift 
over the traps will use two hundred or more traps and run a trap-line for 
100 miles, perhaps taking 2 weeks to make the rounds. On the wind- 
swept barren grounds, where traps are either blown clean or buried in 
hard snow every day or two, the trapper will have hard work to attend 
to much more than twenty traps efficiently during the short days. 
For small mammals the trap-lines will not need to be so long, as the 
mammals may be at your very door. Moreover, the traps must be visited 
every day, and in hot weather at least twice daily if possible, otherwise 
the specimens are apt to decompose, to be polluted by blowflies, or mangled 
by ants. When ants are very annoying, the writer has found that dusting 
the traps with pyrethrum insect powder will often keep them away. The 
powder does not seem to prevent mice and shrews from visiting the traps. 
Ordinarily a trapper of small mammals can run a line of fifty or 
sixty traps, sometimes even a hundred in favourable circumstances. The 
beginner may often obtain good results by placing small traps at random 
until, after a little experience, the most favourable places are located. 
Mammals usually follow natural lines of cover, such as stone and rail 
fences, fallen logs, or edges of thick bush. Slinking along under shelter, 
the animal darts across open spaces to the nearest salient of protection. 
Traps set in such situations may be sprung by the animals in passing 
even if they are not attracted by the bait. In runways, the trap is prefer- 
ably set across the runway, and the floor of the runway may be scooped 
out to bring the trap flush with the surface. Flying squirrels may be 
caught in traps set on logs, stumps, or horizontal branches, or a little 
shelf for the trap may be made by placing a slab of bark across dead 
twigs near the trunk of the tree, or on wooden pegs driven into the 
bark. The latter method is useful where the lower branches of large 
trees are high above the ground, as in the western yellow pine. A certain 
familiarity with mammal habits will soon show what may be expected, 
as how some mammals will step up to reach an elevated bait, or step 
over a low obstacle into a trap set on the other side. The solving of 
exceptions depends upon the keenness or imagination of the collector in 
understanding animal psychology. 
Small traps set in the open should always be tied down. Small 
mammals are almost invariably quickly killed by shock or by choking, 
but larger mammals may be caught accidentally and drag the trap away 
or into a hole. About 2 feet of cord or small copper or brass wire may 
be fastened to the staple that holds down the trigger, but it is better to 
