of capturing all individuals present during a trapping 

 period of three nights (Bole 1939). Influx and de- 

 parture of animals, however, disturbs the accuracy 

 of the measurement. Influx is usually more of a prob- 

 lem than escape from the area, as the trap bait and re- 

 moval of captured individuals encourages invasion 

 (Stickel 1946). Since all animals whose home ranges 

 approach or overlap the boundary of the trapping 

 area are likely to be caught, a correction for this error 

 may be made by considering the census area to in- 

 clude a surrounding belt equal to one-half of the home 

 range of each species concerned (Dice 1952). In 

 order to reduce the boundary of contact with the out- 

 side area to a minimum, square or circular areas are 

 used, rather than rectangular or irregular-shaped 

 areas. Censuses taken in this manner and live-trap 

 censuses sometimes give comparable results (R. M. 

 Wetzel 1949, Buckner 1957), but in neither case can 

 one usually be certain that he has captured all the 

 inhabitants of the area (Fowle and Edwards 1954). 



Live trapping, marking, and release of individuals 

 is a more trustworthy means of censusing small mam- 

 mals, but is more laborious and time-consuming 

 (Blair 1941, Stickel 1946). Traps are usually dis- 

 tributed grid fashion at intervals of 15 to 20 meters, 

 over several acres or hectares. Trapping is continued 

 for a week, or until very few or no unmarked ani- 

 mals are captured. Marking is commonly toe clip- 

 ping, ear notching, tattooing, or tags (Taber 1956). 

 Since the animals are immediately freed, the popula- 

 tion equilibrium is not greatly disturbed, and influx 

 of extraneous individuals is negligible. The method 

 has the further advantage of allowing the determina- 

 tion of home ranges. Individual animals differ, how- 

 ever, in the readiness with which they will enter traps 

 (Geis 1955), and this will affect the accurate deter- 

 mination of home ranges. 



The type of bait used varies with the species be- 

 ing trapped. Seasonal fluctuations in numbers of ani- 

 mals trapped may sometimes be due to variability in 

 the acceptance of bait (Fitch 1954). For mice and 

 shrews, a paste made of peanut butter, oatmeal flakes, 

 and raisins is commonly used. 



The most recent and promising development of 

 technique for determining home ranges is the label- 

 ing of individuals with radioactive material, then fol- 

 lowing the movements of the freed animals by use of 

 geiger counters (Godfrey 1954, Pendleton 1956, 

 L. S. Miller 1957, Harrison 1958). This procedure 

 has also been used with amphibians ( Karlstrom 

 1957). 



Birds 



Airplanes have come into common use for cen- 

 susing large concentrations of waterfowl. Aerial pho- 

 tographs are made and enlarged, and individual birds 



36 Background 



